Operation description - boost section

LED7706

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Operation description - boost section

6.1

Functional description

The LED7706 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 (400 mV typ.). The input voltage

range is from 4.5 V up to the output voltage. In addition, the LED7706 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.

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 (see Table 7), 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

where the first term represents the highest total forward voltage drop over N active rows and

the second is the voltage drop across the leading generator (400 mV typ.).

The device continues to monitor the voltage drop across all the rows and automatically

switches to the current generator having the lowest voltage drop.

VIN

LDO5

AVCC

SGND

LED7706

1u

100n

4R7

LDO

mV

400

)

V

(

max

V

j

,

F

m

1

j

N

1

i

OUT

LEDS

ROWS

+

=

Σ

=

=