LED7707 datasheet(13/48 Pages) STMICROELECTRONICS

13 / 48 page

LED7707

Operation description

13/48

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 (see

Figure 6):

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 (700 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.

Figure 6.

Calculation of the output voltage of the boost regulator

5.1.2

Enable function

The LED7707 is enabled by the EN pin. This pin is active high and, when forced to SGND,

the device is turned off. This pin is connected to a permanently active 2.5 µA current source;

when sudden device turn-on at power-up is required, this pin must be left floating or

connected to a delay capacitor. Starting from an ON state, when the LED7707 is turned off,

it quickly discharges the Soft-Start capacitor and turns off the power-MOSFET, the current

generators and the LDO. The power consumption is thus reduced to 20 µA only.

700

)

(

max

BST

LEDS

ROWS

700 mV

ILED

Row with the highest voltage

drop across LEDs

VBOOST

VIN

max

Σ V

Leading

generator

Boost

controller

Current

generators

section

AM00583v1

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