LED7707 datasheet(12/48 Pages) STMICROELECTRONICS

12 / 48 page

Operation description

LED7707

12/48

5.1

Boost section

5.1.1

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

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

LDO5

AVCC

SGND

CLDO5

CAVCC

100n

Rfilt

LDO

LED7707

AM00581v1

VIN

0.7V

PWM

Minimum voltage drop

selector

COMP

ROWx

SGND

RILIM

E/A

Slope

Error amplifier

AM00582v1

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