AN2983 datasheet(11/32 Pages) STMICROELECTRONICS

11 / 32 page

AN2983

Circuit description

Doc ID 15679 Rev 1

11/32

If we consider the time at which the capacitor voltage crosses the 0.7 V threshold, the

equivalent toff time is 1.32 µS (+150 nS compared to the nominal value, due to the tolerance

of C8 and stray capacitance) while the remaining 250 nS are related to the propagation

delay (during which the voltage falls from 0.7 V to 0.44 V).

Figure 10.

toff delay (nominal)

2.4

Current setting

As already indicated, the CS pin of the L6562A controller is internally connected to the non-

inverting input of the CS comparator, whose threshold (Vth) is fixed to 1.08 V (nom.)

Tying it to the sense resistor forces the comparator’s output high when I_Led = Vth / RS.

This is a very simple way to detect – and limit – the peak load current to a fixed value.

In order to make this value adjustable, we can introduce an auxiliary voltage source (Va),

connected to the CS pin through a resistor (Ra) and another resistor (Rb) between CS and

the sense resistor, as indicated in Figure 11. In this way the auxiliary source can be used to

modify the threshold at which the (peak) load current triggers the comparator.

Figure 11.

Current setting

AM04261v1

Traces:

CH(4) green=I_LED;

CH(2) blue=V_C8;

CH(1) red=V_GD;

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