AN2823 datasheet(9/33 Pages) STMICROELECTRONICS

9 / 33 page

AN2823

The L5973AD in LED driving applications

Doc ID 15015 Rev 1

9/33

The current source schematic in Figure 7 shows a negligible value for the output capacitor

to achieve better performance in the dimming operation. The high switching frequency of the

L5973AD helps to keep the unfiltered current ripple in the LED negligible compared to the

DC current, so the color shift of the emitted light in all operating conditions can be

considered negligible.

The L5973AD is based on a voltage mode control loop. Therefore, the duty ratio of the

internal switch is obtained through a comparison between a sawtooth waveform (generated

by an oscillator) and the output voltage of the error amplifier (see Figure 6). The embedded

error amplifier is a transconductance type so its output stage delivers current to the

compensation network.

Figure 6.

Block diagram of L5973AD

In Figure 7, the output of the embedded transconductance error amplifier is clamped by

switch Q1, in accordance with the PWM signal.

When switch Q1 is ON, the COMP pin is low (see Figure 9). Regardless of the FB input

there is no intersection between the internal sawtooth waveform and the output of the error

amplifier. The internal switch is thus disabled and the inductor current starts decreasing

down to zero.

When the PWM signal disables the Q1 switch (see Figure 10), no current flows in the sense

resistor RS, so the error amplifier is highly unbalanced and free to charge the compensation

network composed of C4, C5 and R5. When the voltage of the COMP pin crosses the

internal sawtooth ramp, the embedded switch is enabled: the current source starts charging

the inductor and the rising edge phase of the LED begins.

The minimum voltage of the internal sawtooth waveform is about 1 V (see the voltage mode

architecture in Figure 8) so there is a time delay between the disabling of the Q1 switch and

the first pulse of the internal MOSFET. The delay is due to the time necessary to charge the

compensation network to the minimum voltage to cross the internal ramp. This provides a

shift between the duty cycle of the input PWM signal and the real PWM dimming performed

on the load current.

Figure 9 and Figure 10 show the dimming operation monitoring the input dimming signal

and the LED current.

AM01357v1

AN2823 Datasheet(PDF) 9 Page - STMicroelectronics