L6566ATR datasheet(25/51 Pages) STMICROELECTRONICS

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L6566A

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 (see "PFC

Interface"), 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 (see "Chapter 5.8: PFC interface on page 31") 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.

L6566A

Vref

Vcso = Vref

R + Rc

Figure 13.

Adaptive UVLO block

VCOMPL

VCOMPO

VCOMP

(pin 9)

Vcc

(pin 5)

VccOFF1

VccOFF2

Vcc_PFC

(pin 6)

Tdelay

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

UVLO

VccOFF1

Vcc

L6566A

COMP

VccOFF2

(*)

VCOMPL

VCOMPO

Vcc_PFC

logic

Vcc_PFC

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