LED6000 datasheet(11/49 Pages) STMICROELECTRONICS

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LED6000

Functional description

The LED6000 device is based on a voltage mode, constant frequency control loop. The

LEDs current, monitored through the voltage drop on the external current sensing resistor,

RSNS, is compared to an internal reference (0.25 V) providing an error signal on the COMP

pin. The COMP voltage level is then compared to a fixed frequency sawtooth ramp, which

finally controls the on- and off-time of the power switch.

The main internal blocks are shown in Figure 2: Block diagram on page 4 and can be

summarized as follow.



The fully integrated oscillator that provides the sawtooth ramp to modulate the duty

cycle and the synchronization signal. Its switching frequency can be adjusted by an

external resistor. The input voltage feedforward is implemented.



The soft-start circuitry to limit the inrush current during the startup phase.



The voltage mode error amplifier.



The pulse width modulator and the relative logic circuitry necessary to drive the internal

power switch.



The high-side driver for the embedded N-channel power MOSFET switch and

bootstrap circuitry. A dedicated high resistance low-side MOSFET, for anti-boot

discharge management purposes, is also present.



The peak current limit sensing block, with a programmable threshold, to handle the

overload including a thermal shutdown block, to prevent the thermal runaway.



The bias circuitry, which includes a voltage regulator and an internal reference, to

supply the internal circuitry and provide a fixed internal reference and manages the

current dimming feature. The switchover function from VCC to VBIAS can be

implemented for higher efficiency. This block also implements voltage monitor circuitry

(UVLO) that checks the input and internal voltages.

4.1

Oscillator and synchronization

Figure 4 shows the block diagram of the oscillator circuit. The internal oscillator provides

a constant frequency clock, whose frequency depends on the resistor externally connected

between the FSW pin and ground.

Figure 4. Oscillator and synchronization

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