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L6668

Figure 41. Hiccup-mode OCP: timing diagram

A second comparator senses the voltage on the current sense input and shuts the IC down if the voltage

at the pin exceeds 1.5 V. Such an anomalous condition is typically generated by either a short circuit of

the secondary rectifier or a shorted secondary winding or a saturated flyback transformer.

This condition is latched as long as the IC is supplied. When the IC is disabled, however, no energy is

coming from the self-supply circuit, then the voltage on the Vcc capacitor will decay and cross the UVLO

threshold after some time, which clears the latch. The internal start-up generator is still off, then the Vcc

voltage still needs to go below its restart voltage before the Vcc capacitor is charged again and the IC re-

started.

Ultimately, either of the above mentioned failures will result in a low-frequency intermittent operation (Hic-

cup-mode operation), with very low stress on the power circuit. The timing diagram of figure 41 illustrates

this operation.

4.6 Power Management

The L6668 is specifically designed to minimize converter's losses under light or no-load conditions, and a

special function has been provided to help the designer meet energy saving requirements even in power-

factor-corrected systems where a PFC pre-regulator precedes the DC-DC converter.

Actually EMC regulations require compliance with low-frequency harmonic emission limits at nominal

load, no limit is envisaged when the converter operates with a light load. Then the PFC pre-regulator can

be turned off, thus saving the no-load consumption of this stage (0.5 to 1W).

To do so, the device provides the PFC_STOP (#14) pin: it is an open collector output, normally low, that

This signal will be externally used for switching off the PFC controller and the pre-regulator as shown in

figure 42. To prevent intermittent operation of the PFC stage, 0.5V hysteresis is provided: the PFC_STOP

A capacitor (and a limiting resistor in the hundred ohms), shown in dotted lines, may be used if one wants

to delay PFC turn-off

When the L6668 is in UVLO (Vcc<8.7V) the pin is kept high so as to ensure that the PFC pre-regulator

will start up only after the DC-DC converter governed by the L6668 is activated.

Vcc

OUT

OCP latch

Vcc ON

Vcc OFF

re

Secondary diode is shorted here

t

t

t

t

V CS

Vcc_OK

t

1.5 V

Vcc

OUT

OCP latch

Vcc ON

Vcc OFF

re

Secondary diode is shorted here

t

t

t

t

V CS

Vcc_OK

t

1.5 V