5

Functional description

The ALED6000 is based on a voltage mode, constant frequency control loop. The LED 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

saw-tooth ramp, which finally controls the on- and off-time of the power switch.

The main internal blocks are shown in the block diagram in Figure 2. Block diagram and can be summarized as

follows:

•

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

feed-forward is implemented

•

The soft-start circuitry to limit inrush current during the start-up 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 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 programmable threshold, to handle overload including a thermal

shutdown block, to prevent thermal runaway

•

The bias circuitry, which includes a voltage regulator and 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 a voltage monitor

circuitry (UVLO) that checks the input and internal voltages

5.1

Oscillator and synchronization

Figure 4. Oscillator and synchronization 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

Clock

Generator

Synchronization

Ramp

Generator

FSW

Sawtooth

SYNCH

180° phase shift

clock

If the FSW pin is left floating, the programmed frequency is 250 kHz (typ.); if FSW pin is connected to an external

resistor the programmed switching frequency can be increased up to 1.5 MHz, as shown in Figure 5. Switching

frequency programmability . The required RFSW value (expressed in kΩ) is estimated by the equation below :

FSW=250kHz+12500RFSW

(1)

ALED6000

Functional description

DS13018 - Rev 4

page 9/45