AN2304 Application note LED driver for single flash Introduction This application note is dedicated to the design of a flash LED driver using STCF02, which is a step-up/down current mode converter. Schematic and functional description, recommendations for PCB Layout and external components selection are also evaluated. This device is dedicated for driving a single LED with forward voltage range 2.7 to 5 V. Photo of reference design board February 2007 Rev 4 1/ www.st.com Contents AN2304 Contents 1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 2 3 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.1 Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.3 LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.4 RFLASH selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.5 RTORCH selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.6 RMF selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.7 CCOMP selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.8 NTC and Rx resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4 Safety shutdown in flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 5 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2/ AN2304 1 Schematic description Schematic description The Flash LED driver is based on STCF02 device with all necessary external components. High operational frequency (1.8MHz) small size external component usage, resulting a very compact design of the Flash LED device. The device can operate in five modes: ● Shutdown mode ● Shutdown mode with activated NTC LED temperature sensing ● Torch mode ● Middle flash mode ● Flash mode These modes can be selected by a combination of logic signals connected to enable pins. Table 1. 1.1 Truth table selection mode ILED_MAX EN1 EN2 EN3 ILED 0 0 x 0 0 Shutdown 1 0 1 0 0 Shutdown+NTC 0 1 1 1 x 1 ITORCH= 160mV/(RTORCH+RFLASH) 250 Torch+NTC 1 0 0 IMFLASH = f(IFLASH, RMFLASH) 500 Medium Flash+NTC 1 1 0 IFLASH = 160mV/RFLASH 600 Flash+NTC (mA) MODE Application schematic Figure 1. Typical application schematic 3/ Schematic description Figure 2. AN2304 STCF02 Demo board electric schematic 4 Table 2. Symbol STCF02 Demo board BOM List Parameter Type Supplier & part number Typ. value Min Typ Unit Max RFLASH Sensing flash current resistor Thick film type (0805) ROHM MCR10EZHFLOR27 0.27 Ohm RTORCH Sensing torch current resistor Thick film type (0603) ROHM MCR03EZPJ6R20 6.2 Ohm RMF Setting Resistor for Thick film type middle flash mode (0402) ROHM MCR01MZSF1K10 1100 Ohm CIN Input filtering capacitor Ceramic type (0603) 6.3 V TDK C1608X5ROJ106MT 10 COUT Output capacitor Ceramic type (0603) 6.3 V TDK C1608X5ROJ106MT 10 4/ 2x10 µF µF AN2304 Table 2. Symbol L Selection of external components STCF02 Demo board BOM List (continued) Parameter 4.5(L) x 4.7 (W) x 1.4 (L) Type Supplier & part number Low profile inductor shielded DCR TDK VFL5014A-4R7 Isat Typ. value Min Typ Unit Max 4.7 µF 0.12 Ohm 1.7 A R1 Anti bump resistor Thick film type (0402) ROHM MCR03EZPJ10KR00 10 KOhm R2 Anti bump resistor Thick film type (0402) ROHM MCR03EZPJ10KR00 10 KOhm R3 Anti bump resistor Thick film type (0402) ROHM MCR03EZPJ10KR00 10 KOhm R4 Resistor Thick film type (0402) ROHM MCR03EZPJ1R00 0 Ohm R7 Safety shutdown resistor Thick film type (0402) ROHM MCR03EZPJ1MR00 1 MOhm C1 Filtering Capacitor Ceramic (0603) TDK C1608C0J1H470J 47 pF C2 Safety shutdown capacitor Ceramic type (0603) 6.3 V TDK C1608X5ROJ105MT 1 µF D1 Schottky diode STPS1L40M D2 Schottky diode STPS1L40M LED Luxeon LED PWF1 LXCL-PW1 2 Selection of external components 2.1 Input and output capacitor selection For input and output capacitors it is recommended to use a ceramic capacitor with low ESR. For a good stability of the device supplied by a low input voltage of 2.7 V at maximum ratings, it is recommended to use 10 uF/6.3 V as a minimum value of input capacitor and 10 uF/6.3 V as a minimum value of output capacitor. Note: See recommended components in Table 2. 2.2 Inductor selection Shielded thick inductor with low DC series resistance of wiring is recommended for this application. For good efficiency it is recommended to use an inductor with series DC resistance RDCL<RD/10, [Ω, Ω,1] where RD is dynamic resistance of LED. 5/ Selection of external components AN2304 For nominal operation, the peak inductor current can be calculated by this formula: Equation 1 2 ⎛ I OUT ( V OUT – V in ) • V IN ⎞ V OUT I PEAK = ⎜ ----------- + ----------------------------------------------------⎟ • -------------V IN 2• L• F• V 2 ⎝ n OUT ⎠ Where: – IPEAK Peak inductor current – IOUT Current sourced at the VOUT pin – n Efficiency of the STCF02 – VOUT Output voltage at the pin Vout – VIN Input voltage at the pin Vin – L Inductance value of the inductor – F Switching frequency Note: See recommended components in Table 2. 2.3 LED selection Any LED with forward voltage from 2.7 V to 5 V is feasible for use with device STCF02. LED forward voltage must include the voltage spread of this value. It is possible to set the LED current in the three different operating modes (torch, medium flash, high flash) through three external sensing resistors. Note: See recommended components in Table 2. 2.4 RFLASH selection RFLASH resistor can be selected by equations RFLASH=160mV/IFLASH and PRFLASH=RFLASH*IFLASH2, where PRFLASH is the dissipated power on RFLASH resistor. It is recommended to use thick metal film resistor 0603 package size with 1% tolerance. Maximum flash LED current for STCF02 device is 600mA in battery voltage range from 2.7 V to 5.5 V. 2.5 RTORCH selection RTORCH resistor can be selected by equations: Equation 2 160mV – ( I TORCH • R FLASH ) R TORCH = ----------------------------------------------------------------------------I TORCH Equation 3 P RTORCH = R TORCH • I 6/ 2 TORCH AN2304 Selection of external components where PRTORCH is the dissipated power on RTORCH resistor. It is recommended to use thick metal film resistor with 1% or 5% tolerance. Maximum torch LED current for STCF02 device is 250 mA in voltage range from 2.7 V to 5.5 V. 2.6 RMF selection With this resistor it is possible to set the LED current value in middle flash mode. The value of RMF resistor is defined by equation: Equation 4 –5 13000 • R MF⎞ 10 I MF = ----------- ⎛ 3000 + --------------------------------R FL ⎝ 13000 + R MF⎠ where IMF is middle flash LED current. It is recommended to use thick film resistor in 0402 package size. 2.7 CCOMP selection This component could optionally be used if the error amplifier bandwidth, which is set to 2kHz default, has to be decreased in case of unstable function of driver, which could cause the use of different inductor and output capacitor values. It is recommended to use a ceramic capacitor in 0402 package size. 2.8 NTC and Rx resistor selection The NTC resistor for sensing LED temperature and the Rx resistor create the voltage divider. Output of this divider is compared to the internal voltage reference 1.192 V. When the voltage of the voltage divider output is increased over the reference, the logic will switch off the power circuit. The maximum current, which could flow through the NTC pin is 1mA. Voltage divider must be supplied from an external voltage source. 7/ PCB design AN2304 3 PCB design 3.1 PCB design rules STCF02 is a powerful switched device, the PCB must be designed in line with rules for designing switched supplies. It is recommended to use at least four layers PCB to save the area on application PCB. The power wirings must be as short as possible and wide, because of large current. Place all external components close to the STCF02. High-energy switched loops should be as small as possible to reduce EMI. Most of LEDs need cooling, which could be done by defined area of copper on the PCB. Use the reference guide of each LED to design the heatsink. Place the RFLASH resistor as close as possible to pin 8. When the change of PCB layer is needed, use enough vias. Place the NTC resistor as close as possible to LED for good temperature sensing. Direct connection of GND and PGND is needed to achieve correct value of output current. The Led current should not flow through this track! Sensing of the voltage on the Rflash resistor has to be done with a wire from pin 7 directly connected to Rflash resistor, no current flows through this track. Pin 6 and pin 7 have to be connected on the pin of Rflash resistor. Expose pad has to be connected to the PGND with a track as wide as possible. Figure 3. PCB layout + - Input supply range = 2.7 V to 5.5 V ● Torch mode selection: pushing the Torch button, we will have 25 mA current flowing through the WLed, no time limit is present. Is possible to reach up to 250 mA of torch current calculating the Rtorch resistor using this formula: Equation 5 16mV I TORCH = ------------------------------------------------R TORCH + R FLASH 8/ AN2304 Safety shutdown in flash mode ● Medium Flash mode selection: pushing the Medium Flash button, we will have 150 mA current flowing through the WLed, safety shutdown is active and after maximum 0.5 sec. the WLed will be turned off. Is possible to reach up to 500 mA of medium Flash current calculating the Rmflash resistor using this formula: Equation 6 –5 10 I MF = --------------------------------------------------------------------------------13000 • R MF ⎛ ⎞ --------------------------------R FL • 3000 + ⎝ 13000 + R MF⎠ ● Full Flash mode selection: pushing the Full Flash button, we will have 600 mA current flowing through the WLed, safety shutdown is active and after maximum 0.5 sec. the WLed will be turned off. Is possible to reach up to 600 mA of full Flash current calculating the Rflash resistor using this formula: Equation 7 160mVI FLASH = -------------------R FLASH 4 Safety shutdown in flash mode Alternatively to NTC temperature sensing and protection, it is possible to properly drive all the STCF02 features with only the EN1 and EN2 pins and include the safety shutdown mode. This feature has to be active only for Mid Flash and Full Flash operating mode because in Torch mode there is no risk of burning the LED because of the low current flowing through it. The purpose of this specific application is to avoid LED burning if the microcontroller does not work properly (this means EN1 and/or EN2 are pins stuck in a high logic level). If EN2 is stuck at high level while EN1 still in low level, the selected mode is Torch, and the safety shutdown in not needed. If EN1 is stuck at high level, the STCF02 is in Mid/Full Flash mode and the LED should be protected from long time operation at high peak current. In this condition, after an RC defined time, the LED automatically goes off and stays off waiting for EN1/EN2 pin resuming at low level. After the microcontroller reset, both EN pins will go at low level (device enters "All off" mode) and the capacitor is discharged. At this point, the STCF02 is ready to resume the normal operating mode. In typical operating conditions safety shutdown time is about 500msec. This is possible by applying an RC filter with R=1 MOhm and C=0.47 uF. 9/ Safety shutdown in flash mode Figure 4. Safety shutdown RC circuit Figure 5. Safety shut down graphics AN2304 Operation for RC filter R=1M Ohm, C=1.0 uF; VLX1 switching voltage (dark blue), during flash mode with EN1 voltage in high level (light blue). The RX pin voltage (violet) increases and the output current (light green) goes down after one second (1.192 V threshold reached). 10/ AN2304 5 Revision history Revision history Table 3. Revision history Date Revision 26-Jan-2006 1 04-Apr-2006 2 25-May-2006 3 06-Feb-2007 4 Changes First issue - Pictures changed - New values added in table “Bill of Material“ - Pictures changed - Pictures changed - BOM changed - Minor text changes 11/ AN2304 Please Read Carefully: Information in this document is provided solely in connection with ST products. STMicroelectronics NV and its subsidiaries (“ST”) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described herein at any time, without notice. All ST products are sold pursuant to ST’s terms and conditions of sale. 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