PM6600 datasheet(23/43 Pages) STMICROELECTRONICS

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PM6600

Operation description

23/43

Operation description

7.1

Boost section

7.1.1

Functional description

The PM6600 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 input voltage range is from 4.7 V up to 28 V.

The boost section is based on a constant switching frequency, Peak Current-Mode

architecture. The boost output voltage is controlled such that the lowest ROWs' voltage,

referred to SGND, is equal to an internal reference voltage (400 mV typ.).

In addition, the PM6600 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 43.

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 (see Figure 49), while an internal current loop regulates the same current

at each ROW according to the set value (RILIM pin).

A dedicated circuit automatically selects the lowest voltage drop among all the ROWs and

provides this voltage the main loop that, in turn, regulates the output voltage. In fact, once

the reference generator has been detected, the error amplifier compares its voltage drop to

the internal reference voltage and varies the COMP output. The voltage at the COMP pin

determines the inductor peak current at each switching cycle. The output voltage of the

boost regulator is thus determined by the total forward voltage of the LEDs strings:

Equation 1

VIN

LDO5

AVCC

SGND

PM6600

Cavcc

100n

Rfilt

4R7

LDO

400

)

(

max

OUT

LEDS

ROWS

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