L6566B datasheet(26/51 Pages) STMICROELECTRONICS

26 / 51 page

Application information

L6566B

26/51

5.5

PWM control block

The device is specific for secondary feedback. Typically, there is a TL431 on the secondary

side and an optocoupler that transfers output voltage information to the PWM control on the

primary side, crossing the isolation barrier. The PWM control input (pin 9, COMP) is driven

directly by the phototransistor’s collector (the emitter is grounded to GND) to modulate the

duty cycle (Figure 14, left-hand side circuit).

In applications where a tight output regulation is not required, it is possible to use a primary-

sensing feedback technique. In this approach the voltage generated by the self-supply

winding is sensed and regulated. This solution, shown in Figure 14, right-hand side circuit,

is cheaper because no optocoupler or secondary reference is needed, but output voltage

regulation, especially as a result of load changes, is quite poor.

Figure 14.

Possible feedback configurations that can be used with the L6566B

Ideally, the voltage generated by the self-supply winding and the output voltage should be

related by the Naux/Ns turn ratio only. Actually, numerous non-idealities, mainly

transformer's parasites, cause the actual ratio to deviate from the ideal one. Line regulation

is quite good, in the range of ± 2 %, whereas load regulation is about ± 5 % and output

voltage tolerance is in the range of ± 10 %.

The dynamics of the pin is in the 2.5 to 5 V range. The voltage at the pin is clamped

downwards at about 2 V. If the clamp is externally overridden and the voltage on the pin is

pulled below 1.4 V the L6566B will shut down. This condition is latched as long as the

device is supplied. While the device is disabled, however, no energy is coming from the self-

supply circuit, thus the voltage on the Vcc capacitor will decay and cross the UVLO

threshold after some time, which clears the latch and lets the HV generator restart. This

function is intended for an externally controlled burst-mode operation at light load with a

reduced output voltage, a technique typically used in multi-output SMPS, such as those for

TVs or monitors (see the timing diagram Figure 15 on page 27).

Vout

TL431

L6566B

COMP

L6566B

Vcc

Naux

COMP

Secondary feedback

Primary feedback

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