STLDC08 datasheet(13/29 Pages) STMICROELECTRONICS

13 / 29 page

STLDC08

Detailed description

Doc ID 18476 Rev 1

13/29

Detailed description

7.1

Main control loop

The STLDC08 is an LED driver step-up controller dedicated to handheld equipment, having

a typical voltage ranging from 0.8 V to 1.5 V. The controller drives an N-channel Power

MOSFET and implements a hysteretic current mode control with constant OFF time.

Hysteretic operation eliminates the need for small signal control loop compensation. The

control loop adapts the value of the inductor peak current as needed to deliver the desired

current on the LED branch. The LED current is set by an external sense resistor RFB

inserted between the feedback pin (FB) and GND. When the current mode control system

operates in continuous mode the control peak current is almost equivalent to the average

current control.

7.2

Start up

At the startup phase, when the device is connected to the battery or when the EN pin is

pulled high, the internal 2x charge pump starts to work, boosting the voltage on the 2VCC

pin. When the 2VCC pin reaches 1.7 V a soft-start cycle begins. The external main MOSFET

is switched on/off allowing the charging of the output capacitor.

If the optional PWMOUT MOSFET is used for the dimming operation, the PWMOUT pin is

held low, further assuring that no current is flowing. The PWMOUT pin starts to follow the

PWM input when the soft-start cycle is ended.

When VOUT voltage exceeds 1.9 V, the chip starts drawing its supply current from VOUT

rather than from VCC, the charge pump is turned off and the voltage on the 2VCC pin goes to

zero. When VOUT exceeds the forward voltage of LED VLED, the current starts flowing trough

the LED, but, at this point, the voltage on the DRV pin is high enough to allow the main

MOSFET to carry the necessary current.

STLDC08 データシート(PDF) 13 Page - STMicroelectronics