MIC2871 1.2A High-Brightness LED Flash Driver with Single-Wire Serial Interface General Description Features The MIC2871 is a high-current, high-efficiency flash LED driver. The LED driver current is generated by an integrated inductive boost converter with a 2MHz switching frequency which allows the use of very small inductor and output capacitor. These features make the MIC2871 an ideal solution for high-resolution camera phone LED flash light driver applications. The MIC2871 operates in either flash or torch modes that can be controlled through the single-wire serial interface and/or external control pins. Default flash and torch brightness can be adjusted via an external resistor. A robust single-wire serial interface allows simple control by the host processor to support typical camera functions such as auto-focus, white balance, and image capture (flash mode). The MIC2871 is available in a 14-pin 3mm x 2mm LDFN package with a junction temperature range of -40C to +125C. * * * * Datasheets and support documentation are available on Micrel's web site at: www.micrel.com. * * * * * * * Up to 1.2A flash LED driving current Highly-efficient, synchronous boost driver (up to 94%) 5% LED current accuracy Control through single-wire serial interface or external control pins Input voltage range: 2.7V to 5.5V True load disconnect Configurable safety time-out protection Output overvoltage protection (OVP) LED short detection and protection. 1A shutdown current Available in 14-pin 3mm x 2mm LDFN package Applications * * * * * * Camera phones/mobile handsets Cell phones/smartphones LED light for image capture/auto focus/white balance Handset video light (torch light) Digital cameras Portable applications Typical Application Micrel Inc. * 2180 Fortune Drive * San Jose, CA 95131 * USA * tel +1 (408) 944-0800 * fax + 1 (408) 474-1000 * http://www.micrel.com May 29, 2013 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Ordering Information Part Number Marking Code Temperature Range MIC2871YMK 2871 -40C to +125C Package (1) Lead Finish 14-pin 3mm x 2mm LDFN Pb-Free Note: 1. Package is a GREEN RoHS-compliant package. Lead finish is Pb-Free. Mold compound is Halogen Free. Pin Configuration 14-Pin 3mm x 2mm LDFN (MK) (Top View) Pin Description Pin Number Pin Name 1 AGND1 Analog Ground. LED current return path. 2 DC Single-wire serial interface control input. 3 LED LED Current Sink Pin. Connect the LED anode to OUT and cathode to this pin. 4 FEN1 5 AGND2 Pin Function Flash Mode Enable Pin. Toggling FEN1 from LOW to HIGH enables MIC2871 into the flash mode. FEN1 is logic-OR with FEN2. If this pin is left floating, it is pulled-down internally by a builtin 1A current source when the device is enabled. Analog Ground. Reference ground of the FRSET pin. 6 VIN 7 PGND1 8 OUT 9, 12 NC No Connect. Connect this pin to AGND or leave it floating. 10 SW Inductor Connection Pin. It is connected to the internal power MOSFETs. 11 FEN2 13 PGND2 Power Ground. 14 FRSET Flash Mode Current-Level Programming Pin. Connect a resistor from this pin to AGND2 to set the maximum current in the flash mode. This pin may be grounded if the default flash mode current (1A) is desired. This pin cannot be left floating and the recommended resistance range is from 17k to 60k. EP ePad May 29, 2013 Supply Input Pin. Connect a low-ESR ceramic capacitor of at least 2.2F to AGND2. Power Ground. Inductor current return path. Boost Converter Output Pin. This is connected to the anode of the LED. Connect a low ESR ceramic capacitor of at least 4.7F to PGND1. Additional Flash Mode Enable Pin. FEN2 is logic-OR with FEN1. If this pin is left floating, it is pulled-down internally by a built-in 1A current source when the device is enabled. Exposed Heat Sink Pad. Connect to ground for best thermal performance. 2 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Absolute Maximum Ratings(2) Operating Ratings(3) Input Voltage (VIN) ........................................ -0.3V to +6.0V General I/O Voltage (VFEN1, VFEN2) .................... -0.3V to VIN VOUT and VLED Voltage .................................. -0.3V to +6.0V Single-Wire I/O Voltage (VDC) ........................... -0.3V to VIN VFRSET Voltage ................................................... -0.3V to VIN VSW Voltage .................................................. -0.3V to +6.0V Lead Temperature (soldering, 10s) .......................... +260C Junction Temperature ................................... 0C to +150C Storage Temperature (Ts) ......................... -40C to +150C (5) ESD Rating ............................... 2kV, HBM and 200V, MM Input Voltage (VIN) .......................................... 2.7V to +5.5V Enable Input Voltage (VFEN1, VFEN2) ....................... 0V to VIN Single-Wire I/O Voltage (VDC) ................................ 0V to VIN Junction Temperature (TJ) ........................ -40C to +125C (4) Power Dissipation (PD) ........................... Internally Limited Package Thermal Resistance (4) 3mm x 2mm LDFN (JA) ............................ 65.83C/W (4) .............................................. 38.9C/W 3mm x 2mm LDFN (JC) Electrical Characteristics(6) VIN = 3.6V, L = 1H, COUT = 4.7F, RFRSET = 20.5k, IOUT = 100mA, TA = 25C, bold values indicate -40C TJ +125C, unless otherwise noted.. Symbol Parameter Condition Min. Typ. Max. Units 5.5 V Power Supply 2.7 VIN Supply Voltage Range VSTART Start-Up Voltage 2.65 2.95 V VUVLO UVLO Threshold (falling) 2.30 2.5 V ISTB Standby Current VDC = HIGH, boost regulator and LED current driver both OFF. ISD Shutdown Current VDC = 0V VOVP Overvoltage Protection (OVP) Threshold 1 2 A 5.37 5.55 V 60 mV OVP Blanking Time 23 s Maximum Duty Cycle ISW Switch Current Limit DMIN Minimum Duty Cycle NMOS 5.2 A OVP Hysteresis DMAX PMOS 230 Switch On-Resistance ISW Switch Leakage Current FSW Oscillator Frequency VIN = VOUT = 2.7V 82 86 90 % 3.5 4.5 5.5 A 4 6.4 9 % ISW = 100mA 100 ISW = 100mA VDC = 0V, VSW = 5.5V 0.01 m 1 2 -10 Oscillator Frequency Variation A MHz 10 % Notes: 2. Exceeding the absolute maximum rating may damage the device. 3. The device is not guaranteed to function outside its operating rating. 4. The maximum allowable power dissipation of any TA (ambient temperature) is PD(max) = (TJ(max) - TA) / JA. Exceeding the maximum allowable power dissipation will result in excessive die temperature, and the regulator will go into thermal shutdown. 5. Devices are ESD sensitive. Handling precautions recommended. Human body model, 1.5k in series with 100pF. 6. Specification for packaged product only. May 29, 2013 3 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Electrical Characteristics(6) VIN = 3.6V, L = 1H, COUT = 4.7F, RFRSET = 20.5k, IOUT = 100mA, TA = 25C, bold values indicate -40C TJ +125C, unless otherwise noted. Symbol Parameter Conditions Min. Typ. Max. Units TSD Overtemperature Shutdown Threshold 155 C Overtemperature Shutdown Hysteresis 15 C TTO Safety Timeout Shutdown Default timer setting 1.25 s ITO Safety Timer Current Threshold Default current threshold setting 250 mA Safety Timer Current Resolution 50 mA Safety Timer Current-Threshold Accuracy 25 mA 3.6 V 50 mV 1.7 V VLBVD Low-Battery Voltage Detection Threshold Default LBVD threshold setting Low-Battery Voltage Detection Threshold Accuracy VSHORT LED Short-Circuit Detection Voltage Threshold ITEST LED Short-Circuit Detection Test Current VOUT - VLED 1 2 3 mA 5 % Current Sink Channels VLED Channel Current Accuracy 3.5V < VIN <4.2V, ILED = 1A Current Sink Voltage Dropout Boost regulator ON, ILED = 1A -5 160 mV FEN1, FEN2 Control Pins FEN1/FEN2 Threshold Voltage FEN1/FEN2 Pull-down Current 1.5 FLASH ON V 0.4 FLASH OFF FEN1 = FEN2 = 5.5V 1 5 A Electrical Characteristics - Single-Wire Interface (Guaranteed by Design) VIN = 3.6V, L = 1H, COUT = 4.7F, IOUT= 100mA, TA = 25C, bold values indicate -40C TJ +125C, unless otherwise noted. Symbol Parameter Conditions Min. Typ. LOW-Level Input Voltage Units 0.4 V 1.5 HIGH-Level Input Voltage DC Pull-Down Current Max. V DC = 5.5V 2.5 5 A TON ON Time 0.1 72 s TOFF OFF Time 0.1 72 s TLAT Latch Time 97 324 s TEND END Time 405 May 29, 2013 4 s 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Typical Characteristics Shutdown Current vs. Temperature Standby Current vs. Temperature 2.20 0.5 0.4 0.3 0.2 0.1 0.0 SWITCHING FREQUENCY (MHz) 245 STANDBY CURRENT (A) 240 235 230 225 -40 -20 0 20 40 60 80 100 120 -40 -20 1.95 ILED = 400mA ILED = 250mA ILED = 100mA L = 1H COUT = 4.7F 50 0 20 40 60 80 100 120 3.0 3.4 3.8 4.2 4.6 255 250 245 240 L = 1H COUT = 4.7F ILED = 250mA RFRSET = 20k 235 -40 Flash Mode ILED(MAX) vs. FRSET Resistor -20 0 20 40 60 80 100 TORCH MODE ILED(MAX) (mA) 800 600 400 L = 1 H COUT = 4.7F 0 20 30 40 FRSET RESISTOR (k) May 29, 2013 1.00 0.95 0.90 L = 1H COUT = 4.7F ILED = 1A RFRSET = 20k 0.85 -40 50 60 200 150 100 L = 1 H COUT = 4.7F 0 10 20 30 40 FRSET RESISTOR (k) 5 0 20 40 60 80 100 120 Flash Mode ILED(MAX) Accuracy vs. Input Voltage 250 50 -20 TEMPERATURE (C) 0 10 1.05 0.80 300 0 1.10 120 Torch Mode ILED(MAX) vs. FRSET Resistor 1000 4.5 1.15 TEMPERATURE (C) 1200 4.0 Flash Mode LED Current vs. Temperature 260 5.0 3.5 3.0 2.5 1.20 INPUT VOLTAGE (V) 200 ILED = 1A INPUT VOLTAGE (V) 265 230 2.6 125C L = 1 H COUT = 1F 1.85 FLASH MODE LED CURRENT (A) TORCH MODE LED CURRENT (mA) ILED = 1.0A ILED = 1.2A ILED = 640mA 60 75C 1.90 270 90 EFFICIENCY (%) 2.00 Torch Mode LED Current vs. Temperature 100 70 25C 2.05 TEMPERATURE (C) WLED Power Efficiency vs. Input Voltage 80 -40C 2.10 1.80 TEMPERATURE (C) FLASH MODE ILED(MAX) (mA) 2.15 220 FLASH MODE ILED(MAX) ACCURACY (%) SHUTDOWN CURRENT (A) 0.6 Boost Switching Frequency vs. Input Voltage 50 60 3.5 3.0 RFRSET = 17k 2.5 2.0 1.5 1.0 RFRSET = 20k 0.5 0.0 -0.5 -1.0 -1.5 RFRSET = 30k RFRSET = 39k RFRSET = 51k RFRSET = 62k 3.5 3.7 3.9 4.1 4.3 INPUT VOLTAGE (V) 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Typical Characteristics (Continued) TORCH MODE ILED(MAX) ACCURACY (%) Torch Mode ILED(MAX) Accuracy vs. Input Voltage 0.4 0.2 0.0 RFRSET = 17k -0.2 RFRSET = 20k -0.4 -0.6 RFRSET = 30k -0.8 RFRSET = 39k -1.0 RFRSET = 62k -1.2 RFRSET = 51k -1.4 3.5 3.7 3.9 4.1 4.3 INPUT VOLTAGE (V) May 29, 2013 6 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Functional Characteristics May 29, 2013 7 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Functional Diagram Figure 1. Simplified MIC2871 Functional Block Diagram May 29, 2013 8 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Functional Description VIN The input supply provides power to the internal MOSFETs gate drive and controls circuitry for the switchmode regulator. The operating input voltage range is from 2.7V to 5.5V. A 2.2F low-ESR ceramic input capacitor should be connected from VIN to AGND2 as close to the MIC2871 as possible to ensure a clean supply voltage for the device. The minimum voltage rating of 10V is recommended for the input capacitor. OUT Boost converter output pin which is connected to the anode of the LED. A low-ESR ceramic capacitor of 4.7F or larger should be connected from OUT to PGND1 as close as possible to the MIC2871. The minimum voltage rating of 10V is recommended for the output capacitor. LED The current sink pin for the LED. The LED anode is connected to the OUT pin and the LED cathode is connected to this pin. SW The MIC2871 has internal low-side and synchronous MOSFET switches. The switch node (SW) between the internal MOSFET switches connects directly to one end of the inductor and provides the current paths during switching cycles. DC The DC is a single multiplexed device enable and serial data control pin used for functional control and communication in GPIO-limited applications. When the DC pin is used as a hardware device enable pin, a logic high signal on the DC pin enables the device, and a logic low signal on the DC pin disables the device. When the DC pin is used as the single-wire serial interface digital control pin, a combination of bit edges and the period between edges is used to communicate a variable length data word across the single wire. Each word is transmitted as a series of pulses, each pulse incrementing an internal data counter. A stop sequence consisting of an inactive period is used to latch the data word internally. The data word received is then used to set the value of the corresponding register for controlling specific function. The MIC2871 supports five writeable registers for controlling flash mode, torch mode, safety timer duration, safety timer threshold current, and lowbattery threshold. The other end of the inductor is connected to the input supply voltage. Due to the high-speed switching on this pin, the switch node should be routed away from sensitive nodes wherever possible. AGND1 This is the ground path of the LED current sink. It should be connected to the AGND2, but not via exposed pad, on the PCB. The current loop of the analog ground should be separated from that of the power ground (PGND1 and PGND2). AGND1 and AGND2 should be connected to PGND1 and PGND2 at a single point. AGND2 This is the ground path for the internal biasing and control circuitry. AGND2 should be connected to the PCB pad for the package exposed pad. AGND2 should be connected to the AGND1 directly without going through the exposed pad. The current loop of the analog ground should be separated from that of the power ground (PGND1 and PGND2). The AGND2 and AGND1 should be connected to PGND1 and PGND2 at a single point. An address/data frame is used to improve protection against erroneous writes where communications are in error. When DC is in a low state and no data is detected for an extended period of time, the MIC2871 will automatically go into a low-power SHUTDOWN state, simultaneously resetting all internal registers to their default states. PGND1 and PGND2 The power ground pins are the ground path for the high current in the boost switch and they are internally connected together. The current loop for the power ground should be as small as possible and separate from the analog ground (AGND) loop as applicable. May 29, 2013 FEN1 and FEN2 FEN1 and FEN2 are hardware enable pins for flash mode. FEN1 is logic-OR with FEN2. A logic low-to-high transition on either FEN1 pin or FEN2 pin can initiate the MIC2871 in flash mode. If FEN1 or FEN2 is left floating, it is pulled down internally by a built-in 1A current source when the device is enabled. Flash mode is terminated when both FEN1 and FEN2 are pulled low or left floating, and the flash register is cleared. 9 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 FRSET The flash mode maximum LED current level is programmed through the FRSET pin. A resistor connected from the FRSET pin to AGND2 set the maximum current in the flash mode. This pin can be grounded if the default flash mode current of 1A is desired. For the best current accuracy, 0.1% or smaller tolerance resistor for setting the maximum flash mode LED current is recommended. This pin cannot be left floating and the minimum resistance is limited to 17k. The maximum flash mode current to maximum Torch mode current ratio is internally fixed as 4 to1. May 29, 2013 10 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Application Information The MIC2871 can drive a high-current flash WLED in either flash mode or torch mode. Like the flash mode current, the torch mode current can be tuned to a fraction of the maximum torch mode level by selecting the desired torch current level percentage in the torch control register (address 2) through the singlewire serial interface. Boost Converter The internal boost converter is turned on/off automatically when the LED driver is activated/de-activated without any exception. The torch current is the product of the maximum Torch current setting and the percentage selected in the torch register. The boost converter is an internally-compensated current-mode PWM boost converter running at 2MHz. It is for stepping up the supply voltage to a high enough value at the OUT pin to drive the LED current. If the supply voltage is high enough, the synchronous switch of the converter is then fully turned on. In this case, all the excessive voltage is dropped over the linear LED driver. Configurable Safety Timer The flash safety timeout feature automatically shuts down the LED current after the safety timer duration is expired if the programmed LED current exceeds a certain current threshold. Both the current threshold and the timer duration are programmable via the safety timer registers (addresses 3 and 5). Flash Mode The maximum current level in the flash mode is 1.2A. This current level can be adjusted through an external resistor connecting to the FRSET pin according to Equation 1: ILED(MAX) = 20500 Low-Battery Voltage Detection (LBVD) When the VIN voltage drops below the LBVD threshold (default = 3.6V) in flash or torch mode, the LED current driver is disabled. The LED driver can be resumed by toggling the corresponding input control signal. The LBVD threshold is adjustable thru the LBVD control register (address 4). Eq. 1 R FRSET Overvoltage Protection When the output voltage rises above the OVP threshold, MIC2871 is latched off automatically to avoid permanent damage to the IC. To clear the latched off condition, either power cycle the MIC2871 or assert the DC pin LOW. Alternatively, the default value of 1A is used when the FRSET pin is grounded. The flash mode current can be initiated at this preset FRSET brightness level by asserting FEN1 or FEN2 pin HIGH, or by setting the flash control register (address 1), for the desired flash duration, subjected to the safety timeout setting. The flash mode current is terminated when the FEN1 and FEN2 pins are brought LOW and the flash register is cleared. Short-Circuit Detection Each time before enabling the LED driver, the MIC2871 performs the short circuit test by driving the flash LED with a small (2mA typical) current for 200s. If (VOUT - VLED) < 1.7V at the end of the short-circuit test, the LED is considered to be shorted and MIC2871 will ignore the flash and/or torch mode command. Note that the shortcircuit test is carried out every time prior to flash and torch mode but the result is not latched. Flash mode current can be adjusted to a fraction of the maximum flash mode current level by selecting the desired percentage in the flash control register through the single-wire serial interface. The flash current is the product of the maximum flash current setting and the percentage selected in the flash register. Thermal Shutdown When the internal die temperature of MIC2871 reaches 155C, the LED driver is disabled until the die temperature falls below 140C. Torch Mode By default, the maximum torch mode level is one-fourth (1/4) of the maximum flash mode current. The torch mode operation is activated by setting the torch control register (address 2) for the desired duration. The torch mode current is terminated when the torch register is cleared or when the configurable safety timer expires. May 29, 2013 11 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Single-Wire Interface The single-wire interface allows the use of a single multiplexed enable and data pin (DC) for control and communication in GPIO limited applications. The interface is implemented using a simple mechanism allowing any open drain or directly driven GPIO to control the MIC2871. Idle mode is entered automatically at the end of a communication frame by holding DC high for TEND, by enabling the device by bringing DC high when in shutdown mode, or when an error is detected by the single-wire interface logic. Shutdown mode can be entered at any time by pulling down DC for TEND, discarding any current communication and resetting the internal registers. If a communication is received before the shutdown period but after the TLAT period, the communication is discarded. This state is also used to create an internal error state to avoid erroneously latching data where the communication process cannot be serviced in time. Additionally, each register has a maximum value associated with it. If the number of bits clocked in exceeds the maximum value for the register, the data is assumed to be in error and the data is discarded. The MIC2871 uses the single-wire interface for simple command and control functions. The interface provides fast access to write only registers with protection features to avoid potentially erroneous data writes and improve robustness. When DC is in a low state and no data is detected for an extended period of time, the MIC2871 will automatically go into a low-power SHUTDOWN state, simultaneously resetting internal registers to default states. Overview The single-wire interface relies on a combination of bit edges and the period between edges in order to communicate across a single wire. Each word is transmitted as a series of pulses, with each pulse incrementing an internal data counter. A stop sequence consisting of an inactive period is used to latch the data word internally. An address and data framing format is used to improve protection against erroneous writes by enforcing address and data field lengths as well as the timing duration between them. Timing is designed such that when communicating with a device using a low cost on chip oscillator, the worst case minimum and maximum conditions can be easily met within the wide operating range of the oscillator. Using this method guarantees that the device can always detect the delay introduced by the communication master. Idle States and Error Conditions In shutdown mode, the MIC2871 is in a reset condition with all functions off while consuming minimal power. Register settings are reset to default state when coming out of shutdown state. In idle mode, all register settings persist and all MIC2871 functions continue in their current state. Table 1 summarises the difference between the two idle modes: Figure 2. Abort, Shutdown, and Idle Timing Waveforms Communication Details The serial interface requires delimiters to indicate the start of frame, data as a series of pulses, and end of frame indicated by a lack of activity. The start of frame is the first high to low transition of DC when in idle mode. The first rising edge resets the internal data counter to 0. Table 1. Differences between Idle Modes Shutdown Idle Low High 1A 230A Register State Default Persist Start-Up Time 1s 100ns DC ISUPPLY (all functions off) May 29, 2013 12 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 MIC2871 Registers The MIC2871 supports five writeable registers for controlling the torch and the flash modes of operation as shown in Table 2. Note that register addressing starts at 1. Writing any value above the maximum value shown for each registers will cause an invalid data error and the frame will be discarded. Figure 3. Data Word Pulse Timing Table 2. Five Writable Registers of MIC2871 A pulse is delimited by the signal first going below VL and then above VH within the latch timeout TLAT. During this transition, minimum on (TON) and off (TOFF) periods are observed to improve tolerance to glitches. Each rising edge increments the internal data register. Data is automatically latched into internal shadow address and data registers after an inactivity period of >TLAT. To send register write commands, the address and data are entered in series as two data words using the above pattern, with the second word starting after the first latch period has expired. After the second word is entered, the IDLE command should be issued by leaving the DC pins high for TEND. Address Name Max. Value 1 FEN/FCUR 31 Flash Enable/Current 2 TEN/TCUR 31 Torch Enable/Current 3 STDUR 7 Safety Timer Duration 4 LB_TH 9 Low Battery Voltage Detection Threshold 5 ST_TH 5 Safety Timer Threshold Description Flash Current Register (FEN/FCUR: default 0) The flash current register enables and sets the flash mode current level. Valid values are 0 to 31; values 0 - 15 will set the flash current without enabling the flash (such that it can be triggered externally), values 16 - 31 will set the flash current and enable the flash. The flash current register maps into the internal FEN and FCUR registers as shown in the table below. Table 3 describes the relationship between the flash current as a percentage of maximum current, and the FCUR register setting. After receiving the stop sequence, the internal registers decode and update cycle is started, with the shadow register values being transferred to the decoder. Figure 4 shows an example of entering a write of data 5 to address 3. Figure 4. Communication Timing Example of Entering Write for Data 5 to Address 3 Only correctly formatted address/data combination will be treated as a valid frame and processed by the MIC2871. Any other input, such as a single data word followed by TEND, or three successive data words will be discarded by the target hardware as an erroneous entry. Additionally, any register write to either an invalid register or with invalid register data will also be discarded. May 29, 2013 13 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Torch Current Register (TEN/TCUR: default 0) The torch current register enables and sets the torch mode current level. Valid values are 0 to 31; values 0 - 15 will set the torch current without enabling the torch (such that it can be triggered by setting the internal TEN register value to 1), values 16 - 31 will set the torch current and enable the torch. A value of 0 at the internal TEN register will disable the torch. The torch current register maps into the internal TEN and TCUR registers as shown in Table 4. The table also describes the relationship between the torch current as a percentage of maximum current, and the TCUR register setting. Table 3. Flash Current Register Mapping into Internal FEN and FCUR Registers, and Relationship between Flash Current as % of Maximum Current and the FCUR Register Setting Value FEN/FCUR[4:0] Dec. Binary FEN[4] FCUR[3:0] % of IMAX 0 00000 0 100.00 1 00001 0 88.96 2 00010 0 79.04 3 00011 0 70.72 4 00100 0 63.04 5 00101 0 56.00 6 00110 0 49.92 7 00111 0 44.64 8 01000 0 39.68 9 01001 0 35.52 10 01010 0 31.68 11 01011 0 28.16 12 01100 0 25.12 13 01101 0 22.40 14 01110 0 20.00 15 01111 0 17.92 16 10000 1 100.00 17 10001 1 88.96 18 10010 1 79.04 19 10011 1 70.72 20 10100 1 63.04 21 10101 1 56.00 22 10110 1 49.92 23 10111 1 44.64 24 11000 1 39.68 25 11001 1 35.52 26 11010 1 31.68 27 11011 1 28.16 28 11100 1 25.12 29 11101 1 22.40 30 11110 1 20.00 31 11111 1 17.92 May 29, 2013 14 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Safety Timer Duration Register (STDUR: default 7) The safety timer duration register sets the duration of the flash and torch mode when the LED current exceeds the programmed threshold current. Valid values are 0 for the minimum timer duration to 7 for the maximum duration. Table 4. Torch Current Register Mapping into Internal TEN and TCUR Registers, and Relationship between Torch Current as % of Maximum Current and the TCUR Register Setting Value TEN/TCUR[4:0] Dec. Binary TEN[4] TCUR[3:0] % of IMAX 0 00000 0 100.00 1 00001 0 88.96 Table 5. Safety Timer Duration Register Setting and Safety Timer Duration Value Binary FDUR[2:0] (binary) Timeout (ms) 2 00010 0 79.04 Dec. 3 00011 0 70.72 0 000 000 156.25 4 00100 0 63.04 1 001 001 312.5 5 00101 0 56.00 2 010 010 468.75 6 00110 0 49.92 3 011 011 625 7 00111 0 44.64 4 100 100 781.25 8 01000 0 39.68 5 101 101 937.5 9 01001 0 35.52 6 110 110 1093.75 10 01010 0 31.68 7 111 111 1250 11 01011 0 28.16 12 01100 0 25.12 13 01101 0 22.40 14 01110 0 20.00 15 01111 0 17.92 16 10000 1 100.00 17 10001 1 88.96 18 10010 1 79.04 19 10011 1 70.72 20 10100 1 63.04 21 10101 1 56.00 Dec. Binary 22 10110 1 49.92 0 23 10111 1 44.64 24 11000 1 25 11001 26 27 Low-Battery Threshold Register (LB_TH: default 7) The LB_TH register sets the supply threshold voltage below which the internal low battery flag is asserted and flash functions are inhibited. Table 6 shows the threshold values that correspond to the register settings. Setting 0 is reserved for disabling the function, and settings between 1 and 9 inclusively enable and set the LB_TH value between 3.0V and 3.8V with 100mV resolution. Table 6. Low-Battery Threshold Register Setting and Supply Threshold Voltage Value LB_TH[3:0] VBAT Threshold (V) 0000 0000 Disabled 1 0001 0001 3.0 39.68 2 0010 0010 3.1 1 35.52 3 0011 0011 3.2 11010 1 31.68 4 0100 0100 3.3 11011 1 28.16 5 0101 0101 3.4 0110 0110 3.5 28 11100 1 25.12 6 29 11101 1 22.40 7 0111 0111 3.6 30 11110 1 20.00 8 1000 1000 3.7 31 11111 1 17.92 9 1001 1001 3.8 May 29, 2013 15 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Safety Timer Threshold Current Register (ST_TH: default 4) Safety timer threshold current determines the amount of LED current flowing through the external LED before the internal LED safety timer is activated. Setting ST_TH to 0 disables the safety timer function, and setting the register to values 1 to 5 set the safety time threshold current 100mA to 300mA in 50mA steps. Table 7. Safety Timer Threshold Current Register Setting and Safety Timer Threshold Current Value ST_TH[2:0] Safety Timer Threshold Current (mA) 000 000 Disabled 1 001 001 100mA 2 010 010 150mA 3 011 011 200mA 4 100 100 250mA 5 101 101 300mA Dec. Binary 0 May 29, 2013 16 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Component Selection Inductor Inductor selection is a balance between efficiency, stability, cost, size, and rated current. Since the boost converter is compensated internally, the recommended inductance of L is limited from 1H to 2.2H to ensure system stability. It is usually a good balance between these considerations. A large inductance value reduces the peak-to-peak inductor ripple current hence the output ripple voltage and the LED ripple current. This also reduces both the DC loss and the transition loss at the same inductor's DC resistance (DCR). However, the DCR of an inductor usually increases with the inductance in the same package size. This is due to the longer windings required for an increase in inductance. Since the majority of the input current passes through the inductor, the higher the DCR the lower the efficiency is, and more significantly at higher load currents. On the other hand, inductor with smaller DCR but the same inductance usually has a larger size. The saturation current rating of the selected inductor must be higher than the maximum peak inductor current to be encountered and should be at least 20% to 30% higher than the average inductor current at maximum output current. The Y5V and Z5U type ceramic capacitors are not recommended due to their wide variation in capacitance over temperature and increased resistance at high frequencies. The rated voltage of the output capacitor should be at least 20% higher than the maximum operating output voltage over the operating temperature range. FRSET Resistor Since FRSET resistor is used for setting the maximum LED current, resistor type with 0.1% tolerance is recommended for more accurate maximum LED current setting. Input Capacitor A ceramic capacitor of 2.2F or larger with low ESR is recommended to reduce the input voltage ripple to ensure a clean supply voltage for the device. The input capacitor should be placed as close as possible to the MIC2871 VIN pin with short trace for good noise performance. X5R or X7R type ceramic capacitors are recommended for better tolerance over temperature. The Y5V and Z5U type temperature rating ceramic capacitors are not recommended due to their large reduction in capacitance over temperature and increased resistance at high frequencies. These reduce their ability to filter out highfrequency noise. The rated voltage of the input capacitor should be at least 20% higher than the maximum operating input voltage over the operating temperature range. Output Capacitor Output capacitor selection is also a trade-off between performance, size, and cost. Increasing output capacitor will lead to an improved transient response, however, the size and cost also increase. The output capacitor is preferred in the range of 2.2F to 10F with ESR from 10m to 50m. X5R or X7R type ceramic capacitors are recommended for better tolerance over temperature. May 29, 2013 17 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Power Dissipation Consideration As with all power devices, the ultimate current rating of the output is limited by the thermal properties of the device package and the PCB on which the device is mounted. There is a simple, 's law type relationship between thermal resistance, power dissipation and temperature which are analogous to an electrical circuit: Now replacing the variables in Equation 2, we can find the junction temperature (TJ) from the power dissipation, ambient temperature and the known thermal resistance of the PCB (CA) and the package (JC). TJ = PDISS x ( JC + CA ) + TA Eq. 3 As can be seen in the diagram, total thermal resistance JA = JC + CA. Hence this can also be written as in Equation 4: TJ = PDISS x ( JA ) + TA Figure 5. Series Electrical Resistance Circuit From this simple circuit we can calculate VX if we know ISOURCE, VZ and the resistor values, RXY and RYZ using Equation 2: Eq. 4 Since effectively all of the power losses (minus the inductor losses) in the converter are dissipated within the MIC2871 package, PDISS can be calculated thus: 1 V X = ISOURCE x (R XY + R YZ ) + VZ Linear Mode: PDISS = [POUT x Eq. 2 2 - 1 ] - IOUT x DCR Eq. 5 Thermal circuits can be considered using this same rule and can be drawn similarly by replacing current sources with power dissipation (in watts), resistance with thermal resistance (in C/W) and voltage sources with temperature (in C). 1 Boost Mode: PDISS = [POUT x 2 IOUT x DCR 1- D - 1] - Eq. 6 Duty Cycle in Boost Mode: D = VOUT - VIN VOUT Eq. 7 where: = Efficiency taken from efficiency curves and DCR = inductor DCR. JC and JA are found in the operating ratings section of the datasheet. Figure 6. Series Thermal Resistance Circuit May 29, 2013 18 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Where the real board area differs from 1" square, CA (the PCB thermal resistance) values for various PCB copper areas can be taken from Figure 7. Figure 7 is taken from Designing with Low Dropout Voltage Regulators available from the Micrel website ("LDO Application Hints"). Figure 7. Graph to Determine PC Board Area for a Given PCB Thermal Resistance Figure 7 shows the total area of a round or square pad, centered on the device. The solid trace represents the area of a square, single-sided, horizontal, solder-masked, copper PC board trace heat sink, measured in square millimeters. No airflow is assumed. The dashed line shows PC boards trace heat sink covered in black oil-based paint and with 1.3m/sec (250 feet per minute) airflow. This approaches a "best case" pad heat sink. Conservative design dictates using the solid trace data, which indicates 2 a maximum pad size of 5000 mm is needed. This is a pad 71mm by 71mm (2.8 inches per side). May 29, 2013 19 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 PCB Layout Guidelines PCB layout is critical to achieve reliable, stable and efficient performance. A ground plane is required to control EMI and minimize the inductance in power, signal and return paths. The following guidelines should be followed to ensure proper operation of the device: Output Capacitor IC (Integrated Circuit) * Use wide and short traces to connect the output capacitor to the OUT and PGND1 pins. * Place several vias to the ground plane close to the output capacitor ground terminal. * Use either X5R or X7R temperature rating ceramic capacitors. Do not use Y5V or Z5U type ceramic capacitors. * Place the IC close to the point-of-load (in this case, the flash LED). * Use fat traces to route the input and output power lines. * Analog grounds (AGND1 and AGND2) and power grounds (PGND1 and PGND2) should be kept separate and connected at a single location. * Use wide and short trace to connect the LED anode to the OUT pin. * * The exposed pad (EPAD) on the bottom of the IC must be connected to the analog grounds AGND2 of the IC. Use wide and short trace to connect the LED cathode to the LED pin. * * 8 to 12 thermal vias must be placed on the PCB pad for exposed pad and connected it to the ground plane to ensure a good PCB thermal resistance can be achieved. Make sure that the LED's PCB land pattern can provide sufficient PCB pad heat sink to the flash LED. FRSET Resistor The FRSET resistor should be placed close to the FRSET pin and connected to AGND2. Flash LED VIN Decoupling Capacitor * The VIN decoupling capacitor must be placed close to the VIN pin of the IC and preferably connected directly to the pin and not through any via. The capacitor must be located right at the IC. * The VIN decoupling capacitor should be connected to analog ground (AGND2). * The VIN terminal is noise sensitive and the placement of capacitor is very critical. Inductor * Keep both the inductor connections to the switch node (SW) and input power line short and wide enough to handle the switching current. Keep the areas of the switching current loops small to minimize the EMI problem. * Do not route any digital lines underneath or close to the inductor. * Keep the switch node (SW) away from the noise sensitive pins. * To minimize noise, place a ground plane underneath the inductor. May 29, 2013 20 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Typical Application Schematic Bill of Materials Item C1 C4 L1 Part Number GRM188R61A225KE34D LMK107BJ475KA-T PIFE25201B-1R0MS-39 SLSW6R007 LED1 Manufacturer Murata (7) Taiyo Yuden Cyntec (8) (9) Samsung (10) (11) LXCL-MN06-3002 Philips R4 ERA3AEB2052V Panasonic U1 MIC2871YMK Micrel, Inc. (12) (13) Description Qty. 2.2F, 10V, 10%, X5R, 0603 Capacitor 1 4.7F, 10V, 10%, X5R, 0603 Capacitor 1 1.0H, 3.55A, 2.5mm x 2.0mm x 1.2mm Inductor 1 4mm x 4mm x 2.2mm High-Power Flash LED LUXEON Flash 6 Module, 4mm x 4mm x 2.2mm, 180lux @ ILED = 1A LED 1 20.5k, 1/10W, 0.1%, 0603 Resistor 1 1.2A High-Brightness LED Flash Driver with Single-Wire Serial Interface 1 Notes: 7. Murata: www.murata.com. 8. Taiyo Yuden: www.t-yuden.com. 9. Cyntec: www.cyntec.com. 10. Samsung: www.samsung.com. 11. Philips: www.philipslumileds.com. 12. Panasonic: www.panasonic.com. 13. Micrel, Inc.: www.micrel.com. May 29, 2013 21 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 PCB Layout Recommendations Top Layer Bottom Layer May 29, 2013 22 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 Package Information and Recommended Landing Pattern(14, 15) 14-Pin 3mm x 2mm LDFN (MK) Notes: 14. Package information is correct as of the publication date. For updates and most current information, go to www.micrel.com. 15. Disclaimer: This is only a recommendation based on information available to Micrel from its suppliers. Actual land pattern may have to be significantly different due to various materials and processes used in PCB assembly. Micrel makes no representation or warranty of performance based on the recommended land pattern. May 29, 2013 23 052913-1.0 (while in progress) Revision 1.0 (final document) Micrel, Inc. MIC2871 MICREL, INC. 2180 FORTUNE DRIVE SAN JOSE, CA 95131 USA TEL +1 (408) 944-0800 FAX +1 (408) 474-1000 WEB http://www.micrel.com Micrel makes no representations or warranties with respect to the accuracy or completeness of the information furnished in this data sheet. This information is not intended as a warranty and Micrel does not assume responsibility for its use. Micrel reserves the right to change circuitry, specifications and descriptions at any time without notice. No license, whether express, implied, arising by estoppel or otherwise, to any intellectual property rights is granted by this document. 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A Purchaser's use or sale of Micrel Products for use in life support appliances, devices or systems is a Purchaser's own risk and Purchaser agrees to fully indemnify Micrel for any damages resulting from such use or sale. (c) 2013 Micrel, Incorporated. May 29, 2013 24 052913-1.0 (while in progress) Revision 1.0 (final document) Mouser Electronics Authorized Distributor Click to View Pricing, Inventory, Delivery & Lifecycle Information: Micrel: MIC2871YMK T5 MIC2871YMK-T5