Order Now Product Folder Support & Community Tools & Software Technical Documents LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 LM7705 Low-Noise Negative Bias Generator 1 Features * * * * * * * * * * 1 3 Description Regulated Output Voltage -0.232 V Output Voltage Tolerance 5% Output Voltage Ripple 4 mVPP Supply Voltage 3 V to 5.25 V Conversion Efficiency Up to 98% Quiescent Current 78 A Shutdown Current 20 nA Turnon Time 500 s Operating Temperature Range -40C to 125C 8-Pin VSSOP Package 2 Applications * * * True Zero Amplifier Outputs Portable Instrumentation Low-Voltage Split-Power Supplies The LM7705 device is a switched capacitor voltage inverter with a low noise, -0.23 V fixed negative voltage regulator. This device is designed to be used with low voltage amplifiers to enable the amplifiers output to swing to zero volts. The -0.23 V is used to supply the negative supply pin of an amplifier while maintaining less then 5.5 V across the amplifier. Railto-Rail output amplifiers cannot output zero volts when operating from a single-supply voltage and can result in error accumulation due to amplifier output saturation voltage being amplified by following gain stages. A small negative supply voltage will prevent the amplifiers output from saturating at zero volts and will help maintain an accurate zero through a signal processing chain. Additionally, when an amplifier is used to drive an input of the ADC, the amplifier can output a zero voltage signal and the full input range of an ADC can be used. The LM7705 device has a shutdown pin to minimize standby power consumption. Device Information(1) PART NUMBER LM7705 PACKAGE VSSOP (8) BODY SIZE (NOM) 3.00 mm x 3.00 mm (1) For all available packages, see the orderable addendum at the end of the data sheet. Typical Application +V +V + In + - In -V CFLY CF+ VDD shutdown SD VOUT LM7705 VSS true zero output voltage CF-0.23V COUT CRES CRES low voltage amplifier VSS 1 An IMPORTANT NOTICE at the end of this data sheet addresses availability, warranty, changes, use in safety-critical applications, intellectual property matters and other important disclaimers. PRODUCTION DATA. LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com Table of Contents 1 2 3 4 5 6 7 Features .................................................................. Applications ........................................................... Description ............................................................. Revision History..................................................... Pin Configuration and Functions ......................... Specifications......................................................... 1 1 1 2 3 3 6.1 6.2 6.3 6.4 6.5 6.6 6.7 3 3 4 4 4 5 6 Absolute Maximum Ratings ...................................... ESD Ratings.............................................................. Recommended Operating Conditions....................... Thermal Information .................................................. 3.3-V Electrical Characteristics ................................. 5-V Electrical Characteristics .................................... Typical Characteristics .............................................. Detailed Description ............................................ 10 7.1 Overview ................................................................. 10 7.2 Functional Block Diagram ....................................... 10 7.3 Feature Description................................................. 10 7.4 Device Functional Modes........................................ 10 8 Application and Implementation ........................ 14 8.1 Application Information............................................ 14 8.2 Typical Application .................................................. 16 9 Power Supply Recommendations...................... 18 10 Layout................................................................... 19 10.1 Layout Guidelines ................................................. 19 10.2 Layout Examples................................................... 19 11 Device and Documentation Support ................. 20 11.1 11.2 11.3 11.4 Community Resources.......................................... Trademarks ........................................................... Electrostatic Discharge Caution ............................ Glossary ................................................................ 20 20 20 20 12 Mechanical, Packaging, and Orderable Information ........................................................... 20 4 Revision History NOTE: Page numbers for previous revisions may differ from page numbers in the current version. Changes from Revision C (September 2015) to Revision D Page * Deleted 'Maximum Output Current 26 mA' from Features list ............................................................................................... 1 * Deleted IO_MAX spec from 3.3-V Electrical Characteristics and 5-V Electrical Characteristics tables.................................. 4 Changes from Revision B (March 2013) to Revision C * Added Pin Configuration and Functions section, ESD Ratings table, Feature Description section, Device Functional Modes, Application and Implementation section, Power Supply Recommendations section, Layout section, Device and Documentation Support section, and Mechanical, Packaging, and Orderable Information section .............................. 1 Changes from Revision A (November 2008) to Revision B * 2 Page Page Changed layout of National Semiconductor Data Sheet to TI format .................................................................................. 19 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 5 Pin Configuration and Functions DGK Package 8-Pin VSSOP Top View 8 1 LM7705 4 5 Pin Functions PIN NAME NO. TYPE DESCRIPTION CF+ 1 Analog CFLY Positive Capacitor Connection VSS 2 Ground Power Ground SD 3 Input VDD 4 Power Positive Supply Voltage VSS 5 Ground Power Ground VOUT 6 Output Output Voltage CRES 7 Analog Reserve Capacitor Connection CF- 8 Analog CFLY Negative Capacitor Connection Shutdown Pin If SD pin is LOW, device is ON If SD pin is HIGH, device is OFF 6 Specifications 6.1 Absolute Maximum Ratings over operating free-air temperature range (unless otherwise noted) (1) MIN MAX UNIT 5.75 V VDD + 0.3 VSS - 0.3 V 150 C 260 C 150 C Supply voltage VDD - VSS SD Junction temperature (2) Mounting temperature Infrared or Convection (20 sec) -65 Storage temperature, Tstg (1) (2) Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. These are stress ratings only, which do not imply functional operation of the device at these or any other conditions beyond those indicated under Recommended Operating Conditions. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability. Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not ensured on shipped production material. 6.2 ESD Ratings VALUE Human body model (HBM), per ANSI/ESDA/JEDEC JS-001 V(ESD) (1) (2) (3) Electrostatic discharge (1) UNIT 2000 Charged-device model (CDM), per JEDEC specification JESD22C101 (2) 750 Machine model (3) 200 V Human Body Model, applicable std. MIL-STD-883, Method 3015.7. Field induced Charge-Device Model, applicable std. JESD22-C101-C. (ESD FICDM std of JEDEC). Machine model, applicable std JESD22-A115-A (ESSD MM srd of JEDEC). Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 3 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com 6.3 Recommended Operating Conditions over operating free-air temperature range (unless otherwise noted) Supply voltage (VDD to GND) MIN MAX UNIT V 3 5.25 Supply voltage (VDD wrt VOUT) 3.23 5.48 V Temperature range -40 125 C 6.4 Thermal Information LM77005 THERMAL METRIC (1) DGK (VSSOP) UNIT 8 PINS RJA (1) Junction-to-ambient thermal resistance 253 C/W For more information about traditional and new thermal metrics, see the Semiconductor and IC Package Thermal Metrics application report, SPRA953. 6.5 3.3-V Electrical Characteristics Unless otherwise specified, all limits are ensured for TA = 25C, VDD = 3.3 V, VSS = 0 V, SD = 0 V, CFLY= 5 F, CRES = 22 F, COUT = 22 F. PARAMETER TEST CONDITIONS TA = 25C -0.24 2 -40C to 125C -0.25 1 TA = 25C -0.24 2 -40C to 125C -0.25 1 IOUT = 0 mA VOUT MIN (1) Output Voltage IOUT = -20 mA TYP (2) MAX (1) -0.232 UNIT -0.219 -0.209 V -0.226 -0.219 -0.209 VR Output Voltage Ripple IOUT = -20 mA IS Supply Current No Load ISD Shutdown Supply Current SD = VDD POWER Current Conversion Efficiency -5 mA IOUT -20 mA 98% POWER Current Conversion Efficiency IOUT = -5 mA 98% tON Turnon Time IOUT = -5 mA 500 s t OFF Turnoff Time IOUT = -5 mA 700 s tOFF CP Turnoff Time Charge Pump IOUT = -5 mA 11 ZOUT Output Impedance fOSC Oscillator Frequency -1 mA IOUT -20 mA 50 78 -40C to 125C mVPP 100 150 20 TA = 25C 0.23 -40C to 125C s 0.8 1.3 VIH Shutdown Input High IC Shutdown Pin Input Current SD = VDD Load Regulation 0 mA IOUT -20 mA 1.25 TA = 25C 1.85 -40C to 125C 2.15 -40C to 125C V V 50 TA = 25C kHz 1.6 -40C to 125C A nA 92 Shutdown Input Low 4 TA = 25C TA = 25C VIL (1) (2) 4 0.12 pA 0.6 0.85 %/mA All limits are specified by testing or statistical analysis. Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not specified on shipped production material. Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 3.3-V Electrical Characteristics (continued) Unless otherwise specified, all limits are ensured for TA = 25C, VDD = 3.3 V, VSS = 0 V, SD = 0 V, CFLY= 5 F, CRES = 22 F, COUT = 22 F. PARAMETER Line Regulation TEST CONDITIONS 3 V VDD 5.25 V TA = 25C (No Load) -40C to 125C MIN (1) -0.2 TYP (2) MAX (1) 0.29 0.7 1.1 UNIT %/V 6.6 5-V Electrical Characteristics Unless otherwise specified, all limits are ensured for TA = 25C, VDD = 5.0V, VSS = 0V, SD = 0V, CFLY = 5 F, CRES = 22 F, COUT = 22 F. PARAMETER MIN (1) TYP (2) MAX (1) TA = 25C -0.24 2 -0.233 -0.219 -40C to 125C -0.25 1 TA = 25C -0.24 2 -40C to 125C -0.25 1 TEST CONDITIONS IOUT = 0 mA VOUT Output Voltage IOUT = -20 mA UNIT -0.209 V -0.226 -0.219 -0.209 VR Output Voltage Ripple IOUT = -20 mA IS Supply Current No Load ISD Shutdown Supply Current SD = VDD POWER Current Conversion Efficiency -5 mA IOUT -20 mA 98% POWER Current Conversion Efficiency IOUT = -5 mA 98% tON Turnon Time IOUT = -5 mA 200 s t OFF Turnoff Time IOUT = -5 mA 700 s tOFF CP Turnoff Time Charge Pump IOUT = -5 mA 11 ZOUT Output Impedance -1 mA IOUT -20 mA fOSC Oscillator Frequency 4 TA = 25C 103 135 240 20 TA = 25C 0.26 -40C to 125C s 0.8 1.3 91 -40C to 125C 1.95 VIH Shutdown Input High IC Shutdown Pin Input Current SD = VDD Load Regulation 0 mA IOUT -20 mA TA = 25C 3 V VDD 5.25 V (No Load) TA = 25C Line Regulation TA = 25C 2.8 -40C to 125C 50 0.14 -40C to 125C -40C to 125C V V 3.25 pA 0.6 0.85 -0.2 kHz 2.55 Shutdown Input Low A nA TA = 25C VIL (1) (2) 60 -40C to 125C mVPP 0.29 %/mA 0.7 1.1 %/V All limits are specified by testing or statistical analysis. Typical values represent the most likely parametric norm as determined at the time of characterization. Actual typical values may vary over time and will also depend on the application and configuration. The typical values are not tested and are not specified on shipped production material. Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 5 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com 6.7 Typical Characteristics VDD = 3.3 V and TA = 25C unless otherwise noted. 300 SUPPLY CURRENT (eA) OUTPUT VOLTAGE (V) -0.19 -0.20 -0.21 IOUT=10 mA IOUT = 20 mA -0.22 -0.23 250 200 125C 85C 150 25C 100 50 0 -0.24 IOUT = 5 mA IOUT = 0 mA 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) -40C 5.0 3.0 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) Figure 1. Output Voltage vs. Supply Voltage Figure 2. Supply Current vs. Supply Voltage 125C -0.20 OUTPUT VOLTAGE (V) OUTPUT VOLTAGE (V) -0.20 85C -0.21 -0.22 -40C -0.23 -0.24 25C -0.25 125C -0.21 -0.22 85C 25C -0.23 -40C -0.24 -0.25 SUPPLY VOLTAGE = 5.0V SUPPLY VOLTAGE = 3.3V 0 5 10 15 20 25 30 0 OUTPUT CURRENT (mA) 40 50 60 15 SUPPLY VOLTAGE = 3.3V 12 CRES = CFILTER = 10 eF 9 6 CRES = CFILTER = 22 eF 3 0 40 80 SUPPLY VOLTAGE = 5.0V OUTPUT VOLTAGE RIPPLE (mVPP) OUTPUT VOLTAGE RIPPLE (mVPP) 30 Figure 4. Output Voltage vs. Output Current 15 12 CRES = CFILTER = 10 eF 9 6 CRES = CFILTER = 22 eF 3 0 -40 120 TEMPERATURE (C) 0 40 80 120 TEMPERATURE (C) Figure 5. Output Voltage Ripple vs. Temperature 6 20 OUTPUT CURRENT (mA) Figure 3. Output Voltage vs. Output Current 0 -40 10 Figure 6. Output Voltage Ripple vs. Temperature Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 Typical Characteristics (continued) VDD = 3.3 V and TA = 25C unless otherwise noted. SUPPLY VOLTAGE = 5.0V SUPPLY VOLTAGE = 3.3V 20 SUPPLY CURRENT (mA) 16 -40C 12 25C 8 85C 4 0 16 -40C 12 25C 8 0 125C 0 4 85C 4 8 12 16 20 125C 0 4 SUPPLY VOLTAGE = 3.3V 110 85C 25C 100 95 125C 90 85 80 0 4 8 12 16 100 95 90 85C 25C 125C 85 80 0 4 8 12 16 20 OUTPUT CURRENT (mA) Figure 10. Current Conversion Efficiency vs. Output Current SUPPLY VOLTAGE = 5.0V 0V OUTPUT VOLTAGE (0.2V/DIV) ENABLE PULSE ENABLE VOLTAGE OUTPUT VOLTAGE (0.2V/DIV) -40C 105 SUPPLY VOLTAGE = 3.3V 0 mA 20 SUPPLY VOLTAGE = 5.0V 20 Figure 9. Current Conversion Efficiency vs. Output Current 10 mA 20 mA 16 110 OUTPUT CURRENT (mA) 0V 12 Figure 8. Supply Current vs. Output Current CURRENT CONVERSION EFFICIECY (%) CURRENT CONVERSION EFFICIECY (%) Figure 7. Supply Current vs. Output Current 105 -40C 8 OUTPUT CURRENT (mA) OUTPUT CURRENT (mA) ENABLE PULSE 0V 0 mA 0V 5 mA 10 mA 20 mA ENABLE VOLTAGE SUPPLY CURRENT (mA) 20 5 mA TURN ON TIME (200 es/DIV) TURN ON TIME (100 es/DIV) Figure 11. Turnon Time Figure 12. Turnon Time Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 7 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com Typical Characteristics (continued) VDD = 3.3 V and TA = 25C unless otherwise noted. SUPPLY VOLTAGE = 3.3V SUPPLY VOLTAGE = 5.0V 0.4 LOAD REGULATION (%/mA) 0.3 0.2 0.1 0.0 0.3 0.2 0.1 0.0 0 40 80 120 -40 0 Figure 13. Load Regulation vs. Temperature -0.210 20 +85/+125C 10 OUTPUT VOLTAGE (V) -0.226 OUTPUT CURRENT (mA) OUTPUT VOLTAGE (V) 30 -0.234 0 -0.242 30 -0.226 20 +25C 10 0 -0.242 -10 -10 -0.250 TIME (20 us/DIV) TIME (20 us/DIV) Figure 15. Transient Response Figure 16. Transient Response 40 -0.210 20 10 -0.234 +85/+125C -0.242 OUTPUT VOLTAGE (V) 30 OUTPUT CURRENT (mA) SUPPLY VOLTAGE = 5V +25C -0.226 -0.218 -40C 30 +25C 20 -0.226 10 -0.234 +85/+125C 0 -0.242 -10 -0.250 OUTPUT CURRENT -0.250 40 -0.210 SUPPLY VOLTAGE = 3.3V OUTPUT VOLTAGE (V) +85/+125C -0.234 OUTPUT CURRENT -0.250 0 OUTPUT CURRENT TIME (20 us/DIV) -10 TIME (20 us/DIV) Figure 17. Transient Response 8 SUPPLY VOLTAGE = 5V -0.218 OUTPUT CURRENT -40C 40 -40C -0.218 -0.218 120 -0.210 SUPPLY VOLTAGE = 3.3V +25C 80 Figure 14. Load Regulation vs. Temperature 40 -40C 40 TEMPERATURE (C) TEMPERATURE (C) OUTPUT CURRENT (mA) -40 OUTPUT CURRENT (mA) LOAD REGULATION (%/mA) 0.4 Figure 18. Transient Response Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 Typical Characteristics (continued) VDD = 3.3 V and TA = 25C unless otherwise noted. 300 SUPPLY VOLTAGE = 5V 0 -0.05 SUPPLY CURRENT (eA) OUTPUT VOLTAGE (V) 250 SUPPLY VOLTAGE = 3.3V -0.10 -0.15 SUPPLY VOLTAGE = 5V -0.20 200 150 SUPPLY VOLTAGE = 3.3V 100 50 -0.25 0 1 2 3 4 0 0 5 SHUTDOWN VOLTAGE (V) 1 2 3 4 5 SHUTDOWN VOLTAGE (V) Figure 19. Output voltage vs. Shutdown Voltage Figure 20. Supply Current vs. Shutdown Voltage 100 OSCILLATOR FREQUENCY (kHz) SUPPLY VOLTAGE = 3.3V 95 90 85 80 SUPPLY VOLTAGE = 5V 75 70 -40 0 40 80 120 TEMPERATURE (C) Figure 21. Oscillator Frequency vs. Temperature Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 9 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com 7 Detailed Description 7.1 Overview The LM7705 is a switched capacitor voltage inverter with a low-noise, -0.23-V fixed negative bias output. The part will operate over a supply voltage range of 3 V to 5.25 V. Applying a logical low level to the SD input will activate the part, and generate a fixed -0.23-V output voltage. The part can be disabled; the output is switched to ground level, by applying a logical high level to the SD input of the part. 7.2 Functional Block Diagram CFLY VCP,OUT VCP,IN VDD CHARGE PUMP INVERTOR PRE REGULATOR VSS VOUT CRESERVE POST REGULATOR Cout VSS fosc VREF1 VREF2 7.3 Feature Description 7.3.1 Supply Voltage The LM7705 will operate over a supply voltage range of 3 V to 5.25 V, and meet the specifications given in the 3.3-V Electrical Characteristics Table. Supply voltage lower than 3.3 V will decrease performance (The output voltage will shift towards zero, and the current sink capabilities will decrease) A voltage higher than 5.25 V will exceed the Absolute Maximum Ratings ratings and therefore damage the part. 7.3.2 Output Voltage and Line Regulation The fixed and regulated output voltage of -0.23 V has tight limits, as indicated in the 3.3-V Electrical Characteristics table, to ensure a stable voltage level. The usage of the pre- and post regulator in combination with the charge pump inverter ensures good line regulation of 0.29%/V 7.3.3 Output Current and Load Regulation The LM7705 can sink currents more than 26 mA, causing an output voltage shift to -200 mV. A specified loadregulation of 0.14% mA/V ensures a minor voltage deviation for load current up to 20 mA. 7.3.4 Quiescent Current The LM7705 consumes a quiescent current less than 100 A. Sinking a load current, will result in a current conversion efficiency better than 90%, even for load currents of 1 mA, increasing to 98% for a current of 5mA. 7.4 Device Functional Modes 7.4.1 General Amplifier Application This section will discuss a general DC coupled amplifier application. First, one of the limitations of a DC coupled amplifier is discussed. This is illustrated with two application examples. A solution is a given for solving this limitation by using the LM7705. Due to the architecture of the output stage of general amplifiers, the output transistors will saturate. As a result, the output of a general purpose op amp can only swing to a few 100 mV of the supply rails. Amplifiers using CMOS technology do have a lower output saturation voltage. This is illustrated in Figure 22. For example, Texas Instruments' LM7332 can swing to 200 mV to the negative rail, for a 10-k load, over all temperatures. 10 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 Device Functional Modes (continued) OUTPUT VOLTAGE (V) V+ OUTPUT SATURATION VDSAT 0 0 V+ INPUT VOLTAGE (V) Figure 22. Limitation of the Output of an Amplifier The introduction of operational amplifiers with output rail-to-rail drive capabilities is a strong improvement and the (output) performance of op amps is for many applications no longer a limiting factor. For example, Texas Instruments' LMP7701 (a typical rail-to-rail op amp), has an output drive capability of only 50 mV over all temperatures for a 10-k load resistance. This is close to the lower supply voltage rail. However, for true zero output applications with a single supply, the saturation voltage of the output stage is still a limiting factor. This limitation has a negative impact on the functionality of true zero output applications. This is illustrated in Figure 23. +V VIN VOUT + VDSAT - 0V 0V Figure 23. Output Limitation for Single-Supply True Zero Output Application In the One-Stage, Single-Supply True Zero Amplifier section, two applications will be discussed, showing the limitations of the output stage of an op amp in a single supply configuration: * A single stage true zero amplifier, with a 12-bit ADC back end. * A dual stage true zero amplifier, with a 12-bit ADC back end. 7.4.1.1 One-Stage, Single-Supply True Zero Amplifier This application shows a sensor with a DC output signal, amplified by a single supply op amp. The output voltage of the op amp is converted to the digital domain using an Analog to Digital Converter (ADC). Figure 24 shows the basic set-up of this application. +V LMP7701 VREF ADC122S021 + ADC - SENSOR RG1 RF1 GAIN = 50x Figure 24. Sensor With DC Output and a Single-Supply Op Amp Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 11 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com Device Functional Modes (continued) The sensor has a DC output signal that is amplified by the op amp. For an optimal signal-to-noise ratio, the output voltage swing of the op amp must be matched to the input voltage range of the Analog to Digital Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However, the low side of the range cannot be adjusted and is affected by the output swing of the op amp. Example: Assume the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7701, using a 0/+5V supply voltage, having an output drive of 50 mV from both rails. This results in an output range of 50 mV to 4.95V. Select two resistors values for RG1 and RF1 that result in a gain of 50x. The output of the LMP7701 must swing from 0 mV to 4.5 V. The higher value is no problem, however the lower swing is limited by the output of the LM7701 and won't go below 50 mV instead of the desired 0 V, causing a non-linearity in the sensor reading. When using a 12-bit ADC, and a reference voltage of 5 V (having an ADC step size of approximate 1.2 mV), the output saturation results in a loss of the lower 40 quantization levels of the ADCs dynamic range. 7.4.1.2 Two-Stage, Single-Supply True Zero Amplifier This sensor application produces a DC signal, amplified by a two cascaded op amps, having a single supply. The output voltage of the second op amp is converted to the digital domain. Figure 25 shows the basic setup of this application. +V 1/2 LMP7702 VREF +V + A1 1/2 LMP7702 + A2 ADC122S021 - ADC - SENSOR RG1 RF1 RG2 GAIN = 10x RF2 GAIN = 5x Figure 25. Sensor With DC Output and a 2-Stage, Single-Supply Op Amp The sensor generates a DC output signal. In this case, a DC coupled, 2-stage amplifier is used. The output voltage swing of the second op amp must me matched to the input voltage range of the Analog to Digital Converter (ADC). For the high side of the range this can be done by adjusting the gain of the op amp. However, the low side of the range can't be adjusted and is affected by the output drive of the op amp. Example: Assume; the output voltage range of the sensor is 0 to 90 mV. The available op amp is a LMP7702 (Dual LMP7701 op amp) that can be used for A1 and A2. The op amp is using a 0/+5-V supply voltage, having an output drive of 50 mV from both rails. This results in an output range of 50 mV to 4.95 V for each individual amplifier. Select two resistors values for RG1 and RF1 that result in a gain of 10x for the first stage (A1) and a gain of 5x for the second stage (A2) The output of the A2 in the LMP7702 must swing from 0V to 4.5 V. This swing is limited by the 2 different factors: 1. The high voltage swing is no problem; however the low voltage swing is limited by the output saturation voltage of A2 from the LM7702 and will not go below 50 mV instead of the desired 0 V. 2. Another effect has more impact. The output saturation voltage of the first stage will cause an offset for the input of the second stage. This offset of A1 is amplified by the gain of the second stage (10x in this example), resulting in an output offset voltage of 500mV. This is significantly more that the 50 mV (VDSAT) of A2. When using a 12-bit ADC, and a reference voltage of 5 Volt (having an ADC step size of approximate 1.2 mV), the output saturation results in a loss of the lower 400 quantization levels of the ADCs dynamic range. This will cause a major non-linearity in the sensor reading. 12 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 Device Functional Modes (continued) 7.4.1.3 Dual-Supply, True Zero Amplifiers The limitations of the output stage of the op amp, as indicated in both examples, can be omitted by using a dual supply op amp. The output stage of the used op amp can then still swing from 50 mV of the supply rails. However, the functional output range of the op amp is now from ground level to a value near the positive supply rail. Figure 26 shows the output drive of an amplifier in a true zero output voltage application. +V VIN VOUT + 0V - 0V -V Figure 26. Amplifier Output Drive With a Dual-Supply Disadvantages of this solution are: * The usage of a dual-supply instead of a simple single supply is more expensive. * A dual supply voltage for the op amps requires parts that can handle a larger operating range for the supply voltage. If the op amps used in the current solution cannot handle this, a redesign can be required. A better solution is to use the LM7705. This low-noise negative bias generator has some major advantages with respect to a dual-supply solution: * Operates with only a single positive supply, and is therefore a much cheaper solution. * The LM7705 generates a negative supply voltage of only -0.23 V. This is more than enough to create a Truezero output for most op amps. * In many applications, this small extension of the supply voltage range can be within the abs max rating for many op amps, so an expensive redesign is not necessary. In the Typical Application section, a typical amplifier application will be evaluated. The performance of an amplifier will be measured in a single supply configuration. The results will be compared with an amplifier using a LM7705 supplying a negative voltage to the bias pin. Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 13 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com 8 Application and Implementation NOTE Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI's customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality. 8.1 Application Information 8.1.1 Functional Description The LM7705, low-noise negative bias generator, can be used for many applications requiring a fixed negative voltage. A key application for the LM7705 is an amplifier with a true zero output voltage using the original parts, while not exceeding the maximum supply voltage ratings of the amplifier. The voltage inversion in the LM7705 is achieved using a switched capacitor technique with two external capacitors (CFLY and CRES). An internal oscillator and a switching network transfers charge between the two storage capacitors. This switched capacitor technique is given in Figure 27. V+ CAP+ S3 S1 CFLY S4 S2 OUT=V- CAP- CRES O1 O2 OSCILLATOR Figure 27. Voltage Inverter The internal oscillator generates two anti-phase clock signals. Clock 1 controls switches S1 and S2. Clock 2 controls switches S3 and S4. When Switches S1 and S2 are closed, capacitor CFLY is charged to V+. When switches S3 and S4 are closed (S1 and S2 are open) charge from CFLY is transferred to CRES and the output voltage OUT is equal to -V+. Due to the switched capacitor technique, a small ripple will be present at the output voltage with a frequency of the oscillator. The magnitude of this ripple will increase for increasing output currents. The magnitude of the ripple can be influenced by changing the values of the used capacitors. 8.1.2 Technical Description As indicated in Functional Description, the main function of the LM7705 is to supply a stabilized negative bias voltage to a load, using only a positive supply voltage. A general block diagram for this charge pump inverter is given in Figure 28. The external power supply and load are added in this diagram as well. LM7705 POWER SUPPLY PRE REGULATOR CHARGE PUMP POST REGULATOR LOAD Figure 28. LM7705 Architecture The architecture given in Figure 28 shows that the LM7705 contains 3 functional blocks: * Pre-regulator * Charge pump inverter * Post-regulator 14 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 Application Information (continued) The output voltage is stabilized by: * Controlling the power supplied from the power supply to the charge pump input by the pre-regulator * The power supplied from the charge pump output to the load by the post-regulator. A more detailed block diagram of the negative bias generator is given in Figure 29. The control of the preregulator is based on measuring the output voltage of the charge pump. The goal of the post-regulator is to provide an accurate controlled negative voltage at the output, and acts as a lowpass filter to attenuate the output voltage ripple. The voltage ripple is a result of the switching behavior of the charge pump and is dependent of the output current and the values of the used capacitors. CFLY VCP,OUT VCP,IN VDD CHARGE PUMP INVERTOR PRE REGULATOR VSS VOUT CRESERVE POST REGULATOR Cout VSS fosc VREF1 VREF2 Figure 29. Charge Pump Inverter With Input and Output Control In Charge Pump Theory, a simple equation will be derived that shows the relation between the ripple of the output current, the frequency of the internal clock generator and the value of the capacitor placed at the output of the LM7705. 8.1.3 Charge Pump Theory This section uses a simplified but realistic equivalent circuit that represents the basic function of the charge pump. The schematic is given in Figure 30. A B V2 V1 CFLY CRES RL Figure 30. Charge Pump When the switch is in position A, capacitor CFLY will charge to voltage V1. The total charge on capacitor CFLY is Q1 = CFLY x V 1. The switch then moves to position B, discharging CFLY to voltage V2. After this discharge, the charge on CFLY will be Q2 = CFLY x V2. The charge has been transferred from the source V1 to the output V2. The amount of charge transferred is: Aq = q1 -q2 = CFLY (V1 V2) (1) When the switch changes between A and B at a frequency f, the charge transfer per unit time, or current is: I = f Aq = f CFLY (V1 V2) (2) The switched capacitor network can be replaced by an equivalent resistor, as indicated in Figure 31. Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 15 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com Application Information (continued) REQ V2 V1 CRES RL Figure 31. Switched Capacitor Equivalent Circuit The value of this resistor is dependent on both the capacitor value and the switching frequency as given in Equation 3 I= V1 V2 V1 V2 = REQ 1 * (c)f CFLY (3) The value for REQ can be calculated from Equation 3 and is given in Equation 4 REQ = 1 * (c) f CFLY (4) Equation 4 show that the value for the resistance at an increased internal switching frequency, allows a lower value for the used capacitor. 8.2 Typical Application This section shows the measurement results of a true zero output amplifier application with an analog to digital converter (ADC) used as back-end. The biasing of the op amp can be done in two ways: * A single supply configuration * A single supply in combination with the LM7705, extending the negative supply from ground level to a fixed -0.23 Voltage. VREF +V LMP7701 VIN ADC122S021 + ADC SDO - CRES -V A B +V LM7705 CFLY COUT Figure 32. Typical True Zero Output Voltage Application With or Without LM7705 16 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 Typical Application (continued) 8.2.1 Design Requirements The key specifications of the used components are shown in Table 1. Table 1. Design Parameters PARAMETERS EXAMPLE VALUE SUPPLY VOLTAGE/REFERENCE VOLTAGE Supply voltage 5V ADC Voltage Reference 5V LMP7701 VDSAT (typical) 18 mV VDSAT (over temperature) 50 mV LM7705 Output voltage ripple 4 mVPP Output voltage noise 10 mVPP ADC Type ADC122S021 Resolution 12-bit Quantization level 5V/4096 = 1.2 mV 8.2.2 Detailed Design Procedure 8.2.2.1 Basic Setup The basic setup of this true zero output amplifier is given in Figure 32. The LMP7701 op amp is configured as a voltage follower to demonstrate the output limitation, due to the saturation of the output stage. The negative power supply pin of the op amp can be connected to ground level or to the output of the negative bias generator, to demonstrate the VDSAT effect at the output voltage range. The output voltage of the LMP7701 is converted to the digital domain using an ADC122S021. This is an 12-bit analog to digital converter with a serial data output. Data processing and graphical displaying is done with a computer. The negative power supply pin of the op amp can be connected to ground level or to the output of the negative bias generator, to demonstrate the effect at the output voltage range of the op amp. Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 17 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com 8.2.3 Application Curves The output voltage range of the LMP7701 has been measured, especially the range to ground level. A small DC signal, with a voltage swing of 50 mVPP is applied to the input. The digitized output voltage of the op amp is measured over a given time period, when its negative supply pin is connected to ground level or connected to the output of the LM7705. Figure 33 shows the digitized output voltage of the op amp when its negative supply pin is connected to ground level. The output of the amplifier saturates at a level of 14 mv (this is in line with the typical value of 18 mV given in the datasheet) The graph shows some fluctuations (1-bit quantization error). Figure 34 show the digitized output voltage of the op amp when its negative supply pin is connected to the output of the LM7705. Again, the graph shows some 1-bit quantization errors caused by the voltage ripple and output noise. In this case the op amps output level can reach the true zero output level. Figure 33 and Figure 34 show that: * With a single supply, the output of the amplifier is limited by the VDSAT of the output stage. * The amplifier can be used as a true zero output using a LM7705. * The quantization error of the digitized output voltage is caused by the noise and the voltage ripple. * Using the LM7705 does not increase the quantization error in this set up. 0.050 DIGITIZED OUTPUT VOLTAGE (V) DIGITIZED OUTPUT VOLTAGE (V) 0.050 0.040 0.030 0.020 0.010 0.040 0.030 0.020 0.010 VDSAT 0.000 0 80 160 240 320 0.000 0 400 TIME (SAMPLES) 80 160 240 320 400 TIME (SAMPLES) Figure 33. Digitized Output Voltage Without LM7705 Figure 34. Digitized Output Voltage With LM7705 9 Power Supply Recommendations To prevent large variations at the VDD pin of the package it is recommended to add a decouple capacitor as close to the pin as possible. 18 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 LM7705 www.ti.com SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 10 Layout 10.1 Layout Guidelines The LM7705 is a switched capacitor voltage inverter. This means that charge is transferred from different external capacitors, to generate a negative voltage. For this reason the part is very sensitive for contact resistance between the package and external capacitors. TI also recommends to use low ESR capacitors for CFLY, CRES and COUT in combination with short traces. The output voltage noise can be suppressed using a small RF capacitor, will a value of, for example, 100 nF. 10.2 Layout Examples Figure 35 contains a layout example for the LM7705. CBYPASS COUT CRES CFLY Figure 35. Example PCB Layout: Top layer Figure 36. Schematics for Example PCB Layout Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 19 LM7705 SNVS420D - NOVEMBER 2008 - REVISED MAY 2018 www.ti.com 11 Device and Documentation Support 11.1 Community Resources The following links connect to TI community resources. Linked contents are provided "AS IS" by the respective contributors. They do not constitute TI specifications and do not necessarily reflect TI's views; see TI's Terms of Use. TI E2ETM Online Community TI's Engineer-to-Engineer (E2E) Community. Created to foster collaboration among engineers. At e2e.ti.com, you can ask questions, share knowledge, explore ideas and help solve problems with fellow engineers. Design Support TI's Design Support Quickly find helpful E2E forums along with design support tools and contact information for technical support. 11.2 Trademarks E2E is a trademark of Texas Instruments. All other trademarks are the property of their respective owners. 11.3 Electrostatic Discharge Caution These devices have limited built-in ESD protection. The leads should be shorted together or the device placed in conductive foam during storage or handling to prevent electrostatic damage to the MOS gates. 11.4 Glossary SLYZ022 -- TI Glossary. This glossary lists and explains terms, acronyms, and definitions. 12 Mechanical, Packaging, and Orderable Information The following pages include mechanical, packaging, and orderable information. This information is the most current data available for the designated devices. This data is subject to change without notice and revision of this document. For browser-based versions of this data sheet, refer to the left-hand navigation. 20 Submit Documentation Feedback Copyright (c) 2008-2018, Texas Instruments Incorporated Product Folder Links: LM7705 PACKAGE OPTION ADDENDUM www.ti.com 6-Feb-2020 PACKAGING INFORMATION Orderable Device Status (1) Package Type Package Pins Package Drawing Qty Eco Plan Lead/Ball Finish MSL Peak Temp (2) (6) (3) Op Temp (C) Device Marking (4/5) LM7705MM/NOPB ACTIVE VSSOP DGK 8 1000 Green (RoHS & no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 F26A LM7705MME/NOPB ACTIVE VSSOP DGK 8 250 Green (RoHS & no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 F26A LM7705MMX/NOPB ACTIVE VSSOP DGK 8 3500 Green (RoHS & no Sb/Br) SN Level-1-260C-UNLIM -40 to 125 F26A (1) The marketing status values are defined as follows: ACTIVE: Product device recommended for new designs. LIFEBUY: TI has announced that the device will be discontinued, and a lifetime-buy period is in effect. NRND: Not recommended for new designs. Device is in production to support existing customers, but TI does not recommend using this part in a new design. PREVIEW: Device has been announced but is not in production. Samples may or may not be available. OBSOLETE: TI has discontinued the production of the device. (2) RoHS: TI defines "RoHS" to mean semiconductor products that are compliant with the current EU RoHS requirements for all 10 RoHS substances, including the requirement that RoHS substance do not exceed 0.1% by weight in homogeneous materials. Where designed to be soldered at high temperatures, "RoHS" products are suitable for use in specified lead-free processes. TI may reference these types of products as "Pb-Free". RoHS Exempt: TI defines "RoHS Exempt" to mean products that contain lead but are compliant with EU RoHS pursuant to a specific EU RoHS exemption. Green: TI defines "Green" to mean the content of Chlorine (Cl) and Bromine (Br) based flame retardants meet JS709B low halogen requirements of <=1000ppm threshold. Antimony trioxide based flame retardants must also meet the <=1000ppm threshold requirement. (3) MSL, Peak Temp. - The Moisture Sensitivity Level rating according to the JEDEC industry standard classifications, and peak solder temperature. (4) There may be additional marking, which relates to the logo, the lot trace code information, or the environmental category on the device. (5) Multiple Device Markings will be inside parentheses. Only one Device Marking contained in parentheses and separated by a "~" will appear on a device. If a line is indented then it is a continuation of the previous line and the two combined represent the entire Device Marking for that device. (6) Lead/Ball Finish - Orderable Devices may have multiple material finish options. Finish options are separated by a vertical ruled line. Lead/Ball Finish values may wrap to two lines if the finish value exceeds the maximum column width. Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties. TI has taken and continues to take reasonable steps to provide representative and accurate information but may not have conducted destructive testing or chemical analysis on incoming materials and chemicals. TI and TI suppliers consider certain information to be proprietary, and thus CAS numbers and other limited information may not be available for release. Addendum-Page 1 Samples PACKAGE OPTION ADDENDUM www.ti.com 6-Feb-2020 In no event shall TI's liability arising out of such information exceed the total purchase price of the TI part(s) at issue in this document sold by TI to Customer on an annual basis. Addendum-Page 2 PACKAGE MATERIALS INFORMATION www.ti.com 8-May-2018 TAPE AND REEL INFORMATION *All dimensions are nominal Device Package Package Pins Type Drawing LM7705MM/NOPB VSSOP DGK 8 LM7705MME/NOPB VSSOP DGK LM7705MMX/NOPB VSSOP DGK SPQ Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) B0 (mm) K0 (mm) P1 (mm) W Pin1 (mm) Quadrant 1000 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 8 250 178.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 8 3500 330.0 12.4 5.3 3.4 1.4 8.0 12.0 Q1 Pack Materials-Page 1 PACKAGE MATERIALS INFORMATION www.ti.com 8-May-2018 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) LM7705MM/NOPB VSSOP DGK 8 1000 210.0 185.0 35.0 LM7705MME/NOPB VSSOP DGK 8 250 210.0 185.0 35.0 LM7705MMX/NOPB VSSOP DGK 8 3500 367.0 367.0 35.0 Pack Materials-Page 2 IMPORTANT NOTICE AND DISCLAIMER TI PROVIDES TECHNICAL AND RELIABILITY DATA (INCLUDING DATASHEETS), DESIGN RESOURCES (INCLUDING REFERENCE DESIGNS), APPLICATION OR OTHER DESIGN ADVICE, WEB TOOLS, SAFETY INFORMATION, AND OTHER RESOURCES "AS IS" AND WITH ALL FAULTS, AND DISCLAIMS ALL WARRANTIES, EXPRESS AND IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT OF THIRD PARTY INTELLECTUAL PROPERTY RIGHTS. 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