ST AN2809 Application note

AN2809

Application note

6-row / 30-mA LED driver with boost converter for the backlight of LCD panels

Introduction

The LED7706 consists of a high-efficiency monolithic boost converter and six controlled current generators (rows) specifically designed to supply the LED arrays used in the backlights of LCD panels. The device can manage an output voltage of up to 36 V (ten white LEDs per row).

The generators can be externally-programmed to sink up to 30 mA and can be dimmed via a PWM signal (a 1% dimming duty-cycle at 20 kHz can be managed). The device detects and manages the open and shorted LED faults and leaves unused rows floating. Basic protections (output over-voltage, internal MOSFET over-current and thermal shutdown) are provided.

Figure 1. LED7706 demonstration board

February 2009

Rev 2

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Contents	AN2809

Contents

1	LED7706 main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 5
	1.1	Boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
	1.2	Backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	LED7706 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		6
3	Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		7
4	Components assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		8
5	I/O interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		9
6	Recomended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
7	Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10

7.1 SW1 fixed or adjustable switching frequency (FSW pin) . . . . . . . . . . . . . 10 7.2 SW2 fault management mode (MODE pin) . . . . . . . . . . . . . . . . . . . . . . . 11 7.3 SW3 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

8	Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		12
9	Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		13
	9.1	Quick startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	9.2	Open and shorted WLEDs fault testing . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	9.3	Device synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	9.4	Efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
10	WLEDs test board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		18
11	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		21

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AN2809	List of figures

List of figures

Figure 1. LED7706 demonstration board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Figure 2. LED7706 board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 3. Top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Figure 4. Bottom side test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Figure 5. SW1 (FSW) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Figure 6. SW2 (MODE) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 7. LED7706 demonstration board and white LEDs test board assembly . . . . . . . . . . . . . . . . 12 Figure 8. LED7706 board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 9. LED7706 synchronization setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Figure 10. Efficiency measurements setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 11. Efficiency vs DIM duty cycle, VIN=12V, 10 white LEDs in series, IOUT = 120mA . . . . . . . . 17 Figure 12. Efficiency vs DIM duty cycle, VIN=24V, 10 white LEDs in series, IOUT = 120mA . . . . . . . . 17 Figure 13. WLEDs test board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Figure 14. WLEDs test board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

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List of tables	AN2809

List of tables

Table 1. LED7706 performance summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Table 2. LED7706 component list. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Table 3. LED7706 demonstration board test points description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Table 4. Faults management summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Table 5. Test board jumpers function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 6. Test boards switches functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 7. Test board test-points function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Table 8. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

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AN2809	LED7706 - main features

1 LED7706 - main features

1.1Boost section

●4.5- to 36-V input voltage range

●Internal power MOSFET

●Internal +5 V LDO for device supply

●Up to 36-V output voltage

●Constant frequency peak current-mode control

●200-kHz to 1-MHz adjustable switching frequency

●External sync for multi-device application

●Pulse-skip power-saving mode at light loads

●Programmable soft-start

●Programmable OVP protection

●Single ceramic output capacitor

●Non-latched thermal shutdown

1.2Backlight driver section

●Six rows with 30 mA maximum current capability (adjustable)

●Up to 10 white LEDs per row

●Row disabling option

●Less than 500 ns minimum dimming time (1% minimum dimming duty-cycle at 20 kHz dimming frequency)

●±2.0% current matching between rows

●LED failure (open and short circuit) detection

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LED7706 demonstration board	AN2809

2 LED7706 demonstration board

The LED7706 demonstration board has been designed to manage six strings of 8 to 10 white LEDs each.

Table 1 summarizes the board features and Figure 2 shows the schematic of the LED7706 demonstration board. The input voltage range is limited to 32 V because of the 35 V rated input capacitor. Extended operating input voltage ranges (up to 36 V) can be achieved by using a 50-V rated MLCC.

	Table 1.	LED7706 performance summary
		Parameter	Conditions	Value
	Minimum input voltage			4.5 V
	Maximum input voltage			32 V
		Output voltage		VIN<VBOOST<36 V
	Output OVP threshold		R1 = 510 kΩ, R2 = 16 kΩ	38 V
	Internal MOSFET OCP		R7 = 180 kΩ	3.3 A
	Boost section switching frequency		FSW pin to AVCC	660 kHz
			FSW pin to R5 = 330 kΩ	825 kHz
	Minimum dimming on-time		400 Hz < FDIM < 20 kHz	500 ns
	Output current (each row)		R6=51 kΩ	19.6 mA
	Output current accuracy			±2.0%
		Efficiency	VIN = 12 V, VBOOST = 34 V,	91%
			FSW = 660 kHz

Figure 2. LED7706 board schematic

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AN2809	Component list

3 Component list

Table 2.		LED7706 component list
Qty	Component		Description	Package	Part number	MFR	Value
1		C1	Ceramic, 35V, X5R, 20%	SMD 1210	UMK325BJ106KM-T	Taiyo	10µF
						Yuden
1		C2	Ceramic, 50V, X7R, 20%	SMD 1206	GRM31CR71H475KA88B	MURATA	4.7µF
2		C3, C4	Ceramic, 50V, X7R, 20%	SMD 1206			N.M.
1		C5	Ceramic, 25V, X5R, 20%	SMD 0603		Standard	1µF
1		C6	Ceramic, 25V, X5R, 20%	SMD 0603		Standard	100nF
1		C7	Ceramic, 25V, X5R, 20%	SMD 0603		Standard	3.3nF
1		C8	Ceramic, 25V, X5R, 20%	SMD 0603		Standard	4.7nF
1		C9	Ceramic, 25V, X5R, 20%	SMD 0603		Standard	N.M.
1		C10	Ceramic, 25V, X5R, 20%	SMD 0402		Standard	220pF
1		C11	Ceramic, 25V, X5R, 20%	SMD 0603		Standard	4.7nF
1		R1	Chip resistor, 0.1W, 1%	SMD 0603		Standard	510kΩ
1		R2	Chip resistor, 0.1W, 1%	SMD 0603		Standard	16kΩ
1		R3	Chip resistor, 0.1W, 1%	SMD 0603		Standard	2.4kΩ
1		R4	Chip resistor, 0.1W, 1%	SMD 0603		Standard	4.7Ω
1		R5	Chip resistor, 0.1W, 1%	SMD 0603		Standard	330kΩ
1		R6	Chip resistor, 0.1W, 1%	SMD 0603		Standard	51kΩ
1		R7	Chip resistor, 0.1W, 1%	SMD 0603		Standard	180kΩ
1		R8	Chip resistor, 0.1W, 1%	SMD 0603		Standard	680kΩ
2		R9, R10	Chip resistor, 0.1W, 1%	SMD 0603		Standard	100kΩ
1		R11	Chip resistor, 0.1W, 1%	SMD 0603		Standard	1.2kΩ
1		L1	6µ8, 75mH, 2.7A	6x6mm	LPS6235-682MLC	Coilcraft	6.8µH
1		D1	Schottky, 40V, 1A	DO216-AA	STPS1L40M	ST	STPS1L40M
1		D2	Red LED, 3mA	SMD 0603		Standard
1		U1	Integrated circuit	QFN4x4	LED7706	ST	LED7706
1		J2	PCB pad jumper
1		J8	Header 8	SIL 8		Standard
1	SW1, SW2		Jumper 3	SIL 3		Standard
1		SW3	Pushbutton	6x6mm	FSM4JSMAT	TYCO

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