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
May 2009
Doc ID 14893 Rev 1
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www.st.com
Contents
AN2810
Contents
1
LED7707 main features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1
1.2
Boost section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Backlight driver section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2
3
4
5
6
7
LED7707 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Component list . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Component assembly and layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
I/O interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Recommended equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1
7.2
7.3
SW1 fixed or adjustable switching frequency (FSW pin) . . . . . . . . . . . . . . 8
SW2 fault management mode (MODE pin) . . . . . . . . . . . . . . . . . . . . . . . . 9
SW3 enable function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8
9
Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Getting started . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1
9.2
9.3
9.4
Quick startup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Open and shorted WLEDs fault testing . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Device synchronization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Efficiency measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
10
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
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Doc ID 14893 Rev 1
AN2810
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.
LED7707 demonstration board . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
LED7707 demonstration board schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Top side component placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Bottom side test points . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
SW1 (F
SW
) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
SW2 (MODE) setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
LED7707 demonstration board and white LED test board assembly . . . . . . . . . . . . . . . . . 11
LED7707 demonstration board test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
LED7707 synchronization setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Efficiency measurement setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Efficiency vs. DIM duty cycle, V
IN
=12 V, 6 rows, 10 white LEDs in series, I
OUT
= 360 mA 15
Efficiency vs. DIM duty cycle, V
IN
=24 V, 6 rows, 10 white LEDs in series, I
OUT
= 360 mA 15
Doc ID 14893 Rev 1
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LED7707 main features
AN2810
1
1.1
LED7707 main features
Boost section
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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
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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
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Doc ID 14893 Rev 1
AN2810
LED7707 demonstration board
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.
Table 1.
LED7707 performance summary
Parameter
Minimum input voltage
Maximum input voltage
Output voltage
Output OVP threshold
Internal MOSFET OCP
Boost section switching frequency
FSW pin to R
5
=330 kΩ
Minimum dimming on-time
Output current (each row)
R
6
=24 kΩ
Output current accuracy
Efficiency
V
IN
=12V , V
BOOST
=34 V,
F
SW
=660 kHz
±2%
91%
400 Hz<F
DIM
<20 kHz
825 kHz
10 µs
75 mA
R
1
=510 kΩ, R2=16 kΩ
R
7
=360 kΩ
FSW pin to AVCC
Conditions
Value
6V
32 V
V
IN
<V
BOOST
<36 V
38 V
3.3 A
660 kHz
Figure 2.
LED7707 demonstration board schematic
Doc ID 14893 Rev 1
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