L6566B datasheet(25/51 Pages) STMICROELECTRONICS

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L6566B

Application information

25/51

Figure 12.

Addition of an offset to the current sense lowers the burst-mode

operation threshold

5.4

Adaptive UVLO

A major problem when optimizing a converter for minimum no-load consumption is that the

voltage generated by the auxiliary winding under these conditions falls considerably as

compared even to a few mA load. This very often causes the supply voltage Vcc of the

control IC to drop and go below the UVLO threshold so that the operation becomes

intermittent, which is undesired. Furthermore, this must be traded off against the need of

generating a voltage not exceeding the maximum allowed by the control IC at full load.

To help the designer overcome this problem, the device, besides reducing its own

consumption during burst-mode operation, also features a proprietary adaptive UVLO

function. It consists of shifting the UVLO threshold downwards at light load, namely when

the voltage at pin COMP falls below a threshold VCOMPO internally fixed, so as to have more

headroom. To prevent any malfunction during transients from minimum to maximum load

the normal (higher) UVLO threshold is re-established when the voltage at pin COMP

exceeds VCOMPL and Vcc has exceeded the normal UVLO threshold (see Figure 13). The

normal UVLO threshold ensures that at full load the MOSFET will be driven with a proper

gate-to-source voltage.

L6566B

Vref

Vcso = Vref

R + Rc

Figure 13.

Adaptive UVLO block

VCOMPL

VCOMPO

VCOMP

(pin 9)

Vcc

(pin 5)

VccOFF1

VccOFF2

(*) VccOFF2 < VccOFF1 is selected when Q is high

UVLO

VccOFF1

Vcc

L6566B

COMP

VccOFF2

(*)

VCOMPL

VCOMPO

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