AN3065 datasheet(21/33 Pages) STMICROELECTRONICS

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AN3065

Test results and significant waveforms

Doc ID 16279 Rev 2

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For the EVL6563S-100W we have:

●

For the EVL6563H-100W we have:

–VO = 400 V

–VOVP = 434 V

–

Select: R3+R4+R11 = 8.8 M

Ω

then:

–

R15 = 8.8 M

Ω ·2.5/(434-2.5) = 51 kΩ

Once this function is triggered, the gate drive activity is immediately stopped until the

voltage on the pin PFC_OK drops below 2.4 V. An example is given in

Figure 29.

Notice that both feedback dividers connected to L6563S V_INV (pin #1) and PFC_OK (pin

#7) can be selected without any constraints. The unique criterion is that both dividers have

to sink a current from the output bus which needs to be significantly higher than the current

biasing the error amplifier and PFC_OK comparator.

The OVP function described above is able to handle "normal" overvoltage conditions, i.e.

those resulting from an abrupt load/line change or occurring at startup. In case the

overvoltage is generated by a feedback disconnection, for instance, when one of the upper

resistors of the output divider fails open, an additional circuitry detects the voltage drop of

pin INV. If the voltage on pin INV is lower than 1.66 V and at the same time OVP is active, a

feedback failure is assumed. Thus, the gate drive activity is immediately stopped, the device

is shut down, its quiescent consumption is reduced below 180 µA and the condition is

latched as long as the supply voltage of the IC is above the UVLO threshold. To restart the

system, it is necessary to recycle the input power, so that the Vcc voltage of the L6563S

goes below 6 V and that one of the PWM controllers goes below its UVLO threshold.

Note that this function offers a complete protection against not only feedback loop failures or

erroneous settings, but also against a failure of the protection itself. Either resistor of the

PFC_OK divider failing short or open or a PFC_OK pin floating results in shutting down the

IC and stopping the pre-regulator.

Moreover, the pin PFC_OK doubles its function as a non-latched IC disable. A voltage

below 0.23V shuts down the IC, reducing its consumption below 2 mA. To restart the IC,

simply let the voltage at the pin go above 0.27 V.

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