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LED7707 Schematic ( PDF Datasheet ) - STMicroelectronics

Teilenummer LED7707
Beschreibung 6-rows 85 mA LEDs driver
Hersteller STMicroelectronics
Logo STMicroelectronics Logo 




Gesamt 30 Seiten
LED7707 Datasheet, Funktion
LED7707
6-rows 85 mA LEDs driver with boost regulator
for LCD panels backlight
Features
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 synchronization for multi-device
application
– Pulse-skip power saving mode at light load
– Programmable soft-start
– Programmable OVP protection
– Stable with ceramic output capacitors
– Thermal shutdown
Backlight driver section
– Six rows with 85 mA maximum current
capability (adjustable)
– Rows disable option
– Less than 10 μs minimum dimming time
– ±2 % current matching between rows
– LED failure (open and short-circuit)
detection
Applications
LCD monitors and TV panels
PDAs panel backlight
GPS panel backlight
VFQFPN-24 4x4
Description
The LED7707 consists of a high efficiency
monolithic boost converter and six controlled
current generators (rows) specifically designed to
supply LEDs arrays used in the backlighting of
LCD panels. The device can manage an output
voltage up to 36 V (i.e. 10 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 % dimming duty-cycle at 1 kHz can be
managed). The device allows to detect and
manage the open and shorted LED faults and to
let unused rows floating. Basic protections (output
over-voltage, internal MOSFET over-current and
thermal shutdown) are provided.
Table 1.
Device summary
Order codes
LED7707
LED7707TR
Package
VFQFPN-24 4x4 (exposed pad)
Packaging
Tube
Tape and reel
September 2008
Rev 1
1/48
www.st.com
48
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LED7707 Datasheet, Funktion
Pin settings
LED7707
2.2
Pin description
Table 2. Pin functions
N° Pin
Function
1
COMP
Error amplifier output. A simple RC series between this pin and ground is
needed to compensate the loop of the boost regulator.
2
RILIM
Output generators current limit setting. The output current of the rows can be
programmed connecting a resistor to SGND.
3
BILIM
Boost converter current limit setting. The internal MOSFET current limit can
be programmed connecting a resistor to SGND.
Switching frequency selection and external sync input. A resistor to SGND is
4 FSW used to set the desired switching frequency. The pin can also be used as
external synchronization input. See Section 5.1.5 on page 16 for details.
5
MODE
Current generators fault management selector. It allows to detect and manage
LEDs failures. See Section 5.3.2 on page 23 for details.
6 AVCC + 5 V analog supply. Connect to LDO5 through a simple RC filter.
7
LDO5
+ 5V LDO output and power section supply. Bypass to SGND with a
1 µF ceramic capacitor.
8 VIN Input voltage. Connect to the main supply rail.
Slope compensation setting. A resistor between the output of the boost
9 SLOPE converter and this pin is needed to avoid sub-harmonic instability.
Refer to Section 6.1 on page 26 for details.
10
SGND
Signal ground. Supply return for the analog circuitry and the current
generators.
11 ROW1 Row driver output #1.
12 ROW2 Row driver output #2.
13 ROW3 Row driver output #3.
14 ROW4 Row driver output #4.
15 ROW5 Row driver output #5.
16 ROW6 Row driver output #6.
17 PGND Power ground. Source of the internal Power MOSFET.
18
OVSEL
Over-voltage selection. Used to set the desired 0 V threshold by an external
divider. See Section 5.1.4 on page 15 for details.
19 LX Switching node. Drain of the internal Power MOSFET.
20 DIM Dimming input. Used to externally set the brightness by using a PWM signal.
21
EN
Enable input. When low, the device is turned off. If tied high or left open, the
device is turned on and a soft-start sequence takes place.
22
FAULT
Fault signal output. Open drain output. The pin goes low when a fault condition
is detected (see Section 5.3.1 on page 23 for details).
23 SYNC Synchronization output. Used as external synchronization output.
24 SS Soft-start. Connect a capacitor to SGND to set the desired soft-start duration.
6/48
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LED7707 pdf, datenblatt
Operation description
LED7707
5.1
5.1.1
Boost section
Functional description
The LED7707 is a monolithic LEDs driver for the backlight of LCD panels and it consists of a
boost converter and six PWM-dimmable current generators.
The boost section is based on a constant switching frequency, peak current-mode
architecture. The boost output voltage is controlled such that the lowest row's voltage,
referred to SGND, is equal to an internal reference voltage (700 mV typ.). The input voltage
range is from 4.5 V up to 36 V. In addition, the LED7707 has an internal LDO that supplies
the internal circuitry of the device and is capable to deliver up to 40 mA. The input of the
LDO is the VIN pin.
The LDO5 pin is the LDO output and the supply for the power MOSFET driver at the same
time. The AVCC pin is the supply for the analog circuitry and should be connected to the
LDO output through a simple RC filter in order to improve the noise rejection.
Figure 4. AVCC filtering
VIN
LDO5
LDO
CLDO5
1u
Rfilt
4R AVCC
CAVCC
100n
LED7707
SGND
AM00581v1
Two loops are involved in regulating the current sunk by the generators.
The main loop is related to the boost regulator and uses a constant frequency peak current-
mode architecture to regulate the power rail that supplies the LEDs (Figure 5), while an
internal current loop regulates the same current (flowing through the LEDs) at each row
according to the set value (RILIM pin).
Figure 5. Main loop and current loop diagram
VIN
12/48
Slope
COMP
LX ROWx
PWM
E/A 0.7V
Error amplifier
Minimum voltage drop
selector
SGND
RILIM
AM00582v1
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