L6566BH datasheet(29/51 Pages) STMICROELECTRONICS

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L6566BH

Application information

Doc ID 16610 Rev 2

29/51

Figure 18.

Hiccup-mode OCP: timing diagram

To distinguish an actual malfunction from a disturbance (e.g. induced during ESD tests), the

first time the comparator is tripped the protection circuit enters a “warning state”. If in the

next switching cycle the comparator is not tripped, a temporary disturbance is assumed and

the protection logic is reset in its idle state; if the comparator is again tripped, a real

malfunction is assumed and the L6566BH is stopped. Depending on the time relationship

between the detected event and the oscillator, the device may occasionally stop after the

third detection.

This condition is latched as long as the device is supplied. While it is disabled, however, no

energy comes from the self-supply circuit; hence the voltage on the VCC capacitor decays

and crosses the UVLO threshold after some time, which clears the latch. The internal

startup generator is still off, and the VCC voltage still needs to go below its restart voltage

before the VCC capacitor is charged again and the device restarted. Ultimately, this results in

a low-frequency intermittent operation (Hiccup-mode operation), with very low stress on the

power circuit. This special condition is illustrated in the timing diagram of

Figure 18.

5.8

Frequency modulation

To alleviate the converter’s EMI emissions and reduce cost and size of the line filter, it is

advantageous to modulate its switching frequency, so that the resulting spread-spectrum

action distributes the energy of each harmonic of the switching frequency over a number of

side-band harmonics. Their overall energy is unchanged but the individual amplitudes are

smaller. This is what naturally occurs with QR operation, due to the twice-mains-frequency

ripple appearing on the input bulk capacitor, which translates into different DCM-CCM

boundary frequencies.

The L6566BH is provided with a dedicated pin, FMOD (6), to perform this function if FF mode

is selected.

AM11494v1

Vcc

(pin 5)

(pin 4)

OCP latch

VccON

VccOFF

Vccrestart

Secondary diode is shorted here

VCS

(pin 7)

Vcc_OK

1.5 V

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