ST AN2507 Application note

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

Version A

April 2007

Rev 1

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Contents	AN2507

Contents

1	Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 6
	1.1	Application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	6
2	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
3	PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10

3.1 PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.2 PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3.2.1A four-layer PCB with application area 45.1 mm2 for BGA package, version B 10

3.2.2A two-layer PCB with application area 72.4 mm2 for QFN package . . . 13

3.2.3A four-layer PCB with application area 45.1 mm2 for BGA package, version C 14

4	Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		16
	4.1	Accessing the internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
5	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

6	The STATUS register and the ATN pin . . . . . . . . . . . . . . . . . . . . . . . .	. . 21
	6.1 The STATUS register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . 21
7	Reading and writing to the STCF03 registers through the I2C bus	. . 22
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AN2507	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

8	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
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		30

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List of tables	AN2507

List of tables

Table 1. Recommended components. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 2. Accessibility of internal registers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Table 3. COMMAND register data to enter SHUTDOWN mode (version B) . . . . . . . . . . . . . . . . . . 17 Table 4. COMMAND register data to enter SHUTDOWN mode (version A and C) . . . . . . . . . . . . . 17 Table 5. COMMAND register data to enter SHUTDOWN mode with NTC activated (version A and C)

Table 6. COMMAND register data to enter READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Table 7. COMMAND register data to enter READY mode with NTC ON . . . . . . . . . . . . . . . . . . . . . 18 Table 8. COMMAND register data to enter TORCH mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 9. COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 Table 10. STATUS register bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Table 11. Effect of the STATUS register bits on the operation of the device . . . . . . . . . . . . . . . . . . . 21 Table 12. TORCH mode and FLASH mode dimming registers settings. . . . . . . . . . . . . . . . . . . . . . . 24 Table 13. COMMAND register data to enter FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Table 14. DIMMING register data for the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Table 15. I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Table 16. COMMAND register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Table 17. DIMMING register data for the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Table 18. I2C data packet to activate TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 19. I2C data packet for terminating the TORCH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Table 20. COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Table 21. COMMAND register data for the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 Table 22. I2C data packet for activating the READY mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Table 23. I2C data packet for activating the AUX_LED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Table 24. COMMAND register data for FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Table 25. DIMMING register data for the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Table 26. I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Table 27. I2C data packet for activating the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Table 28. 1st I2C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Table 29. 2nd I2C data packet for restart of the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 Table 30. 3rd I2C data packet to restart the FLASH mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Table 31. Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

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AN2507	List of figures

List of figures

Figure 1. A typical application schematic. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 2. NTC connection for versions with internal voltage reference . . . . . . . . . . . . . . . . . . . . . . . . 9 Figure 3. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 4. Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 5. Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Figure 6. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 7. Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 8. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 9. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 10. Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 11. Top layer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 Figure 12. Middle layer 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 13. Middle layer 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 14. Bottom layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 15. Top overlay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 16. Splitting the FLASH pulse into several shorter pulses . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Figure 17. Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 18. Writing to multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Figure 19. Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 20. Reading from multiple registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 Figure 21. Multiple flashes handled by the TRIG pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Figure 22. I2C bus packets timing for a FLASH lasting longer than FTIM max . . . . . . . . . . . . . . . . . . 30

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Schematic description	AN2507

1 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.

1.1Application 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
	● 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

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AN2507	Selection of external components

2 Selection of external components

2.1Input 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

OUT

– V

) • V

•

OUT

IPEAK

-----------

--------------

----------------------------------------------------

2 • L • F •

VIN

OUT

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.

2.3LED 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.

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Selection of external components	AN2507

2.4RFL 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.

2.5RTR selection

The value of the RTR resistor can be calculated by following equations:

Equation 2

RTR	=	VREF – ITORCH • RFL		and	2
			------------------------------------------------------- I TORCH -		PRTORCH = RTR • ITORCH

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

2.6NTC 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.

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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
STCF03
RX	To optional A/D converter
	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

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PCB design	AN2507

3 PCB design

3.1PCB 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.

3.2PCB layout

3.2.1A 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. Top layer

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