LED7708TR datasheet(21/53 Pages) STMICROELECTRONICS

21 / 53 page

LED7708

Recommended operating conditions

Doc ID 022698 Rev 1

21/53

8.2

Device shutdown

The device is turned off by clearing the DEN bit of the DEVCFG0 register. The content of

most of the internal registers is not cleared acting on the DEN bit.

The LREN pin can be also used to turn the device on and off whenever the internal 3.3 V

LDO is used to supply the LED7708 (POR). In this case the content of the internal registers

is lost.

8.3

Boost controller section and output voltage control

The boost section of the LED7708 consists of:

●

Fixed-frequency current-mode step-up controller

●

Output voltage optimization loop

●

External MOSFET gate driver

●

Related protection circuitry.

8.3.1

Output voltage optimization

The output voltage of the boost section is the supply rail for the LED strings and is regulated

in an adaptive way to improve the overall efficiency. A dedicated digital-to-analog converter

(DAC) is used to change the reference voltage of the loop and the output voltage is adjusted

in order to keep the power dissipation of the LED driving section to the minimum level.

The optimization is achieved by keeping the voltage drop across the channels inside a fixed

window (regulation window) and its amplitude can be selected between two values (300 mV

and 600 mV) by acting on the internal registers. The lower threshold of the regulation

window (VRWL) is a function of the set LED current (Equation 1) and the upper threshold

(VRWH) is therefore determined.

Equation 1

Figure 5 shows the simplified regulation circuit.

RWL

ISETH

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