L6566ATR View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
'L6566ATR' PDF : 51 Pages View PDF
L6566A
Application information
5.5
PWM control block
The device is specific to 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. Ideally, the voltage
generated by the self-supply winding and the output voltage should be related by the
Naux/Ns turn ratio only. In fact, numerous non-idealities, mainly transformer parasitics,
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 dynamic 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 L6566A shuts 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, therefore the voltage on the Vcc capacitor decays and crosses 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 CRT TVs
or monitors (see the timing diagram Figure 15).
Figure 14. Possible feedback configurations that can be used with the L6566A
Vout
5 Vcc
L6566A
9
L6566A
COMP
9
Cs
COMP
Naux
TL431
Secondary feedback
Primary feedback
Doc ID 13794 Rev 4
25/52
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