STLD40DPUR datasheet(7/16 Pages) STMICROELECTRONICS

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STLD40D

Functional description

Doc ID 12203 Rev 7

7/16

Functional description

5.1

Boost controller

The STLD40D is a boost converter operating in PFM (pulsed frequency modulation) mode.

The converter monitors the LED current through the resistor RLED and when the feedback

voltage falls below the reference voltage REF2, the boost switch TSW turns ON and the

current ramps up. The inductor current is measured by sensing the temperature

compensated drain voltage of the boost MOSFET. The boost turns off when its drain voltage

reaches the internally reference REF1, the main switch remains off until the minimum off

time (250 ns typical) has passed and the feedback voltage is below the reference again. A

maximum ON time of 4 µs typical prevents the switch TSW to stay ON during a too long

period of time.

5.2

Adjustable peak inductor current limit

The peak inductor current is monitored by sensing the drain voltage of the switch TSW. Since

it exceeds the temperature compensated and supply voltage compensated reference REF1,

the RS flip flop is reset and TSW is turned OFF. By connecting a resistance to the pin RSET

the peak current limit can be adjusted from 200 mA to 1 A. When RSET resistor value is

about 12 kΩ is connected directly to GND, the default value is 1 A.

5.3

Enable

The ENABLE pin is a high logic input signal and allows turning on/off the controller without

cutting the input voltage from the boost regulator circuit. The pin ENABLE can be used to

dim the LED by applying a low frequency PWM signal.

5.4

OVP

If the regulation loop is cut, there is no signal at the feedback pin, the PFM controller will then

continue to switch without control and generate an output voltage at the SW, and VO pin

exceeding the breakdown value VBSW, and VBO.

The over voltage protection (OVP) senses the voltage at the VO pin. When the voltage

exceed 38 V min. the controller is automatically turned OFF.

A hysteresis control allows the device restarting automatically since the output voltage drops

down of 1.5 V.

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