L6566BH datasheet(28/51 Pages) STMICROELECTRONICS

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Application information

L6566BH

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Doc ID 16610 Rev 2

In applications where this function is not wanted, e.g. because of a narrow input voltage

range, the VFF pin can be simply grounded, directly or through a resistor, depending on

whether the user wants the OVP function to be auto-restart or latched mode (see

“

Section 5.11: OVP block”). The overcurrent setpoint is then fixed at the maximum value of 1

V. If a lower setpoint is desired to reduce the power dissipation on Rs, the pin can be also

biased at a fixed voltage using a divider from VREF (pin 10).

If the FF option is selected the line feedforward function can be still used to compensate for

the total propagation delay Td of the current sense chain (internal propagation delay td(H-L)

plus the turn-off delay of the external MOSFET), which in standard current mode PWM

controllers is done by adding an offset on the current sense pin proportional to the input

voltage. In that case the divider ratio k, which is much smaller as compared to that used with

the QR option selected, can be calculated with the following equation:

Equation 8

where Lp is the inductance of the primary winding. In case a constant maximum power

capability vs. the input voltage is not required, the VFF pin can be grounded, directly or

through a resistor (see

Section 5.11: OVP block), therefore fixing the overcurrent setpoint at

1 V, or biased at a fixed voltage through a divider from VREF to obtain a lower setpoint.

It is possible to bypass the pin to ground with a small film capacitor (e.g. 1-10 nF) to ensure

a clean operation of the IC even in a noisy environment.

The pin is internally forced to ground during UVLO, after activating any latched protection

and when the COMP pin is pulled below its low clamp voltage (see

Section 5.5: PWM

control block).

5.7

Hiccup-mode OCP

A third comparator senses the voltage on the current sense input and shuts down the device

if the voltage on the pin exceeds 1.5 V, a level well above that of the maximum overcurrent

setpoint (1 V). Such an anomalous condition is typically generated by either a short-circuit of

the secondary rectifier or a shorted secondary winding or a hard-saturated flyback

transformer.

opt =

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