EVALUATION KIT AVAILABLE MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller General Description Features The MAX1846 is available in an ultra-compact 10-pin MAX(R) package. Operation at high frequency, compatibility with ceramic capacitors, and inverting topology without transformers allow for a compact design. Compatibility with electrolytic capacitors and flexibility to operate down to 100kHz allow users to minimize the cost of external components. The high-current output drivers are designed to drive a P-channel MOSFET and allow the converter to deliver up to 30W. Ordering Information MAX1846/MAX1847 high-efficiency PWM inverting controllers allow designers to implement compact, lownoise, negative-output DC-DC converters for telecom and networking applications. Both devices operate from +3V to +16.5V input and generate -500mV to -200V output. To minimize switching noise, both devices feature a current-mode, constant-frequency PWM control scheme. The operating frequency can be set from 100kHz to 500kHz through a resistor. The MAX1847 features clock synchronization and shutdown functions. The MAX1847 can also be configured to operate as an inverting flyback controller with an N-channel MOSFET and a transformer to deliver up to 70W. The MAX1847 is available in a 16-pin QSOP package. Current-mode control simplifies compensation and provides good transient response. Accurate current-mode control and over current protection are achieved through low-side current sensing. 90% Efficiency +3.0V to +16.5V Input Range -500mV to -200V Output Drives High-Side P-Channel MOSFET 100kHz to 500kHz Switching Frequency Current-Mode, PWM Control Internal Soft-Start Electrolytic or Ceramic Output Capacitor The MAX1847 also offers: Synchronization to External Clock Shutdown N-Channel Inverting Flyback Option PART TEMP RANGE PIN-PACKAGE MAX1846EUB -40C to +85C 10 MAX MAX1846EUB+ -40C to +105C 10 MAX MAX1847EEE -40C to +85C 16 QSOP MAX1847EEE+ -40C to +85C 16 QSOP +Denotes a lead(Pb)-free/RoHS-compliant package. Typical Operating Circuit POSITIVE VIN Applications Cellular Base Stations Networking Equipment Optical Networking Equipment SLIC Supplies CO DSL Line Driver Supplies Industrial Power Supplies Servers VOIP Supplies P VL MAX is a registered trademark of Maxim Integrated Products, Inc. 19-2091; Rev 4; 7/16 EXT MAX1846 MAX1847 COMP CS FREQ PGND REF Pin Configurations appear at end of data sheet. IN GND FB NEGATIVE VOUT MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Absolute Maximum Ratings IN, SHDN to GND..................................................-0.3V to +20V PGND to GND.......................................................-0.3V to +0.3V VL to PGND for VIN 5.7V.........................-0.3V to (VIN + 0.3V) VL to PGND for VIN > 5.7V......................................-0.3V to +6V EXT to PGND..............................................-0.3V to (VIN + 0.3V) REF, COMP to GND.....................................-0.3V to (VL + 0.3V) CS, FB, FREQ, POL, SYNC to GND.......................-0.3V to +6V Continuous Power Dissipation (TA = +70C) 10-Pin MAX (derate 5.6mW/C above +70C)...........444mW 16-Pin QSOP (derate 8.3mW/C above +70C)..........696mW Operating Temperature Range.......................... -40C to +105C Junction Temperature.......................................................+150C Storage Temperature Range............................. -65C to +150C Lead Temperature (soldering, 10s).................................. +300C Soldering Temperature (reflow) Lead(Pb)-free...............................................................+260C Containing lead(Pb)......................................................+240C Stresses beyond those listed under "Absolute Maximum Ratings" may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Electrical Characteristics (VSHDN = VIN = +12V, SYNC = GND, PGND = GND, RFREQ = 147k 1%, CVL = 0.47F, CREF = 0.1F, TA = 0C to +85C, unless otherwise noted.) PARAMETER CONDITIONS MIN TYP MAX UNITS 16.5 V PWM CONTROLLER Operating Input Voltage Range UVLO Threshold 3.0 VIN rising VIN falling -40C to ~+85C 2.8 2.95 -40C to ~+105C 2.8 2.96 -40C to ~+85C 2.6 2.74 -40C to ~+105C 2.59 2.74 UVLO Hysteresis 60 V mV FB Threshold No load -12 0 12 mV FB Input Current VFB = -0.1V CCOMP = 0.068F, VOUT = -48V, -50 -6 50 nA 0 % Load Regulation Line Regulation Current-Limit Threshold IOUT = 20mA to 200mA (Note 1) CCOMP = 0.068F, VOUT = -48V, VIN = +8V to +16.5V, IOUT = 100mA CS Input Current CS = GND Supply Current VFB = -0.1V, VIN = +3.0V to +16.5V SHDN = GND, VIN = +3.0V to +16.5V Shutdown Supply Current -1 0.04 85 100 % 115 mV 10 20 A 0.75 1.2 mA 10 25 A 1.264 V mV VIN = +3.0V to +16.5V REFERENCE AND VL REGULATOR REF Output Voltage IREF = 50A 1.236 1.25 -2 -15 VL Output Voltage 3.85 4.25 4.65 V -20 -60 mV REF Load Regulation VL Load Regulation www.maximintegrated.com IREF = 0 to 500A IVL = 100A IVL = 0.1mA to 2.0mA Maxim Integrated 2 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Electrical Characteristics (continued) (VSHDN = VIN = +12V, SYNC = GND, PGND = GND, RFREQ = 147k 1%, CVL = 0.47F, CREF = 0.1F, TA = 0C to +85C, unless otherwise noted.) PARAMETER OSCILLATOR Oscillator Frequency CONDITIONS RFREQ = 500k 1% SYNC Input Signal Duty-Cycle TYP UNITS 88 100 112 255 300 345 RFREQ = 500k 1% 93 96 98 85 88 92 % 93 % 200 ns 200 ns 550 kHz RFREQ = 147k 1% 80 7 50 Pulse Width (Note 2) SYNC Input Frequency Range kHz 500 RFREQ = 76.8k 1% Range Minimum SYNC Input Logic-Low SYNC Input Rise/Fall Time MAX RFREQ = 147k 1% RFREQ = 76.8k 1% Maximum Duty Cycle MIN 100 DIGITAL INPUTS POL, SYNC, SHDN Input High 2.0 Voltage POL, SYNC, SHDN Input Low V 0.45 V 20 40 A -4 0 1.5 6 Voltage POL, SYNC Input Current SHDN Input Current SOFT-START POL, SYNC = GND or VL VSHDN = +5V or GND VSHDN = +16.5V Soft-Start Clock Cycles -12 A 1024 Soft-Start Levels 64 EXT OUTPUT EXT Sink/Source Current EXT On-Resistance VIN = +5V, VEXT forced to +2.5V 1 3 7.5 EXT high or low, tested with 100mA load, VIN = +3V 5 12 EXT high or low, tested with 100mA load, VIN = +5V A Note 1: Production test correlates to operating conditions. Note 2: Guaranteed by design and characterization. www.maximintegrated.com Maxim Integrated 3 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Electrical Characteristics (VSHDN = VIN = +12V, SYNC = GND, PGND = GND, RFREQ = 147k 1%, CVL = 0.47F, CREF = 0.1F, TA = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER PWM CONTROLLER Operating Input Voltage Range UVLO Threshold FB Threshold FB Input Current Load Regulation Current Limit Threshold CONDITIONS VIN rising MAX UNITS 3.0 16.5 V 2.95 VIN falling 2.6 No load VFB = -0.1V MIN V -20 +20 -40C to ~+85C -50 +50 -40C to ~+105C -150 +150 -2 0 % 85 115 mV CCOMP = 0.068F, VOUT = -48V, IOUT= 20mA to 200mA (Note 1) mV nA CS Input Current CS = GND 20 A Supply Current VFB = -0.1V, VIN = +3.0V to +16.5V 1.2 mA 25 A Shutdown Supply Current SHDN = GND, VIN = +3.0V to +16.5V REFERENCE AND VL REGULATOR REF Output Voltage REF Load Regulation VL Output Voltage VL Load Regulation OSCILLATOR Oscillator Frequency Maximum Duty Cycle IREF = 50A 1.225 IVL = 100A 3.85 IREF = 0 to 500A IVL = 0.1mA to 2.0mA RFREQ = 500k 1% mV 4.65 V -60 mV 116 RFREQ = 147k 1% 255 345 93 98 RFREQ = 147k 1% 84 93 7 93 % 200 ns RFREQ = 500k 1% Minimum SYNC Input Logic Low Pulse Width SYNC Input Frequency Range V -15 84 SYNC Input Signal Duty-Cycle Range SYNC Input Rise/Fall Time 1.275 (Note 2) 100 kHz % 200 ns 550 kHz DIGITAL INPUTS POL, SYNC, SHDN Input High Voltage POL, SYNC, SHDN Input Low Voltage www.maximintegrated.com 2.0 V 0.45 V Maxim Integrated 4 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Electrical Characteristics (continued) (VSHDN = VIN = +12V, SYNC = GND, PGND = GND, RFREQ = 147k 1%, CVL = 0.47F, CREF = 0.1F, TA = -40C to +85C, unless otherwise noted.) (Note 3) PARAMETER CONDITIONS POL, SYNC Input Current POL, SYNC = GND or VL SHDN Input Current VSHDN = +5V or GND VSHDN = +16.5V EXT OUTPUT MAX UNITS 40 A -12 0 A 6 EXT high or low, IEXT = 100mA, VIN = +5V EXT On-Resistance MIN -40C to ~+85C 7.5 -40C to ~+105C 8.75 EXT high or low, IEXT = 100mA, VIN = +3V 12 Note 3: Parameters to -40C are guaranteed by design and characterization. Typical Operating Characteristics (Circuit references are from Table 1 in the Main Application Circuits section, CVL = 0.47F, CREF = 0.1F, TA = +25C, unless otherwise noted.) VIN = 16.5V 50 40 30 60 40 30 20 10 10 1 10 100 VOUT = -5V 1000 0 10,000 VIN = 12V 70 VIN = 16.5V 60 50 40 30 APPLICATION CIRCUIT B 1 10 10 VOUT = -12V 100 1000 0 10,000 VOUT = -48V APPLICATION CIRCUIT C 1 10 100 LOAD CURRENT (mA) LOAD CURRENT (mA) LOAD CURRENT (mA) OUTPUT VOLTAGE LOAD REGULATION SUPPLY CURRENT vs. SUPPLY VOLTAGE REFERENCE VOLTAGE vs. TEMPERATURE 1.6 -11.94 1.4 1.2 -11.96 -12.02 0.8 1.250 0.6 -12.04 1.258 VREF (V) IIN (mA) -12.00 1.262 1.254 1.0 -11.98 1000 MAX1846/7 toc06 -11.92 1.246 0.4 -12.06 1.242 0.2 -12.08 -12.10 80 20 MAX1846/7 toc05 -11.90 APPLICATION CIRCUIT A VIN = 3.3V VIN = 3V 50 20 0 OUTPUT VOLTAGE (V) 70 90 EFFICIENCY (%) EFFICIENCY (%) 60 80 MAX1846/7 toc04 EFFICIENCY (%) VIN = 5V 70 VIN = 5V 90 EFFICIENCY vs. LOAD CURRENT 100 MAX1846/7 toc02 MAX1846/7 toc01 90 80 100 MAX1846/7 toc03 EFFICIENCY vs. LOAD CURRENT EFFICIENCY vs. LOAD CURRENT 100 APPLICATION CIRCUIT B 0 100 200 300 VIN = 5V 400 LOAD CURRENT (mA) www.maximintegrated.com 500 600 0 VFB = -0.1V 0 2 4 6 8 VIN (V) 10 12 14 16 1.238 -40 -20 0 20 40 60 80 100 TEMPERATURE (C) Maxim Integrated 5 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Typical Operating Characteristics (continued) (Circuit references are from Table 1 in the Main Application Circuits section, CVL = 0.47F, CREF = 0.1F, TA = +25C, unless otherwise noted.) VL VOLTAGE vs. TEMPERATURE 4.340 4.300 4.26 1.250 VL (V) 4.260 VL (V) 4.220 4.24 4.180 1.245 4.23 4.140 100 200 300 400 500 IVL = 0 -40 -20 0 IREF (A) 12 10 8 VIN = 3V 6 4 14 OPERATING CURRENT (mA) VIN = 10V VIN = 16.5V -20 0 20 60 40 TEMPERATURE (C) 80 A 12 300 8 APPLICATION CIRCUIT A B: VIN = 5V, VOUT = -5V C: VIN = 16.5V, VOUT = -5V 6 B 4 298 297 -40 www.maximintegrated.com 100 400 300 200 20 40 60 80 100 0 0 100 200 300 400 500 TEMPERATURE (C) RFREQ (k) EXT RISE/FALL TIME vs. CAPACITANCE EXITING SHUTDOWN 600 MAX1846/7 toc15 FALL TIME 5V/div SHDN 0 VOUT 80 5V/div 60 20 80 0 100 295 RFREQ = 147k 1% -20 120 40 60 40 TEMPERATURE (C) 500 100 140 296 20 SWITCHING FREQUENCY vs. RFREQ C 160 TIME (ns) 299 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 10 0 100 MAX1846/7 toc13 301 -20 0 IVL (mA) A: VIN = 3V, VOUT = -12V SWITCHING FREQUENCY vs. TEMPERATURE -40 4.22 100 MAX1846/7 toc14 -40 302 294 80 2 2 0 60 OPERATING CURRENT vs. TEMPERATURE MAX1846/7 toc10 SHUTDOWN SUPPLY CURRENT (A) 14 40 TEMPERATURE (C) SHUTDOWN SUPPLY CURRENT vs. TEMPERATURE 16 20 fOSC (kHz) 0 4.100 MAX1846/7 toc11 1.240 FREQUENCY (kHz) 4.25 MAX1846/7 toc12 VREF (V) 1.255 VL LOAD REGULATION 4.27 MAX1846/7 toc09 MAX1846/7 toc07 1.260 MAX1846/7 toc08 REFERENCE LOAD REGULATION 0 RISE TIME VIN = 12V 0 2000 4000 6000 CAPACITANCE (pF) 8000 10,000 1A/div IL APPLICATION CIRCUIT B 1ms/div Maxim Integrated 6 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Typical Operating Characteristics (continued) (Circuit references are from Table 1 in the Main Application Circuits section, CVL = 0.47F, CREF = 0.1F, TA = +25C, unless otherwise noted.) HEAVY-LOAD SWITCHING WAVEFORM ENTERING SHUTDOWN MAX1846/7 toc17 MAX1846/7 toc16 SHDN VOUT 5V/div 0 5V/div 1A/div IL VOUT LX 1A/div IL 100mV/div 10V/div APPLICATION CIRCUIT B 1s/div ILOAD = 600mA APPLICATION CIRCUIT B 1ms/div LIGHT-LOAD SWITCHING WAVEFORM MAX1846/7 toc18 VOUT 100mV/div 1A/div IL LX 10V/div APPLICATION CIRCUIT B 1s/div ILOAD = 50mA LOAD-TRANSIENT RESPONSE LOAD-TRANSIENT RESPONSE MAX1846/7 toc19 MAX1846/7 toc20 ILOAD ILOAD VOUT 500mV/div IL 1A/div APPLICATION CIRCUIT B 2ms/div ILOAD = 10mA to 400mA www.maximintegrated.com VOUT 200mV/div IL 500mA/div APPLICATION CIRCUIT C 400s/div ILOAD = 4mA to 100mA Maxim Integrated 7 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Pin Description MAX1846 PIN MAX1847 NAME FUNCTION Sets polarity of the EXT pin. Connect POL to GND to set EXT for use with an external PMOS high-side FET. Connect POL to VL to set EXT for use with an external NMOS lowside FET in transformer-based applications. -- 1 POL 1 2 VL VL Low-Dropout Regulator. Connect 0.47F ceramic capacitor from VL to GND. 2 3 FREQ Oscillator Frequency Set Input. Connect a resistor (RFREQ) from FREQ to GND to set the internal oscillator frequency from 100kHz (RFREQ = 500kW) to 500kHz (RFREQ = 76.8kW). RFREQ is still required if an external clock is used at SYNC. See Setting the Operating Frequency section. 3 4 COMP Compensation Node for Error Amp/Integrator. Connect a series resistor/capacitor network from COMP to GND for loop compensation. See Design Procedure. 4 5 REF 1.25V Reference Output. REF can source up to 500A. Bypass with a 0.1F ceramic capacitor from REF to GND. 5 6 FB Feedback Input. Connect FB to the center of a resistor-divider connected between the output and REF. The FB threshold is 0. -- 7, 9 N.C. -- 8 SHDN 6 10, 11 GND 7 12 PGND 8 13 CS Positive Current-Sense Input. Connect a current-sense resistor (RCS) between CS and PGND. 9 14 EXT External MOSFET Gate-Driver Output. EXT swings from IN to PGND. 10 15 IN -- 16 www.maximintegrated.com SYNC No Connection Shutdown Control. Drive SHDN low to turn off the DC-DC controller. Drive high or connect to IN for normal operation. Analog Ground. Connect to PGND. Negative Rail for EXT Driver and Negative Current-Sense Input. Connect to GND. Power-Supply Input Operating Frequency Synchronization Control. Drive SYNC low or connect to GND to set the internal oscillator frequency with RFREQ. Drive SYNC with a logic-level clock input signal to externally set the converter's operating frequency. DC-DC conversion cycles initiate on the rising edge of the input clock signal. Note that when driving SYNC with an external signal, RFREQ must still be connected to FREQ. Maxim Integrated 8 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Typical Application Circuit 3 x 22F 10V VIN +3V to +5.5V 22k FDS6375 2 0.47F 16 SHDN 10k 150k CS MAX1847 4 0.22F EXT SYNC 220pF 3 5 VOUT -12V AT 400mA IN VL 8 CMSH5-40 15 COMP N.C. PGND 47F 16V 10H DO5022P-103 14 13 47F 16V SANYO 16TPB47M 7, 9 0.02 1W 12 R1 95.3k 1% FREQ REF FB POL 1 GND 10, 11 6 1200pF R2 10.0k 1% 0.1F www.maximintegrated.com Maxim Integrated 9 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Functional Diagram IN EXT SHDN MAX1847 ONLY STARTUP CIRCUITRY PGND EXT DRIVER VL VL REGULATOR UNDERVOLTAGE LOCK OUT SYNC MAX1847 ONLY MAX1846 MAX1847 CONTROL CIRCUITRY POL OSCILLATOR FREQ ERROR COMPARATOR COMP CS FB GM SOFT-START CURRENTSENSE AMPLIFIER ERROR AMPLIFIER PGND X3.3 SLOPE COMP REF REFERENCE GND www.maximintegrated.com Maxim Integrated 10 MAX1846/MAX1847 Detailed Description The MAX1846/MAX1847 current-mode PWM controllers use an inverting topology that is ideal for generating output voltages from -500mV to -200V. Features include shutdown, adjustable internal operating frequency or synchronization to an external clock, soft-start, adjustable current limit, and a wide (+3V to +16.5V) input range. PWM Controller The architecture of the MAX1846/MAX1847 current-mode PWM controller is a BiCMOS multi-input system that simultaneously processes the output-error signal, the current-sense signal, and a slope-compensation ramp (Functional Diagram). Slope compensation prevents subharmonic oscillation, a potential result in current-mode regulators operating at greater than 50% duty cycle. The controller uses fixed-frequency, current-mode operation where the duty ratio is set by the input-to-output voltage ratio. The current-mode feedback loop regulates peak inductor current as a function of the output error signal. Internal Regulator The MAX1846/MAX1847 incorporate an internal lowdropout regulator (LDO). This LDO has a 4.25V output and powers all MAX1846/MAX1847 functions (excluding EXT) for the primary purpose of stabilizing the performance of the IC over a wide input voltage range (+3V to +16.5V). The input to this regulator is connected to IN, and the dropout voltage is typically 100mV, so that when VIN is less than 4.35V, VL is typically VIN minus 100mV. When the LDO is in dropout, the MAX1846/MAX1847 still operate with VIN as low as 3V. For best performance, it is recommended to connect VL to IN when the input supply is less than 4.5V. Undervoltage Lockout The MAX1846/MAX1847 have an undervoltage lockout circuit that monitors the voltage at VL. If VL falls below the UVLO threshold (2.8V typ), the control logic turns the P-channel FET off (EXT high impedance). The rest of the IC circuitry is still powered and operating. When VL increases to 60mV above the UVLO threshold, the IC resumes operation from a start up condition (soft-start). Soft-Start The MAX1846/MAX1847 feature a "digital" soft-start that is preset and requires no external capacitor. Upon startup, the FB threshold decrements from the reference voltage to 0 in 64 steps over 1024 cycles of fOSC or fSYNC. See the Typical Operating Characteristics for a scope picture of the soft-start operation. Soft-start is implemented: 1) when power is first applied to the IC, www.maximintegrated.com High-Efficiency, Current-Mode, Inverting PWM Controller 2) when exiting shutdown with power already applied, and 3) when exiting undervoltage lockout. Shutdown (MAX1847 only) The MAX1847 shuts down to reduce the supply current to 10A when SHDN is low. In this mode, the internal reference, error amplifier, comparators, and biasing circuitry turn off. The EXT output becomes high impedance and the external pullup resistor connected to EXT pulls VEXT to VIN, turning off the P-channel MOSFET. When in shutdown mode, the converter's output goes to 0. Frequency Synchronization (MAX1847 only) The MAX1847 is capable of synchronizing its switching frequency with an external clock source. Drive SYNC with a logic-level clock input signal to synchronize the MAX1847. A switching cycle starts on the rising edge of the signal applied to SYNC. Note that the frequency of the signal applied to SYNC must be higher than the default frequency set by RFREQ. This frequency is required so that the internal clock does not start a switching cycle prematurely. If SYNC is inactive for an entire clock cycle of the internal oscillator, the internal oscillator takes over the switching operation. Choose RFREQ such that fOSC = 0.9 5 fSYNC. EXT Polarity (MAX1847 only) The MAX1847 features an option to utilize an N-channel MOSFET configuration, rather than the typical p-channel MOSFET configuration (Figure 1). In order to drive the different polarities of these MOSFETs, the MAX1847 is capable of reversing the phase of EXT by 180 degrees. When driving a P-channel MOSFET, connect POL to GND. When driving an n-channel MOSFET, connect POL to VL. These POL connections ensure the proper polarity for EXT. For design guidance in regard to this application, refer to the MAX1856 data sheet. Design Procedure Initial Specifications In order to start the design procedure, a few parameters must be identified: the minimum input voltage expected (VIN(MIN)), the maximum input voltage expected (VIN(MAX)), the desired output voltage (VOUT), and the expected maximum load current (ILOAD). Calculate the Equivalent Load Resistance This is a simple calculation used to shorten the verification equations: RLOAD = VOUT / ILOAD Maxim Integrated 11 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller VIN +12V 12F VP1-0190 25V 12.2H 0.47F 8 16 1 2 VL POL SYNC 0.033F 270k 150k 3 5 COMP FREQ REF VOUT -48V AT 100mA IRLL2705 EXT CS MAX1847 4 CMR1U-02 15 IN SHDN 1:4 N.C. PGND GND FB 10, 11 14 470 13 100pF 100V 7, 9 0.05 0.5W 12F 100V 383k 1% 12 6 1800pF 10.0k 1% 0.1F Figure 1. Using an N-Channel MOSFET (MAX1847 only) Calculate the Duty Cycle The duty cycle is the ratio of the on-time of the MOSFET switch to the oscillator period. It is determined by the ratio of the input voltage to the output voltage. Since the input voltage typically has a range of operation, a minimum (DMIN) and maximum (DMAX) duty cycle is calculated by: D MIN = - VOUT + VD VIN(MAX) - VSW - VLIM - VOUT + VD D MAX = - VOUT + VD VIN(MIN) - VSW - VLIM - VOUT + VD where VD is the forward drop across the output diode, VSW is the drop across the external FET when on, and VLIM is the current-limit threshold. To begin with, assume VD = 0.5V for a Schottky diode, VSW = 100mV, and VLIM = 100mV. Remember that VOUT is negative when using this formula. Setting the Output Voltage The output voltage is set using two external resistors to form a resistive-divider to FB between the output and REF (refer to R1 and R2 in Figure 1). VREF is nominally www.maximintegrated.com 1.25V and the regulation voltage for FB is nominally 0. The load presented to the reference by the feedback resistors must be less than 500A to guarantee that VREF is in regulation (see Electrical Characteristics Table). Conversely, the current through the feedback resistors must be large enough so that the leakage current of the FB input (50nA) is insignificant. Therefore, select R2 so that IR2 is between 50A and 250A. IR2 = VREF / R2 where VREF = 1.25V. A typical value for R2 is 10k. Once R2 is selected, calculate R1 with the following equation: R1 = R2 x (-VOUT / VREF) Setting the Operating Frequency The MAX1846/MAX1847 are capable of operating at switching frequencies from 100kHz to 500kHz. Choice of operating frequency depends on a number of factors: 1) Noise considerations may dictate setting (or synchronizing) fOSC above or below a certain frequency or band of frequencies, particularly in RF applications. Maxim Integrated 12 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller 2) Higher frequencies allow the use of smaller value (hence smaller size) inductors and capacitors. to the rate set by RFREQ. Choose RFREQ such that fOSC = 0.9 x fSYNC. 3) Higher frequencies consume more operating power both to operate the IC and to charge and discharge the gate at the external FET, which tends to reduce the efficiency at light loads. Choosing Inductance Value 4) Higher frequencies may exhibit lower overall efficiency due to more transition losses in the FET; however, this shortcoming can often be nullified by trading some of the inductor and capacitor size benefits for lowerresistance components. 5) High-duty-cycle applications may require lower frequencies to accommodate the controller minimum off-time of 0.4s. Calculate the maximum oscillator frequency with the following formula: f OSC(MAX) = x VIN(MIN) - VSW - VLIM VIN(MIN) - VSW - VLIM - VOUT + VD 1 t OFF(MIN) The oscillator frequency is set by a resistor, RFREQ, which is connected from FREQ to GND. The relationship between fOSC (in Hz) and RFREQ (in ) is slightly nonlinear, as illustrated in the Typical Operating Characteristics. Choose the resistor value from the graph and check the oscillator frequency using the following formula: 1 2 5.21x10 -7 + 1.92x10 -11 x R - 4.86 x10 -19 x (R FREQ ) FREQ ( ) ( ) ( ( 0.4 x ILOAD(MAX) x VIN(MAX) - VSW - VLIM - VOUT + VD The SYNC input provides external-clock synchronization (if desired). When SYNC is driven with an external clock, the frequency of the clock directly sets the MAX1847's switching frequency. A rising clock edge on SYNC is interpreted as a synchronization input. If the sync signal is lost, the internal oscillator takes over at the end of the last cycle, and the frequency is returned ) Then calculate an inductance value: L = (VIN(MAX) / IRIPPLE) x (DMIN / fOSC) Choose the closest standard value. Once again, remember that VOUT is negative when using this formula. Determining Peak Inductor Current The peak inductor current required for a particular output is: ILPEAK = ILDC + (ILPP / 2) where ILDC is the average DC inductor current and ILPP is the inductor peak-to-peak ripple current. The ILDC and ILPP terms are determined as follows: ILDC = ) External Synchronization (MAX1847 only) www.maximintegrated.com IRIPPLE = (VIN(MAX) - VSW - VLIM) Remember that VOUT is negative when using this formula. When running at the maximum oscillator frequency (fOSCILLATOR) and maximum duty cycle (DMAX), do not exceed the minimum value of DMAX stated in the Electrical Characteristics table. For designs that exceed the DMAX and fOSC(MAX), an autotransformer can reduce the duty cycle and allow higher operating frequencies. f OSC = The inductance value determines the operation of the current-mode regulator. Except for low-current applications, most circuits are more efficient and economical operating in continuous mode, which refers to continuous current in the inductor. In continuous mode there is a trade-off between efficiency and transient response. Higher inductance means lower inductor ripple current, lower peak current, lower switching losses, and, therefore, higher efficiency. Lower inductance means higher inductor ripple current and faster transient response. A reasonable compromise is to choose the ratio of inductor ripple current to average continuous current at minimum duty cycle to be 0.4. Calculate the inductor ripple with the following formula: ILPP = ILOAD 1 ( - D MAX ) (VIN(MIN) - VSW - VLIM) x DMAX L x f OSC where L is the selected inductance value. The saturation rating of the selected inductor should meet or exceed the calculated value for ILPEAK, although most coil types can be operated up to 20% over their saturation rating without difficulty. In addition to the saturation criteria, the inductor should have as low a series resistance as possible. For continuous inductor current, Maxim Integrated 13 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller the power loss in the inductor resistance (PLR) is approximated by: I PLR ~ R L x LOAD I - D MAX 2 where RL is the inductor series resistance. Once the peak inductor current is calculated, the current sense resistor, RCS, is determined by: RCS = 85mV / ILPEAK For high peak inductor currents (>1A), Kelvin-sensing connections should be used to connect CS and PGND to RCS. Connect PGND and GND together at the ground side of RCS. A lowpass filter between RCS and CS may be required to prevent switching noise from tripping the current-sense comparator at heavy loads. Connect a 100 resistor between CS and the high side of RCS, and connect a 1000pF capacitor between CS and GND. Checking Slope-Compensation Stability In a current-mode regulator, the cycle-by-cycle stability is dependent on slope compensation to prevent subharmonic oscillation at duty cycles greater than 50%. For the MAX1846/MAX1847, the internal slope compensation is optimized for a minimum inductor value (LMIN) with respect to duty cycle. For duty cycles greater then 50%, check stability by calculating LMIN using the following equation: ( ) L MIN = VIN(MIN) xR CS /M S x (2 xD MAX - 1) / (1 - D MAX ) where VIN(MIN) is the minimum expected input voltage, Ms is the Slope Compensation Ramp (41 mV/s) and DMAX is the maximum expected duty cycle. If LMIN is larger than L, increase the value of L to the next standard value that is larger than LMIN to ensure slope compensation stability. Choosing the Inductor Core Choosing the most cost-effective inductor usually requires optimizing the field and flux with size. With higher output voltages the inductor may require many turns, and this can drive the cost up. Choosing an inductor value at LMIN can provide a good solution if discontinuous inductor current can be tolerated. Powdered iron cores can provide the most economical solution but are larger in size than ferrite. Power MOSFET Selection The MAX1846/MAX1847 drive a wide variety of P-channel power MOSFETs (PFETs). The best performance, especially with input voltages below 5V, is achieved with low-threshold PFETs that specify on-resistance with a gate-to-source voltage (VGS) of 2.7V or less. When selecting a PFET, key parameters include: Total gate charge (QG) Reverse transfer capacitance (CRSS) On-resistance (RDS(ON)) Maximum drain-to-source voltage (VDS(MAX)) Minimum threshold voltage (VTH(MIN)) At high-switching rates, dynamic characteristics (parameters 1 and 2 above) that predict switching losses may have more impact on efficiency than RDS(ON), which predicts DC losses. QG includes all capacitance associated with charging the gate. In addition, this parameter helps predict the current needed to drive the gate at the selected operating frequency. The power MOSFET in an inverting converter must have a high enough voltage rating to handle the input voltage plus the magnitude of the output voltage and any spikes induced by leakage inductance and ringing. An RC snubber circuit across the drain to ground might be required to reduce the peak ringing and noise. Choose RDS(ON)(MAX) specified at VGS < VIN(MIN) to be one to two times RCS. Verify that VIN(MAX) < VGS(MAX) and VDS(MAX) > VIN(MAX) - VOUT + VD. Choose the riseand-fall times (tR, tF) to be less than 50ns. Output Capacitor Selection The output capacitor (COUT) does all the filtering in an inverting converter. The output ripple is created by the variations in the charge stored in the output capacitor with each pulse and the voltage drop across the capacitor's equivalent series resistance (ESR) caused by the current into and out of the capacitor. There are two properties of the output capacitor that affect ripple voltage: the capacitance value, and the capacitor's ESR. The output ripple due to the output capacitor's value is given by: VRIPPLE-C = (ILOAD x DMAX x TOSC ) / COUT The output ripple due to the output capacitor's ESR is given by: VRIPPLE-R = ILPP x RESR These two ripple voltages are additive and the total output ripple is: VRIPPLE-T = VRIPPLE-C + VRIPPLE-R www.maximintegrated.com Maxim Integrated 14 MAX1846/MAX1847 The ESR-induced ripple usually dominates this last equation, so typically output capacitor selection is based mostly upon the capacitor's ESR, voltage rating, and ripple current rating. Use the following formula to determine the maximum ESR for a desired output ripple voltage (VRIPPLE-D): RESR = VRIPPLE-D / ILPP Select a capacitor with ESR rating less than RESR. The value of this capacitor is highly dependent on dielectric type, package size, and voltage rating. In general, when choosing a capacitor, it is recommended to use low-ESR capacitor types such as ceramic, organic, or tantalum capacitors. Ensure that the selected capacitor has sufficient margin to safely handle the maximum RMS ripple current. For continuous inductor current the maximum RMS ripple current in the output filter capacitor is: I IRMS = LOAD x D MAX -D MAX 2 I - D MAX Choosing Compensation Components The MAX1846/MAX1847 are externally loop-compensated devices. This feature provides flexibility in designs to accommodate a variety of applications. Proper compensation of the control loop is important to prevent excessive output ripple and poor efficiency caused by instability. The goal of compensation is to cancel unwanted poles and zeros in the DC-DC converter's transfer function created by the power-switching and filter elements. More precisely, the objective of compensation is to ensure stability by ensuring that the DC-DC converter's phase shift is less than 180 by a safe margin, at the frequency where the loop gain falls below unity. One method for ensuring adequate phase margin is to introduce corresponding zeros and poles in the feedback network to approximate a single-pole response with a -20dB/decade slope all the way to unity-gain crossover. Calculating Poles and Zeros The MAX1846/MAX1847 current-mode architecture takes the double pole caused by the inductor and output capacitor and shifts one of these poles to a much higher frequency to make loop compensation easier. To compensate these devices, we must know the center frequencies of the right-half plane zero (zRHP) and the higher frequency pole (pOUT2). Calculate the zRHP frequency with the following formula: High-Efficiency, Current-Mode, Inverting PWM Controller ( The calculations for pOUT2 are very complex. For most applications where VOUT does not exceed -48V (in a negative sense), the pOUT2 will not be lower than 1/8th of the oscillator frequency and is generally at a higher frequency than zRHP. Therefore: pOUT2 0.125 x fOSC A pole is created by the output capacitor and the load resistance. This pole must also be compensated and its center frequency is given by the formula: pOUT1 = 1 / (2 x RLOAD x COUT) Finally, there is a zero introduced by the ESR of the output capacitor. This zero is determined from the following equation: zESR = 1 / (2 x COUT x RESR) Calculating the Required Pole Frequency To ensure stability of the MAX1846/MAX1847, the gain of the error amplifier must roll-off the total loop gain to 1 before ZRHP or POUT2 occurs. First, calculate the DC open-loop gain ADC: A DC = B xG M xR O x (1- D MAX )R LOAD A CS xR CS where: ACS is the current-sense amplifier gain = 3.3 B is the feedback-divider attenuation factor = R2 R1 + R2 GM is the error-amplifier transconductance = 400 A/V RO is the error-amplifier output resistance = 3 M RCS is the selected current-sense resistor Determining the Compensation Component Values Select a unity-gain crossover frequency (fCROS), which is lower than zRHP and pOUT2 and higher than pOUT1. Using fCROS, calculate the compensation resistor (RCOMP). R COMP = www.maximintegrated.com ) 2 - (1- D MAX ) x VIN(MIN) - VOUT xR LOAD Z RHP = (2 x VOUT xL) f CROS x R O A DC xPOUT1 - f CROS Maxim Integrated 15 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Select the next smaller standard value of resistor and then calculate the compensation capacitor required to cancel out the output-capacitor-induced pole (POUT1) determined previously. C COMP = 1 6.28 x POUT1 xR COMP POUT(MAX) = Efficiency x PIN(MAX) In order for pCOMP to compensate the loop, the openloop gain must reach unity at a lower frequency than the right-half-plane zero or the second output pole, whichever is lower in frequency. If the second output pole and the right-half-plane zero are close together in frequency, the higher resulting phase shift at unity gain may require a lower crossover frequency. For duty cycles greater than 50%, slope compensation reduces ADC, reducing the actual crossover frequency from fCROS. It is also a good practice to reduce noise on COMP with a capacitor (CCOMP2) to ground. To avoid adding extra phase margin at the crossover, the capacitor (CCOMP2) should roll-off noise at five times the crossover frequency. The value for CCOMP2 can be found using: R O + R COMP 5 x 6.28 x f CROS x R O x R COMP It might require a couple iterations to obtain a suitable combination of compensation components. Finally, the zero introduced by the output capacitor's ESR must be compensated. This compensation is accomplished by placing a capacitor between REF and FB creating a pole directly in the feedback loop. Calculate the value of this capacitor using the frequency of zESR and the selected feedback resistor values with the formula: R +R 2 C FB = R ESR xC OUT x 1 R 1 xR 2 When using low-ESR, ceramic chip capacitors (MLCCs) at the output, calculate the value of CFB as follows: C FB = Maximum Output Power The maximum output power that the MAX1846/MAX1847 can provide depends on the maximum input power available and the circuit's efficiency: Choose the next larger standard value of capacitor. C COMP2 = Applications Information R1+ R 2 2 x 3.14 x f OSC xR 1xR 2 www.maximintegrated.com Furthermore, the efficiency and input power are both functions of component selection. Efficiency losses can be divided into three categories: 1) resistive losses across the inductor, MOSFET on-resistance, current-sense resistor, rectification diode, and the ESR of the input and output capacitors; 2) switching losses due to the MOSFET's transition region, and charging the MOSFET's gate capacitance; and 3) inductor core losses. Typically, 80% efficiency can be assumed for initial calculations. The required input power depends on the inductor current limit, input voltage, output voltage, output current, inductor value, and the switching frequency. The maximum output power is approximated by the following formula: PMAX = [VIN - (VLIM + ILIM x RDS(ON))] x ILIM x [1 - (LIR / 2)] x [(-VOUT + VD) / (VIN - VSW - VLIM - VOUT + VD)] where ILIM is the peak current limit and LIR is the inductor current-ripple ratio and is calculated by: LIR = ILPP / ILDC Again, remember that VOUT for the MAX1846/MAX1847 is negative. Diode Selection The MAX1846/MAX1847's high-switching frequency demands a high-speed rectifier. Schottky diodes are recommended for most applications because of their fast recovery time and low forward voltage. Ensure that the diode's average current rating exceeds the peak inductor current by using the diode manufacturer's data. Additionally, the diode's reverse breakdown voltage must exceed the potential difference between VOUT and the input voltage plus the leakage-inductance spikes. For high output voltages (-50V or more), Schottky diodes may not be practical because of this voltage requirement. In these cases, use an ultrafast recovery diode with adequate reverse-breakdown voltage. Maxim Integrated 16 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Input Filter Capacitor PC Board Layout Guidelines The input capacitor (CIN) must provide the peak current into the inverter. This capacitor is selected the same way as the output capacitor (COUT). Under ideal conditions, the RMS current for the input capacitor is the same as the output capacitor. The capacitor value and ESR must be selected to reduce noise to an acceptable value and also handle the ripple current (INRMS where: INRMS = Good PC board layout and routing are required in highfrequency-switching power supplies to achieve good regulation, high efficiency, and stability. It is strongly recommended that the evaluation kit PC board layouts be followed as closely as possible. Place power components as close together as possible, keeping their traces short, direct, and wide. Avoid interconnecting the ground pins of the power components using vias through an internal ground plane. Instead, keep the power components close together and route them in a "star" ground configuration using component-side copper, then connect the star ground to internal ground using multiple vias. 1.2 xIO x D MAX - D MAX 2 (I - D MAX ) Bypass Capacitor Main Application Circuits In addition to CIN and COUT, other ceramic bypass capacitors are required with the MAX1846/MAX1847. Bypass REF to GND with a 0.1F or larger capacitor. Bypass VL to GND with a 0.47F or larger capacitor. All bypass capacitors should be located as close to their respective pins as possible. The MAX1846/MAX1847 are extremely versatile devices. Figure 2 shows a generic schematic of the MAX1846. Table 1 lists component values for several typical applications. These component values also apply to the MAX1847. The first two applications are featured in the MAX1846/MAX1847 EV kit. VIN APPLICATION B ONLY CIN 22k P VL EXT CS 9 3 CCOMP RCOMP RFREQ 2 4 COMP FREQ REF GND 6 FB COUT L1 RCS PGND VOUT 8 MAX1846 CCOMP2 D1 10 IN 1 0.47F R1 7 5 R2 CFB 0.1F NOTE: APPLICATIONS A & B USE POS CAPACITORS. APPLICATIONS C & D USE ALUMINUM ELECTROLYTIC CAPACITORS. Figure 2. MAX1846 Main Application Circuit www.maximintegrated.com Maxim Integrated 17 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Table 1. Component List for Main Application Circuits A B C D Input (V) CIRCUIT ID 12 3 to 5.5 12 12 Output (V) -5 -12 -48 -72 Output (A) 2 0.4 0.1 0.1 CCOMP (F) 0.047 0.22 0.1 0.068 3 x 10 3 x 22 10 10 2 x 100 2 x 47 39 39 390 1200 1000 1000 R1 (kW) (1%) 40.2 95.3 383 576 R2 (kW) (1%) 10 10 10 10 RCOMP (k) 8.2 10 220 470 0.02 0.02 0.05 0.05 CIN (F) COUT (F) CFB (pF) RCS (W) RFREQ (kW) D1 150 150 150 150 CMSH5-40 CMSH5-40 CMR1U-02 CMR1U-02 L1 (H) P1 CCOMP2 (pF) 10 10 47 82 FDS6685 FDS6375 IRFR5410 IRFR5410 220 220 22 12 Component Suppliers SUPPLIER COMPONENT PHONE Capacitors 803-946-0690 www.avxcorp.com Diodes 516-435-1110 www.centralsemi.com Coilcraft Inductors 847-639-6400 www.coilcraft.com Dale Resistors 402-564-3131 www.vishay.com/company/brands/dale/ Fairchild MOSFETs 408-721-2181 www.fairchildsemi.com International Rectifier MOSFETs 310-322-3331 www.irf.com IRC Resistors 512-992-7900 www.irctt.com Capacitors 864-963-6300 www.kemet.com AVX Central Semiconductor Kemet On Semiconductor WEBSITE MOSFETs, Diodes 602-303-5454 www.onsemi.com Capacitors, resistors 201-348-7522 www.panasonic.com Sanyo Capacitors 619-661-6835 www.secc.co.jp Siliconix MOSFETs 408-988-8000 www.siliconix.com Sprague Capacitors 603-224-1961 www.vishay.com/company/brands/sprague/ Sumida Inductors 847-956-0666 www.remtechcorp.com Vitramon Resistors 203-268-6261 www.vishay.com/company/brands/vitramon/ Panasonic Note: Indicate that you are using the MAX1846/MAX1847 when contacting these component suppliers. www.maximintegrated.com Maxim Integrated 18 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Chip Information Pin Configurations PROCESS: BiCMOS TOP VIEW VL 1 FREQ 2 COMP 3 REF 4 FB 5 + 10 IN MAX1846 MAX POL 1 + 16 SYNC 15 IN VL 2 9 EXT 8 CS 7 PGND COMP 4 6 GND 14 EXT FREQ 3 MAX1847 12 PGND FB 6 11 GND N.C. 7 10 GND SHDN 8 9 QSOP www.maximintegrated.com 13 CS REF 5 N.C. Package Information For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a "+", "#", or "-" in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE PACKAGE TYPE CODE 10 MAX 16 QSOP U10+2 E16+1 OUTLINE NO. LAND PATTERN NO. 21-0061 21-0055 90-0330 90-0167 Maxim Integrated 19 MAX1846/MAX1847 High-Efficiency, Current-Mode, Inverting PWM Controller Revision History REVISION NUMBER REVISION DATE PAGES CHANGED 2 9/10 Added equation in the Determining the Compensation Component Values section 3 3/14 Removed automotive application from the Applications section 4 7/16 Extended maximum operating temperature from +85C to +105C DESCRIPTION 16 1 1, 2, 4 For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim Integrated's website at www.maximintegrated.com. Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. Maxim Integrated and the Maxim Integrated logo are trademarks of Maxim Integrated Products, Inc. (c) 2016 Maxim Integrated Products, Inc. 20 Mouser Electronics Authorized Distributor Click to View Pricing, Inventory, Delivery & Lifecycle Information: Maxim Integrated: MAX1846EUB+ MAX1847EEE+ MAX1846EUB+T MAX1847EEE+T