ST AN2827 Application note

AN2827

Application note

Driver for double flash LED with I²C interface

Introduction

This application note is dedicated to the design of a flash LED driver using the STCF05 device, which is a boost current mode converter with an I²C interface and internal current source. The schematic, functional description, recommendations for PCB layout and external components selection are also discussed in this application note. The STCF05 device is designed to drive two LEDs in series with a total forward voltage from 5.3 V to 10.2 V.

Figure 1. Demonstration board STCF05 v3: optimized for smallest PCB area (24 mm²)

Figure 2. Demonstration board STCF05 v2: optimized for best efficiency

June 2009

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Contents	AN2827

Contents

1	Schematic description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		4
2	Selection of external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	2.1	Input and output capacitor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	2.2	Inductor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	2.3	LED selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	2.4	NTC and RX resistor selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
3	PCB design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
	3.1	PCB design rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	7
	3.2	PCB layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	8

3.2.1Four-layer PCB with 33.54 mm² application area using VLF4014AT-1R0N2R2 coil 8

3.2.2Four-layer PCB with 23.9 mm² application area using VLS252012T-1R0N1R7 coil 11

4	Internal registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		14
5	Operation modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		15
	5.1	SHUTDOWN mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	5.2	SHUTDOWN mode with NTC feature activated . . . . . . . . . . . . . . . . . . . .	15
	5.3	READY mode NTC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	5.4	Torch mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	16
	5.5	Flash mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	17
6	STATUS register and ATN pin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		19
7	Reading and writing to the STCF05 registers through the I²C bus . . .		20
	7.1	Writing to a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20
	7.2	Writing to multiple registers with incremental addressing . . . . . . . . . . . .	20
	7.3	Reading from a single register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	21
	7.4	Reading from multiple registers with incremental addressing . . . . . . . . .	22

8	Examples of register setup for each mode . . . . . . . . . . . . . . . . . . . . . .	23
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AN2827			Contents
	8.1	Example 1: 400 mA flash with 700 ms duration . . . . . . . . . . . . . . .	. . . . . 23
	8.2	Example 2: 15 mA torch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . . . 24
	8.3	Example 3: auxiliary LED running at 10 mA for 500 ms . . . . . . . . . .	. . . . 25
	8.4	Example 4: red-eye reduction (multiple short flashes) . . . . . . . . . . .	. . . . 25
	8.5	Example 5: flash pulse longer than 1.5 s . . . . . . . . . . . . . . . . . . . . .	. . . . 27
9	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. . . . 29

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Schematic description	AN2827

1 Schematic description

The STCF05 flash LED driver has a high operational frequency (1.8 MHz) which allows the use of small-sized external components.

Figure 3. Typical application schematic

**: Connect to VI, GND, SDA or SCL to choose one of the four different I²C slave addresses.

***: Optional components to support auxiliary functions.

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AN2827	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 also recommended to use 10 µF/6.3 V as a minimum value for the input capacitor, and
	1 µF/16 V as the optimal value for the output capacitor to achieve a good stability of the
	device, for a supply range varying from a low input voltage (2.5 V) to the 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, we recommend using 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 formula:
	IPEAK = [(IOUT / η) + (VOUT - VIN) x VIN²) / (2 x L x F x VOUT²)] x VOUT / VIN
	Where:
	IPEAK	Peak inductor current
	IOUT	Current sourced at the VOUT-pin
	η	Efficiency of the STCF05
	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
	Any string of LEDs with a cumulative forward voltage ranging from 5.3 to 10.2 V is
	compatible with the STCF05. The total LED spread must be taken into account when
	calculating the minimum and maximum voltage of the LEDs that must be inside the
	5.3 –10.2 voltage range. It is possible to set the level of the LED current to flash mode and
	torch mode by setting the corresponding registers through the I²C interface.
Note:	See recommended components in Table 1.
2.4	NTC and RX resistor selection
	Optionally, the STCF05 uses a negative thermistor (NTC) to sense the LED temperature, as
	well as an RX resistor and an external voltage reference in order to use the NTC feature.
	Refer to Figure 3: Typical application schematic for more details.
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Selection of external components	AN2827

Once the NTC feature is activated through the I²C, the internal switch connects the RX resistor to the NTC; this creates a voltage divider supplied by the external reference voltage connected to the NTC.

If the temperature of the NTC thermistor rises as a result of 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 switches automatically to 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 resistor values strongly depends on the power dissipated by the LED and all the components surrounding the NTC thermistor, and on the cooling capabilities of each specific application. The RX and NTC values in Table 1 work well for the demonstration board presented in this application note. A real-life application may require a different type of NTC thermistor to achieve optimal thermal protection.

The procedure to activate the NTC feature is described in Section 5.2: SHUTDOWN mode with NTC feature activated.

Table 1.	List of components
Component		Manufacturer	Part number	Value	Size
	CI	TDK	C1608X5R0J106M	10 µF	0603
	CO	TDK	C1608X5R1A105M	1 µF	0603
	L	TDK	VLF4014AT-1R0N2R2 (1)	1 µH	3.7 x 3.5 x 1.2 mm
			VLS252012T-1R0N1R7 (2)	1 µH	2.5 x 2 x 1.2 mm
	NTC	Murata	NCP21WF104J03RA	100 kΩ	0805
	RX	Rohm	MCR01MZPJ15K	15 kΩ	0402
	LED	Luxeon LED	2x LXCL-PWF1		0805
AUXLED		Rohm	SML-210VT		0805

1.Used in v2 version to achieve best efficiency.

2.Used in v3 version, when low application area down to 23.9 mm² is preferred.

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AN2827	PCB design

3 PCB design

3.1PCB design rules

The STCF05 is a powerful switching device that operates with low input voltages and a high duty cycle. The PCB must be designed in line with switched mode power supply design rules. The power tracks (or wires on the demonstration board) must be as short as possible and wide enough, because of the large currents involved. It is recommended to use a 4-layer PCB to get the best performance. All external components must be placed as close as possible to STCF05. All high-energy switched loops should be as small as possible to reduce EMI.

Most of the LEDs need to be cooled efficiently. This can be achieved by using a dedicated copper area on the PCB. Refer to the selected LED's reference guide to design the proper heatsink. In case a modification of any PCB layer should be required, it is highly recommended to use enough vias. Place the NTC resistor as close as possible to the LED for good temperature sensing. A direct connection between GND and PGND is necessary to achieve a correct output current value. No LED current should flow through this track. Vias connecting the STCF05 pins to the copper tracks (if used) must be 0.1 mm in diameter. It is recommended to use the filled vias.

It is possible to route the STCF05 device with a total PCB area of 23.9 mm² using the TDK inductor VLS252012 1 µH value. When using the VLF4014AT, the application area is increased by 9.6 mm², and the efficiency of the application is improved up to 85%.

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PCB design	AN2827

3.2PCB layout

3.2.1Four-layer PCB with 33.54 mm² application area using VLF4014AT-1R0N2R2 coil

Figure 4. Top layer

Figure 5. Middle layer 1

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AN2827	PCB design
Figure 6.	Middle layer 2

Figure 7. Bottom layer

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