AN2507 Application note High-Power Camera Flash LED Driver with I2C™ Introduction This application note explains the design of a FLASH LED driver using the STCF03 device, which is a Buck-Boost current mode converter with an I2C interface. The schematic, functional description, recommendations for PCB Layout and external components selection are also discussed in this application note. This device is designed for driving a single LED with a forward voltage range from 2.7 to 5 V. A detailed functional description can be found below. Package and demo board top view Version B - Version C April 2007 Version A Rev 1 1/31 www.st.com Contents AN2507 Contents 1 Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1 2 3 4 Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.1 Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2 Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.3 LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.4 RFL selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.5 RTR selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.6 NTC AND RX resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 6 2/31 A four-layer PCB with application area 45.1 mm2 for BGA package, version B 10 3.2.2 A two-layer PCB with application area 72.4 mm2 for QFN package . . . 13 3.2.3 A four-layer PCB with application area 45.1 mm2 for BGA package, version C 14 Accessing the internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.1 SHUTDOWN mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.2 SHUTDOWN mode with the NTC-feature activated . . . . . . . . . . . . . . . . . 17 5.3 READY mode and NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.4 TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5.5 FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 The STATUS register and the ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . 21 6.1 7 3.2.1 Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.1 5 Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 The STATUS register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Reading and writing to the STCF03 registers through the I2C bus . . 22 AN2507 8 9 Contents 7.1 Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 7.2 Writing to multiple registers with incremental addressing . . . . . . . . . . . . 22 7.3 Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 7.4 Reading from multiple registers with incremental addressing . . . . . . . . . 23 Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . . 24 8.1 Example 1: 600 mA FLASH with 700 ms duration . . . . . . . . . . . . . . . . . . 24 8.2 Example 2: 25 mA TORCH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 8.3 Example 3: an Auxiliary LED running at 10 mA for 500 ms . . . . . . . . . . . 26 8.4 Example 4: Red-eye reduction (multiple short flashes) . . . . . . . . . . . . . . 27 8.5 Example 5: A FLASH pulse longer than 1.5 s . . . . . . . . . . . . . . . . . . . . . 28 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3/31 List of tables AN2507 List of tables Table 1. Table 2. Table 3. Table 4. Table 5. Table 6. Table 7. Table 8. Table 9. Table 10. Table 11. Table 12. Table 13. Table 14. Table 15. Table 16. Table 17. Table 18. Table 19. Table 20. Table 21. Table 22. Table 23. Table 24. Table 25. Table 26. Table 27. Table 28. Table 29. Table 30. Table 31. 4/31 Recommended components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Accessibility of internal registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 COMMAND register data to enter SHUTDOWN mode (version B) . . . . . . . . . . . . . . . . . . 17 COMMAND register data to enter SHUTDOWN mode (version A and C) . . . . . . . . . . . . . 17 COMMAND register data to enter SHUTDOWN mode with NTC activated (version A and C) 18 COMMAND register data to enter READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 COMMAND register data to enter READY mode with NTC ON . . . . . . . . . . . . . . . . . . . . . 18 COMMAND register data to enter TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 STATUS register bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Effect of the STATUS register bits on the operation of the device . . . . . . . . . . . . . . . . . . . 21 TORCH mode and FLASH mode dimming registers settings. . . . . . . . . . . . . . . . . . . . . . . 24 COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 DIMMING register data for the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 COMMAND register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 DIMMING register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 I2C data packet to activate TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 I2C data packet for terminating the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 I2C data packet for activating the READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 I2C data packet for activating the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 COMMAND register data for FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 DIMMING register data for the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 1st I2C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 2nd I2C data packet for restart of the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 3rd I2C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 AN2507 List of figures List of figures Figure 1. Figure 2. Figure 3. Figure 4. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Figure 10. Figure 11. Figure 12. Figure 13. Figure 14. Figure 15. Figure 16. Figure 17. Figure 18. Figure 19. Figure 20. Figure 21. Figure 22. A typical application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 NTC connection for versions with internal voltage reference . . . . . . . . . . . . . . . . . . . . . . . . 9 Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Top layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Splitting the FLASH pulse into several shorter pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Writing to multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Reading from multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Multiple flashes handled by the TRIG pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 I2C bus packets timing for a FLASH lasting longer than FTIM max . . . . . . . . . . . . . . . . . . 30 5/31 Schematic description 1 AN2507 Schematic description The FLASH LED driver STCF03 has a high operational frequency (1.8 MHz) which allows the usage of small-sized external components. The three versions (A, B and C) differ in the way the NTC feature is supported. 1.1 Application schematic Figure 1. A typical application schematic Note: ** Connect to VI, or GND or SDA or SCL to choose one of the 4 different I2C Slave Addresses Note: *** Optional components to support auxiliary functions 6/31 ● Version A: STCF03PNR - QFN package, external reference for NTC protection ● Version B: STCF03ITBR - BGA package, internal reference for NTC protection ● Version C: STCF03TBR - BGA package, external reference for NTC protection AN2507 Selection of external components 2 Selection of external components 2.1 Input and output capacitor selection It is recommended to use ceramic capacitors with low ESR as input and output capacitors. It is recommended to use 10 µF/6.3 V as a minimum value of input capacitor and 1 µF/ 6.3 V as an optimal value of output capacitor to achieve good stability of the device supplied from low input voltage (2.7 V) at maximum ratings of output power. Note: see recommended components in Table 1. 2.2 Inductor selection A thin shielded inductor with a low DC series resistance of winding is recommended for this application. To achieve a good efficiency in step-up mode, it is recommended to use an inductor with a DC series resistance RDCL = RD/10 [Ω, Ω, 1], where RD is the dynamic resistance of the LED [Ω, Ω, 1]. For nominal operation, the peak inductor current can be calculated by the following formula: Equation 1 2 ⎛ I OUT ⎛ ( V OUT – V IN ) • V IN ⎞ ⎞ V OUT -⎟ ⎟ • -------------I PEAK = ⎜ ⎛ ------------⎞ + ⎜ ---------------------------------------------------⎝ ⎠ V IN ⎝ n ⎝ 2 • L • F • V OUT 2 ⎠ ⎠ Where: ● IPEAK: Peak inductor current ● IOUT: Current sourced at the VOUT-pin ● n: Efficiency of the STCF03 ● VOUT: Output voltage at the VOUT-pin ● VIN: Input voltage at the VBAT-pin ● L: Inductance value of the inductor ● F: Switching frequency Note: see recommended components in Table 1. 2.3 LED selection All LEDs with a forward voltage range ranging from 2.7 V to 5 V are compatible with STCF03. The forward voltage spread of any selected LED must however lay within this range (2.7 V to 5 V). It is possible to set the level of the LED current in FLASH mode and TORCH mode by setting the values of the corresponding sensing resistors. The level of the LED current in FLASH mode can be set by changing the external FLASH resistor. Note: see recommended components in Table 1. 7/31 Selection of external components 2.4 AN2507 RFL selection The value of the RFL resistor can be calculated by the following equations: RFL = VFB2/IFLASH where VFB2 = 226 mV and PRFLASH = RFL * IFLASH2, where PRFL is the power dissipated on the RFL resistor. It is recommended to use a thin metal film resistor with 0805 package size and 1% tolerance. The maximum LED current in FLASH mode for STCF03 is (800 mA) for a battery voltage ranging from 2.7 V to 5.5 V in VQFPN version. 2.5 RTR selection The value of the RTR resistor can be calculated by following equations: Equation 2 V REF – I TORCH • R FL R TR = ⎛ --------------------------------------------------------⎞ ⎝ ⎠ I TORCH and P RTORCH = R TR • I TORCH 2 where PRTORCH is the power dissipated on the RTR resistor. It is recommended to follow the equation RTR = 6.66* RFL to avoid any jump in the current dimming values. It is recommended to use a thin metal film resistor with 1% or 5% tolerance. The maximum LED current in TORCH mode for SCTF03 is 200 mA for a battery voltage ranging from 2.7 V to 5.5 V 2.6 NTC AND RX resistor selection a) A, C versions without an internal reference voltage for the NTC feature. STCF03 requires a negative thermistor (NTC) for sensing the LED temperature, as well as an RX resistor and an external voltage reference in order to use the NTC feature. Please refer to the typical application schematic in Figure 1 VER A,C for more details. Once the NTC feature is activated, the internal switch connects the RX resistor to the NTC, and this creates a voltage divider supplied by the external reference voltage connected to the NTC. If the temperature of the NTC-thermistor rises due to the heat dissipated by the LED, the voltage on the NTC pin increases. When this voltage exceeds 0.56 V, the NTC_W bit in the STATUS register is set to High, and the ATN pin is set to Low to inform the microcontroller that the LED is becoming hot. The NTC_W bit is cleared by reading the STATUS register. If the voltage on the NTC pin rises further and exceeds 1.2 V, the NTC_H bit in the STATUS register is set to High, and the ATN pin is set to Low to inform the microcontroller that the LED is too hot and the device goes automatically to the READY mode to avoid damaging the LED. This status is latched, until the microcontroller reads the STATUS register. Reading the STATUS register clears the NTC_H bit. The selection of the NTC and RX resistors values strongly depends on the power dissipated by the LED and all components surrounding the NTC-thermistor and on the cooling capabilities of each specific application. The RX and the NTC values in Table 1 below work well in the demo board presented in this application note. A real application may require a different type of NTC-thermistor to achieve optimal thermal protection. 8/31 AN2507 Selection of external components The procedure to activate the NTC-feature is described in Section 5.2. b) Versions with an internal reference voltage for the NTC. This version requires a different connection between the RX and NTC resistors. See Figure 2 below or Figure 1 version B. Figure 2. NTC connection for versions with internal voltage reference NTC STCF03 NTC thermistor To optional A/D converter RX RX resistor Note: Versions with internal reference voltage do not support the SHUTDOWN+NTC mode, because the internal reference voltage is off in SHUTDOWN mode. Table 1. Recommended components Component Manufacturer Part number Value Size CI TDK C1608X5R0J106M 10 µF 0603 CO TDK C1608X5R0J105M 1 µF 0603 L TDK VLF4012AT-4R7M1R1 4.7 µH 3.7 x 3.5 x 1.2 mm NTC Murata NCP21WF104J03RA 100 kΩ 0805 RFL Tyco RL73K1JR27JTD 0.27 Ω 0603 RTR Rohm MCR01MZPJ6R20 1.8 Ω 0402 RX Rohm MCR01MZPJ15K 15 kΩ 0402 LED Luxeon LED LXCL-PW1 9/31 PCB design AN2507 3 PCB design 3.1 PCB design rules STCF03 is a powerful switching device where the PCB must be designed in line with switched supplies design rules. The power tracks (or wires in demo-board) must be as short as possible and wide enough, because of the high currents involved. It is recommended to use a 4 layers PCB to get the best performance. All external components must be placed as close as possible to STCF03. All high-energy switched loops should be as small as possible to reduce EMI. Most of LEDs need efficient cooling, which could be done by using a dedicated copper area on the PCB. Please refer to the selected LED's reference guide to design the heatsink. Place the RFL resistor as close as possible to the PGND pins and the ground pin of the COUT capacitor. In case a modification of any PCB layer is required, it is highly recommended to use enough vias. Place the NTC resistor as close as possible to the LED for good temperature sensing. Direct connection between GND and PGND is necessary in order to achieve correct output current value. No LED current should flow through this track! Voltage sensing on the RFL resistor must to done on a track from ball FB2 and directly connected to the RFL resistor. Again, no current should flow through this track. Pin FB2S must be connected to the RFL resistor pin. Vias connecting the STCF03 pins to the copper tracks (if used) must be 0.1 mm in diameter for BGA version. It is recommended to use the filled vias. 3.2 PCB layout 3.2.1 A four-layer PCB with application area 45.1 mm2 for BGA package, version B (for version C is layout exactly same except the NTC connection, see Figure 1) Figure 3. 10/31 Top layer AN2507 PCB design Figure 4. Middle layer 1 Figure 5. Middle layer 2 11/31 PCB design 12/31 AN2507 Figure 6. Bottom layer Figure 7. Top overlay AN2507 3.2.2 PCB design A two-layer PCB with application area 72.4 mm2 for QFN package Figure 8. Top layer Figure 9. Bottom layer 13/31 PCB design AN2507 Figure 10. Top overlay 3.2.3 A four-layer PCB with application area 45.1 mm2 for BGA package, version C Figure 11. Top layer 14/31 AN2507 PCB design Figure 12. Middle layer 1 Figure 13. Middle layer 2 15/31 Internal registers AN2507 Figure 14. Bottom layer Figure 15. Top overlay 4 Internal registers 4.1 Accessing the internal registers There are 4 internal registers in STCF03 (which are the COMMAND, DIMMING, AUX_LED and STATUS registers). The STATUS register is read-only. The COMMAND register can be accessed in any operation mode. All the other registers can be accessed in any mode, except in SHUTDOWN mode. When the device enters SHUTDOWN mode, the DIMMING, AUX_LED and STATUS registers are cleared. The COMMAND register value remains untouched when entering SHUTDOWN mode. Table 2 shows the accessibility of each register in all operation modes. 16/31 AN2507 Table 2. Operation modes Accessibility of internal registers Mode Register Address SHUTDOWN READY TORCH FLASH SHUTDOWN value PowerON reset value COMMAND 00 Read / Write Read / write Read / write Read / write Untouched Cleared DIMMING 01 Inaccessible Read / write Read / write Read / write Cleared Cleared AUX_LED 02 Inaccessible Read / write Read / write Read / write Cleared Cleared STATUS 03 Inaccessible Read only Read only Read only Cleared Cleared 5 Operation modes 5.1 SHUTDOWN mode SHUTDOWN mode is entered after the Power-ON reset. This mode is mainly used to decrease the power consumption of the device. During this mode, only the I2C interface is alive. The only thing which can be done in SHUTDOWN mode is to access the COMMAND register. Entering SHUTDOWN mode by writing to the COMMAND register aborts any running operation and clears the values of the DIMMING, AUX_LED and STATUS registers. The COMMAND register value is not affected by entering SHUTDOWN mode. The following data must be written to the COMMAND register to enter SHUTDOWN mode. Table 3. CMD_REG COMMAND register data to enter SHUTDOWN mode (version B) PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 0 x x x x x x x MSB Table 4. CMD_REG LSB COMMAND register data to enter SHUTDOWN mode (version A and C) PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 0 x x 0 x x x x MSB 5.2 LSB SHUTDOWN mode with the NTC-feature activated This mode is supported only in version A, which does not have any internal voltage reference for the NTC feature. When this operation mode is activated, the microcontroller can still monitor the NTC voltage through its A/D converter, while STCF03 remains in SHUTDOWN mode and therefore saves power. The following data must be written to the COMMAND register to enter SHUTDOWN mode + NTC. 17/31 Operation modes Table 5. CMD_REG AN2507 COMMAND register data to enter SHUTDOWN mode with NTC activated (version A and C) PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 0 x x 1 x x x x MSB 5.3 LSB READY mode and NTC The READY mode allows the user to access all the internal registers. The NTC feature can be activated in this mode and the temperature of the LED can be sensed by the A/D converter of the microcontroller. The following data must be written to the COMMAND register to enter READY mode. Table 6. CMD_REG COMMAND register data to enter READY mode PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 0 0 0 x x x x MSB LSB The following data must be written to the COMMAND register to activate the NTC feature. Table 7. CMD_REG COMMAND register data to enter READY mode with NTC ON PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 0 0 1 x x x x MSB LSB As soon as the NTC feature is activated, the internal switch connects the NTC resistor to the RX resistor, thereby creating a voltage divider. The voltage on this divider can be, if desired, monitored by the A/D converter of the microcontroller. An external voltage reference must be connected to the NTC to use this feature (only in version A and C). The bits NTC_W and NTC_H of the STATUS register will not be properly set if there is no external reference voltage connected to the NTC (only in version A and C). If the NTC feature is not going to be used, neither the negative thermistor, nor the external reference needs to be connected. In this case, it is recommended to ground the RX pin. As the NTC feature is automatically activated during the FLASH and TORCH mode, leaving the RX pin floating could lead to unwanted interruptions of the light due to non-defined voltages on the RX pin 5.4 TORCH mode This mode is intended to be used for low light intensities. The LED current in the TORCH mode can be adjusted in a range from 15 mA up to 200 mA. The TORCH mode is activated by writing the following data to the COMMAND register. 18/31 AN2507 Table 8. CMD_REG Operation modes COMMAND register data to enter TORCH mode PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 0 1 x x x x x MSB LSB The DIMMING register value (TDIM) must be set as well, unless it has already been set during a previous operation. If TDIM register is not set, then the default output current value will be at the minimum. There is no internal timer which controls the TORCH duration. Therefore, as soon as the TORCH mode is activated, it remains active until a new mode is entered by writing a new data to the COMMAND register. If the TORCH mode was terminated by entering READY or FLASH mode, it can be restarted again by writing the corresponding data to the COMMAND register only, because entering any of the READY and FLASH modes does not influence the TDIM value. If the TORCH mode was terminated by entering into SHUTDOWN mode, then the TDIM value must be set again during the restart of the TORCH, because entering the SHUTDOWN mode clears the TDIM value. As soon as the TORCH mode is activated, the NTC feature is automatically activated too in order to protect the LED against overheating. The NTC feature will be activated even if the NTC_ON bit in the COMMAND register is set to zero. 5.5 FLASH mode This mode is intended to be used for high light intensities. The LED current in the FLASH mode can be adjusted up to 800 mA with the input voltage ranging from 3.3 V up to 5.5 V. The FLASH mode is activated by writing the following data to the COMMAND register. Table 9. CMD_REG COMMAND register data to enter FLASH mode PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 1 x x x x x x MSB LSB The DIMMING register value (FDIM) must be set as well, unless it has already been set during a previous operation. The activation of the FLASH mode requires the TRIG pin to be High. The FLASH mode is active only when the TRIG_EN bit in the COMMAND register is set to 1 and the TRIG pin is High. This gives the user the possibility to choose between a soft and a hard triggering of the FLASH mode. The soft triggering is done by writing data to the internal registers only, while the TRIG pin is permanently kept High, that is, by connecting it to VBAT. This saves one pin of the microcontroller, which can be used for a different purpose, but this way of triggering is less accurate than the hard one. The second disadvantage of this solution is that the FLASH duration can only be set in discrete steps of the internal timer (1 step = approx. 100 ms). 19/31 Operation modes AN2507 Hard-triggering of the FLASH mode requires the microcontroller to manage the TRIG pin. The COMMAND and the DIMMING registers are loaded with data before the TRIG pin is set to High. This allows the user to avoid the I2C-bus latency. FLASH mode then starts as soon as the TRIG pin is set to High. It takes typically about 0.7 ms to ramp-up the LED current to the adjusted value. This time may vary according to the LED current value and the battery voltage. When the TRIG pin is kept High long enough, the internal timer reaches zero and the FLASH mode is over. As soon as the FLASH is timed out, the ATN pin is pulled down for 11µs to inform the microcontroller that the STATUS register was updated and that the flash is over. If the TRIG pin is set to Low before the internal timer reaches zero, the FLASH mode will be interrupted and can be restarted by setting the TRIG pin to High again. The internal timer is stopped while the TRIG pin is Low. This means that the user can split the FLASH into several pulses of a total length equal to the FTIM value. Figure 16 below shows splitting of the FLASH into several shorter pulses. The cumulative length of all the pulses is determined by the FTIM value. Figure 16 shows the case for FTIM = 9 (900 ms FLASH time). The cumulative time when the TRIG pin is High is 1000 ms (5 pulses 200 ms long). The last FLASH pulse will be100ms long only. The reason is that the internal FLASH timer reaches zero and the TRIG_EN bit is set to 0. Figure 16. Splitting the FLASH pulse into several shorter pulses 1300 ms 200 ms Time when the internal flash timer reaches 0 100 ms 2 I C bus packet TRIG_EN bit TRIG pin LED current Internal Flash timer values 9 8 7 6 5 4 3 2 1 0 Hard triggering allows therefore a smooth setting of the FLASH duration. The resolution is about 8.8µs. The minimum FLASH duration is limited by the ramp-up time of the LED current and the maximum is limited by the FTIM value. If it is necessary to make a FLASH pulse longer than the maximum allowed by FTIM, then it is necessary to reload the COMMAND register before the internal timer reaches zero (start a new FLASH before the previous one elapses). See Section 8.5 - Example 5 for more details. 20/31 AN2507 The STATUS register and the ATN pin 6 The STATUS register and the ATN pin 6.1 The STATUS register Table 10. Bit name STATUS register bits N/A F_RUN LED_F NTC_W NTC_H OT_F N/A MSB VOUTOK_N LSB A detailed description of each bit is stated in the datasheet. Table 11. Effect of the STATUS register bits on the operation of the device Bit name Default value F_RUN LED_F NTC_W NTC_H OT_F (STAT_REG) (STAT_REG) (STAT_REG) (STAT_REG) (STAT_REG) VOUTOK_N (STAT_REG) 0 0 0 0 0 0 NO YES YES YES YES YES Forces READY mode when set NO YES NO YES YES YES Sets ATN LOW when set NO YES YES YES YES YES Latched (1) 1. YES means that the bit is set by internal signals and is reset to its default value by an I2C-read operation of STAT_REG; NO means that the bit is set and reset by internal signals in real-time When the status register is latched, reading and writing to the registers is still possible, but the bits TRIG_EN and TCH_ON in the COMMAND register and AUXL register cannot be changed, until the device is unlatched. It is necessary to read the STATUS register to unlatch the device. The ATN pin is also pulled down when the internal timer reaches zero in FLASH mode. In this case the ATN pin is pulled down for 11 µs only. It is recommended to connect the ATN pin to the interrupt input of the microcontroller. If it is not connected to the interrupt input, the ATN pin should be pulled fast enough not to miss the 11 µs pulse, that is, by a programming loop which is entered after start of the FLASH mode. This loop runs until the ATN pin gets Low. It is recommended to make a time-out of such a loop. 21/31 Reading and writing to the STCF03 registers through the I2C bus AN2507 7 Reading and writing to the STCF03 registers through the I2C bus 7.1 Writing to a single register Writing to a single register starts with a START-bit followed by the 7-bit device address of STCF03. The 8-th bit is the R/W bit, which is 0 in this case. R/W = 1 means a Reading operation. Then the master awaits an acknowledgement from STCF03. The 8-bit address of the desired register is sent afterwards to STCF03. It will also be followed by an acknowledge pulse. The last transmitted byte is the data that is going to be written into the register. It is followed again by an acknowledge pulse from STCF03. Then the master generates a STOPbit and the communication is over. See Figure 17 below. Figure 17. Writing to a single register DEVICE ADDRESS 7 bits W R I T E ADDRESS OF REGISTER DATA SDA LINE S M T S A B R T 7.2 L R A M S / C S B W K B L A M S C S B K B L A S S C T B K O P Writing to multiple registers with incremental addressing It would be unpractical to send several times the device address and the address of the register when writing to multiple registers. STCF03 supports writing to multiple registers with incremental addressing. When data is written to a register, the register address is automatically incremented (by one), and therefore the next data can be sent without sending again the device address and the register address. See Figure 18 below. Figure 18. Writing to multiple registers DEVICE ADDRESS 7 bits W R I T E ADDRESS OF REGISTER i DATA i DATA i+1 DATA i+2 DATA i+2 DATA i+n SDA LINE S M T S A B R T 22/31 L R A M S / C S B W K B L A M S C S B K B L A M S C S B K B L A M S C S B K B L A M S C S B K B L A M S C S B K B L A S S C T B K O P AN2507 7.3 Reading and writing to the STCF03 registers through the I2C bus Reading from a single register The reading operation starts with a START-bit followed by 7 bit device address of STCF03. The 8-th bit is the R/W bit, which is 0 in this case. STCF03 confirms the receiving of the address + R/W bit by an acknowledge pulse. The address of the register which should be read is sent after and confirmed by an acknowledge pulse from STCF03 again. Then the master generates a START-bit again and sends the device address followed by the R/W-bit, which is 1 now. STCF03 confirms the receiving of the address + R/W-bit by an acknowledge pulse, and starts to send data to the master. No acknowledge pulse from the master is required after receiving the data. Then the master generates a STOP-bit to terminate the communication. See the Figure 19 below. Figure 19. Reading from a single register DEVICE ADDRESS 7 bits W R I T E DEVICE ADDRESS 7 bits ADDRESS OF REGISTER R E A D DATA SDA LINE S M T S A B R T 7.4 L R A M S / C S B W K B L A S S C T B K A R T R A / C W K L N S S O T O B A P C Reading from multiple registers with incremental addressing Reading from multiple registers starts in the same way as reading from a single register. As soon as the first register is read, the register address is automatically incremented. If the master generates an acknowledge pulse after receiving the data from the first register, then reading from the next register can start immediately without having to send once more the device and the register addresses. The last acknowledge pulse before the STOP-bit is not required. See Figure 20 below. Figure 20. Reading from multiple registers DEVICE ADDRESS 7 bits W R I T E DEVICE ADDRESS 7 bits ADDRESS OF REGISTER i R E A D DATA i DATA i+1 DATA i+2 DATA i+2 DATA i+n SDA LINE S M T S A B R T L R A M S / C S B W K B L A S S C T B K A R T R A / C W K L A M S C S B K B L A M S C S B K B L A M S C S B K B L A M S C S B K B L N S S O T O B A P C K 23/31 Examples of register setup for each mode 8 AN2507 Examples of register setup for each mode A device address 0x62 is used in all the example below. The STCF03 is configured to this device address, if the ADD pin is connected to VBAT pin. In the demoboard the device address is 0x60 because the ADD pin is connected to GND. Table 12. T_DIM (hex) 0 TORCH mode and FLASH mode dimming registers settings 1 2 3 4 5 6 7 F_DIM (hex) 8 9 A B C D E F 0 1 2 3 4 5 6 7 8 9 A B C D E F LED current [mA] 16 19 23 27 32 39 46 55 65 77 92 109 124 147 175 209 248 296 352 418 498 592 705 840 Internal step 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 VREF1 [mV] 33 40 47 56 67 80 95 113 134 160 190 227 33 40 47 56 67 79 95 113 134 160 190 227 Sense Resist. RFL RFL RFL RFL RFL RFL RFL RFL RFL RFL + + + + + + + + + + RTR RTR RTR RTR RTR RTR RTR RTR RTR RTR RFL RFL + + RFL RTR RTR RFL RFL RFL RFL 21 22 23 24 RFL RFL RFL RFL RFL RFL RFL Note: LED current values refer to RFL= 0.27 Ohm, RTR = 1.8 Ohm 8.1 Example 1: 600 mA FLASH with 700 ms duration Let's suppose that RFL = 0.27 Ω. The targeted value of the FLASH current is 600 mA and the FLASH duration should be 700 ms. The reference voltage must be set to 160 mV to achieve a 600 mA FLASH current with a 0.27 Ω sensing resistor. The value of FDIM (4 bits) must be set to 0xD to set up the reference voltage to 160 mV. (See Table 12) The FLASH duration timer can be set to 100ms up to 1500 ms in 100ms increments. If the desired FLASH duration is 700 ms the value FTIM (4 bits) must be set to 0x7. Bit PWR_ON of the command register must be set to 1. Bit TRIG_EN of the command register must be set to 1. Bit TCH_ON of the command register must be set to 0. Bit NTC_ON of the command register can be set to any value, because NTC is automatically ON when the FLASH mode is active. Setting this bit to 0 will not switch off the NTC. Table 13. CMD_REG COMMAND register data to enter FLASH mode PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 1 x x 0 1 1 1 MSB 24/31 LSB AN2507 Table 14. Examples of register setup for each mode DIMMING register data for the FLASH mode TDIM_3 TDIM_2 TDIM_1 TDIM_0 FDIM_3 FDIM_2 FDIM_1 FTIM_0 0 0 0 0 1 1 0 1 DIM_REG MSB LSB It is necessary to write 4 bytes to STCF03 to make a FLASH. Table 15. 8.2 I2C data packet for activating the FLASH mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 D7 1 1 0 1 0 1 1 1 Data of the command register 4 0D 0 0 0 0 1 1 0 1 Data of the dimming register Example 2: 25 mA TORCH Let's suppose that RFL = 0.27 Ω, RTR = 1.8 Ω and the targeted value of the TORCH current is 25 mA. The reference voltage must be set to 56 mV to achieve 25 mA in TORCH mode with the resistor values mentioned above. The value of TDIM (4 bits) must be set to 0x3 to set up the reference voltage to 56 mV. Bit PWR_ON of the command register must be set to 1. Bit TRIG_EN of the command register must be set to 0. Bit TCH_ON of the command register must be set to 0. Bit NTC_ON of the command register can be set to any value, because NTC is automatically ON, when TORCH mode is active. Setting this bit to 0 does not switch off the NTC. Table 16. CMD_REG COMMAND register data for the TORCH mode PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 0 1 1 0 0 0 0 MSB Table 17. DIM_REG LSB DIMMING register data for the TORCH mode TDIM_3 TDIM_2 TDIM_1 TDIM_0 FDIM_3 FDIM_2 FDIM_1 FDIM_0 0 0 1 1 0 0 0 0 MSB LSB 25/31 Examples of register setup for each mode AN2507 It is necessary to write 4 bytes to the STCF03 to run the TORCH mode. Table 18. I2C data packet to activate TORCH mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 B0 1 0 1 1 0 0 0 0 Data of the command register 4 30 0 0 1 1 0 0 0 0 Data of the dimming register The duration of the TORCH mode is "unlimited". TORCH mode is terminated by setting the TCH_ON bit in the COMMAND register to 0. Termination of the TORCH mode can be done by writing the following data to STCF03. Table 19. I2C data packet for terminating the TORCH mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 80 1 0 0 0 0 0 0 0 Data of the command register This puts the STCF03 into READY mode. 8.3 Example 3: an Auxiliary LED running at 10 mA for 500 ms STCF03 must be into READY mode (both bits TRIG_EN and TCH_ON are 0) to activate the Auxiliary LED. A 10 mA output current is reached when AUXI is set to 0x8. AUXT must be set to 0x5 to have a 500 ms duration of the Auxiliary LED lighting. Table 20. CMD_REG COMMAND register data for the AUX_LED PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 0 0 0 0 0 0 0 MSB Table 21. AUX_LED COMMAND register data for the AUX_LED AUX_3 AUX_2 AUX_1 AUX_0 AUX_3 AUX_2 AUX_1 AUX_0 1 0 0 0 0 1 0 1 MSB 26/31 LSB LSB AN2507 Examples of register setup for each mode Writing the 3 bytes below to STCF03 puts it into READY mode. This can be skipped if it already is in READY mode. Table 22. I2C data packet for activating the READY mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 80 1 0 0 0 0 0 0 0 Data of the command register Writing the following 3 bytes to STCF03 will activate the Auxiliary LED for the desired time. Table 23. 8.4 I2C data packet for activating the AUX_LED Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 02 0 0 0 0 0 0 1 0 Auxiliary LED register address 3 85 1 0 0 0 0 1 0 1 Data of the auxiliary LED register Example 4: Red-eye reduction (multiple short flashes) There are two ways to manage this task. The first one is to use hardware triggering of the flashes through the TRIG pin. This is the most suitable and recommended solution, as it reduces the usage of the I2C bus and the length of each FLASH pulse can be adjusted continuously. The second solution is to use the software triggering feature, which means a periodical reloading of the COMMAND register. This however increases traffic on the I2C bus and the flashes can only have length, adjustable in 100 ms increments only. Let's suppose that RFL = 0.27 Ω and the targeted value of the FLASH current is 600 mA. The task is to make 5 flashes of 200 ms duration with 100 ms pause between them. The setting of the reference voltage is identical to the one in Section 8.1. The FLASH timer (FTIM) is set to 0xF, which represents 1.5 s. Table 24. CMD_REG COMMAND register data for FLASH mode PWR_ON TRIG_EN TCH_ON NTC_ON FTIM_3 FTIM_2 FTIM_1 FTIM_0 1 1 0 1 1 1 1 1 MSB Table 25. DIM_REG LSB DIMMING register data for the FLASH mode TDIM_3 TDIM_2 TDIM_1 TDIM_0 FDIM_3 FDIM_2 FDIM_1 FDIM_0 0 0 0 0 1 1 0 1 MSB LSB 27/31 Examples of register setup for each mode AN2507 The data packet which has to be sent is in the table below. Table 26. I2C data packet for activating the FLASH mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 DF 1 1 0 1 1 1 1 1 Data of the command register 4 0D 0 0 0 0 1 1 0 1 Data of the dimming register The picture below shows the TRIG pin and the I2C bus timings. Figure 21. Multiple flashes handled by the TRIG pin 2 I C bus packet TRIG_EN bit TRIG pin 200 ms 100 ms 1400 ms 8.5 Example 5: A FLASH pulse longer than 1.5 s Let's suppose that RFL = 0.27 Ω and the targeted value of the FLASH current is 600 mA. The task is to make a single FLASH pulse with a 4 seconds duration. It is necessary to reload FTIM in the COMMAND REGISTER before the internal FLASH timer reaches zero. This guarantees that the FLASH does go on and does not stop after 1.5 sec. The first packet must contain also the DIMMING REGISTER data, if they are different from those which were used in the previous operation. ● Packet 1 Sets FLASH mode with 1.5 s duration and the proper dimming. Table 27. 28/31 I2C data packet for activating the FLASH mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 DF 1 1 0 1 1 1 1 1 Data of the command register 4 0D 0 0 0 0 1 1 0 1 Data of the dimming register AN2507 Examples of register setup for each mode ● Packet 2 Sets FLASH mode with 1.5 s duration. Dimming is not set again as it is same as before. Table 28. 1st I2C data packet to restart the FLASH mode Byte Hex 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 DF 1 1 0 1 1 1 1 1 Data of the command register ● Binary Comment Packet 3 Sets FLASH mode with 1.5 s duration. Dimming remains untouched. Table 29. 2nd I2C data packet for restart of the FLASH mode Byte Hex 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 DF 1 1 0 1 1 1 1 1 Data of the command register ● Binary Comment Packet 4 Sets FLASH mode with 1 s duration. Dimming remains untouched. Table 30. 3rd I2C data packet to restart the FLASH mode Byte Hex Binary Comment 1 62 0 1 1 0 0 0 1 0 Device address + R/W bit 2 00 0 0 0 0 0 0 0 0 Command register address 3 DA 1 1 0 1 1 0 1 0 Data of the command register Please refer to Figure 22 for more details about the I2C-bus packets timing. The solution described above is using a software termination of the FLASH pulse. (It is timed out by the internal timer.) The FLASH pulse could be also terminated by setting the TRIG pin to low after 4 seconds. In this case, the fourth packet could be the same as packets Packet 2 and Packet 3, because the timing of the FLASH is done by the TRIG pin and it is not necessary to change the value of FTIM in the COMMAND REGISTER. This way of periodical reloading of the COMMAND REGISTER can be used to achieve a continuous FLASH light. In this case, it is very strongly recommended to guarantee an efficient cooling of both the LED and the chip, otherwise the light can be interrupted by activation of the thermal protections. 29/31 Revision history AN2507 Figure 22. I2C bus packets timing for a FLASH lasting longer than FTIM max Timeout of the first Flash Timeout of the second Flash 1.5s Timeout of the third Flash 1.5s Timeout of the fourth Flash – ending of the whole Flash pulse 1.5s 1.0s 1.0s 1.0s 1.0s 2 I C bus packets TRIG_EN bit TRIG pin 4.0s 9 Revision history Table 31. 30/31 Revision history Date Revision 19-Apr-2007 1 Changes Initial release AN2507 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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