L6566BH datasheet(23/51 Pages) STMICROELECTRONICS

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L6566BH

Application information

Doc ID 16610 Rev 2

23/51

Figure 11.

Load-dependent operating modes: timing diagrams

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 to

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

To help the user 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 the

COMP pin falls below a threshold VCOMPO internally fixed, so as to have more headroom. To

AM11487v1

(pin 4)

VCOMPBM

COMP

(pin 9)

20 mV

hyster.

QR Mode

Valley-skipping Mode

Burst-mode

fsw

fosc

QR Mode

FF Mode

Burst-mode

FF Mode

MODE/SC=Open

MODE/SC=VREF

MODE/SC=Open

MODE/SC=VREF

AM11488v1

L6566BH

Vref

Vcso = Vref

R + Rc

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