STMicroelectronics LED7707 Installation and operating instructions

May 2009 Doc ID 14893 Rev 1 1/17

AN2810

Application note

6-row 85 mA LED driver with boost

converter for LCD panel backlighting

Introduction

The LED7707 LED driver from STMicroelectronics consists of a high-efficiency monolithic

boost converter and six controlled current generators (rows), specifically designed to supply

LED arrays used in the backlighting of LCD panels. The device can manage an output

voltage up to 36 V (i.e. ten white-LEDs per row). The generators can be externally

programmed to sink up to 85 mA and can be dimmed via a PWM signal (1% of minimum

dimming duty-cycle at 1 kHz can be managed). The device allows detection and

management of open and shorted LED faults, and permits unused rows to be left floating.

Basic protections (output overvoltage, internal MOSFET overcurrent and thermal shutdown)

are provided.

Figure 1. LED7707 demonstration board

www.st.com

Contents AN2810

2/17 Doc ID 14893 Rev 1

Contents

1 LED7707 main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2 Backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2 LED7707 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

3 Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

4 Component assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

5 I/O interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

6 Recommended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

7 Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

7.1 SW1 fixed or adjustable switching frequency (FSW pin) . . . . . . . . . . . . . . 8

7.2 SW2 fault management mode (MODE pin) . . . . . . . . . . . . . . . . . . . . . . . . 9

7.3 SW3 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

8 Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

9 Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9.1 Quick startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

9.2 Open and shorted WLEDs fault testing . . . . . . . . . . . . . . . . . . . . . . . . . . 13

9.3 Device synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

9.4 Efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

10 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

AN2810 List of figures

Doc ID 14893 Rev 1 3/17

List of figures

Figure 1. LED7707 demonstration board. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Figure 2. LED7707 demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Figure 3. Top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 4. Bottom side test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Figure 5. SW1 (FSW) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 6. SW2 (MODE) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Figure 7. LED7707 demonstration board and white LED test board assembly . . . . . . . . . . . . . . . . . 11

Figure 8. LED7707 demonstration board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 9. LED7707 synchronization setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Figure 10. Efficiency measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Figure 11. Efficiency vs. DIM duty cycle, VIN=12 V, 6 rows, 10 white LEDs in series, IOUT = 360 mA 15

Figure 12. Efficiency vs. DIM duty cycle, VIN=24 V, 6 rows, 10 white LEDs in series, IOUT = 360 mA 15

LED7707 main features AN2810

4/17 Doc ID 14893 Rev 1

1 LED7707 main features

1.1 Boost section

●4.5 V 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

●250 kHz to 1 MHz adjustable switching frequency

●External sync for multi-device application

●Pulse-skip power saving mode at light load

●Programmable soft-start

●Programmable OVP (overvoltage protection)

●Single ceramic output capacitor

●Non-latched thermal shutdown

1.2 Backlight driver section

●Six rows with 85 mA maximum current capability (adjustable)

●Up to 10 WLEDs per row

●Parallelable rows for higher current

●Row disable option

●Less than 10 µs minimum dimming time

●±2% current matching between rows

●LED failure (open and short-circuit) detection

AN2810 LED7707 demonstration board

Doc ID 14893 Rev 1 5/17

2 LED7707 demonstration board

The LED7707 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 LED7707 demonstration

board schematic. The input voltage range is limited to 32 V because of the 35 V rated input

capacitor. An extended operating input voltage range (up to 36 V) can be achieved by using

a 50 V-rated MLCC.

Figure 2. LED7707 demonstration board schematic

Table 1. LED7707 performance summary

Parameter Conditions Value

Minimum input voltage 6 V

Maximum input voltage 32 V

Output voltage VIN<VBOOST<36 V

Output OVP threshold R1=510 kΩ, R2=16 kΩ38 V

Internal MOSFET OCP R7=360 kΩ3.3 A

Boost section switching frequency FSW pin to AVCC 660 kHz

FSW pin to R5=330 kΩ825 kHz

Minimum dimming on-time 400 Hz<FDIM<20 kHz 10 µs

Output current (each row) R6=24 kΩ75 mA

Output current accuracy ±2%

Efficiency VIN=12V , VBOOST=34 V,

FSW=660 kHz 91%

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Component list AN2810

6/17 Doc ID 14893 Rev 1

3 Component list

Table 2. LED7706 demonstration board component list

Qty Componen

tDescription Package Part-number MFR Value

1 C1 Ceramic, 35 V, X5R, 20% SMD 1210 UMK325BJ106KM-T Taiyo Yuden 10 µF

2 C2,C3 Ceramic, 50 V, X7R, 20% SMD 1206 GRM31CR71H475KA88B Murata 4.7 µF

1 C4 Ceramic, 50 V, X7R, 20% SMD 1206 N.M.

1 C5 Ceramic, 25 V, X5R, 20% SMD 0603 Standard 1 µF

1 C6 Ceramic, 25V , X5R, 20% SMD 0603 Standard 100 nF

1 C7 Ceramic, 25 V, X5R, 20% SMD 0603 Standard 3.3 nF

1 C8 Ceramic, 25 V, X5R, 20% SMD 0603 Standard 4.7 nF

1 C9 Ceramic, 25 V, X5R, 20% SMD 0603 Standard N.M.

1 C10 Ceramic, 25 V, X5R, 20% SMD 0402 Standard 220 pF

1 C11 Ceramic, 25 V, X5R, 20% SMD 0603 Standard 4.7 nF

1 C12 Ceramic, 25 V, X5R, 20% SMD 0603 Standard N.M.

1 C13 Ceramic, 25 V, X5R, 20% SMD 0603 Standard N.M.

1 R1 Chip resistor, 0.1 W, 1% SMD 0603 Standard 510 kΩ

1 R2 Chip resistor, 0.1 W, 1% SMD 0603 Standard 16 kΩ

1 R3 Chip resistor, 0.1 W, 1% SMD 0603 Standard 2.4 kΩ

1 R4 Chip resistor, 0.1 W, 1% SMD 0603 Standard 4.7 Ω

1 R5 Chip resistor, 0.1 W, 1% SMD 0603 Standard 330 Ω

1 R6 Chip resistor, 0.1 W, 1% SMD 0603 Standard 24 kΩ

1 R7 Chip resistor, 0.1 W, 1% SMD 0603 Standard 360 kΩ

1 R8 Chip resistor, 0.1 W, 1% SMD 0603 Standard 680 kΩ

2 R9, R10 Chip resistor, 0.1 W, 1% SMD 0603 Standard 100 kΩ

1 R11 Chip resistor, 0.1 W, 1% SMD 0603 Standard 1.2 kΩ

1 R12 Chip resistor, 0.1 W, 1% SMD 0603 Standard N.M.

1 R13 Chip resistor, 0.1 W, 1% SMD 0603 Standard N.M.

1 L1 68 µH, 75 mH, 5.8 A 7x7mm XPL7030-682ML Coilcraft 6.8 µH

1 D1 Schottky, 40 V, 1 A DO216-AA STPS1L40M ST STPS1L40M

1 D2 Red LED, 3 mA SMD 0603 Standard

1 D3 Signal Schottky SOD-523 BAS69 ST N.M.

1 U1 Integrated circuit QFN4x4 LED7707 ST LED7707

1 J2 PCB pad jumper

AN2810 Component assembly and layout

Doc ID 14893 Rev 1 7/17

4 Component assembly and layout

Figure 3. Top side component placement

Figure 4. Bottom side test points

Qty Componen

tDescription Package Part-number MFR Value

1 J8 Header 8 SIL 8 Standard

1 SW1, SW2 Jumper 3 SIL 3 Standard

1 SW3 Pushbutton 6x6mm FSM4JSMAT TYCO

Table 2. LED7706 demonstration board component list (continued)

I/O interface AN2810

8/17 Doc ID 14893 Rev 1

5 I/O interface

The LED7707 demonstration board is equipped with the test points described in Ta bl e 3.

6 Recommended equipment

●4.5 V to 32 V, 2 A capable power supply

●Digital multi-meters

●20 MHz oscilloscope

●Signal generators for PWM dimming and synchronization clock (optional)

7 Configuration

The LED7707 demonstration board allows the user to choose the desired mode of operation

using the SW1 and SW2 selectors (refer to the configuration description in the following

paragraphs). A red LED is connected to the FAULT pin to easily monitor its status; if this

option is not required, the monitor LED can be disconnected opening the J2 jumper.

7.1 SW1 fixed or adjustable switching frequency (FSW pin)

The SW1 selector is used to choose between the fixed switching frequency (660 kHz) and a

user-defined switching frequency in the range 250 kHz to 1 MHz (see Figure 5). When

placed in the “down” position, the fixed switching frequency is selected.

Table 3. LED7707 demonstration board test points description

Test point Description

VIN+ Input voltage, positive terminal

VIN- Input voltage, negative terminal

GND Reference ground

ROW1 to ROW6 Current generator output

VBOOST Boost regulator output voltage

DIM PWM dimming input

EN Enable input (active high)

SYNC Synchronization output

FSW Synchronization input

FAULT Fault signal, active low

AN2810 Configuration

Doc ID 14893 Rev 1 9/17

If SW1 is in the “up” position, the switching frequency is given by:

Equation 1

Figure 5. SW1 (FSW) setting

The R5 resistor is set to 330 kΩ(FSW = 825 kHz) and can be changed by the user.

7.2 SW2 fault management mode (MODE pin)

The SW2 selector is used to connect the MODE to AVCC or ground. When the jumper is set

to the upper position, the MODE pin is connected to ground and the corresponding fault

management is summarized in the first column of Ta bl e 4 .

Otherwise, when SW2 is set to the “down” position, the MODE pin is connected to AVCC

and the corresponding fault management is summarized in the second column of Ta bl e 4 .

Figure 6. SW2 (MODE) setting

FSW 2.5 R5

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Configuration AN2810

10/17 Doc ID 14893 Rev 1

7.3 SW3 enable function

The terminals of switch SW3 are connected on one side to the EN pin and on the other side

to ground. Therefore, when the switch is not pressed, the EN pin is floating, which implies

that the device is working. Pressing the SW3 pin connects the EN pin to ground. When SW3

is released, the device re-starts (a soft-start is performed). The SW3 switch can be activated

whenever a new startup is required or to escape a latched condition.

Table 4. Fault management summary

Fault MODE to GND MODE to VCC

Internal MOSFET

overcurrent

Fault pin HIGH

Power MOSFET turned off

Output overvoltage FAULT pin LOW

Power MOSFET turned off (hysteretic regulation)

Thermal shutdown FAULT pin LOW. Device turned off.

Automatic restart after 30 °C temperature drop.

LED short-circuit

Fault pin LOW

Device turned OFF (100 µA

masking time), latched

condition (Vth=4.0 V)

Open row(s)

Fault pin LOW

Device turned off at first

occurrence, latched condition

FAULT pin HIGH

Faulty row(s) disconnected

(100 µs masking time)

Table of contents

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