AN3142 datasheet(21/44 Pages) STMICROELECTRONICS

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AN3142

Designing a fixed-off-time PFC

Doc ID 17005 Rev 3

21/44

Equation 41

RoutL = 62 kΩ in parallel to a 27 kΩ can be selected. Please note that for RoutH a resistor

with a suitable voltage rating (>400 V) is needed, or more resistors in series have to be

used.

Pin 7 (PFC_OK - feedback failure protection) The PFC_OK pin has been dedicated to

monitor the output voltage with a separate resistor divider. This divider is selected so that

the voltage at the pin reaches 2.5 V if the output voltage exceeds a preset value (Vovp),

usually larger than the maximum Vout that can be expected, including also worst-case

load/line transients. For a maximum output voltage Vout max of 430 V and imaging a 50 µA

current flowing into the divider:

Equation 42

By selecting a commercial value of 51k

Ω:

Equation 43

Connecting in series two 3.3 M

Ω resistors and one 2.2 MΩ resistor a total value of 8.8 MΩ

can be obtained.

Notice that both feedback dividers connected to the L6563S pin #1 (INV) and pin #7

(PFC_OK) 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 can handle “normal” over-voltage conditions, that is,

those resulting from an abrupt load/line change or occurring at start-up. If the over-voltage is

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

output divider fails to open, an additional circuitry detects the voltage drop of pin INV. If the

voltage on pin INV is lower than 1.66V (Typ.) and at same time the OVP is active, a feedback

failure is assumed.

Therefore, the gate drive activity is immediately stopped, the device is shut down, its

quiescent consumption is reduced to 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 complete protection not only against 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

preregulator. Moreover, the PFC_OK pin doubles its function as a not-latched IC disable: a

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

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

159

outH

outL =

Ω

159

outL

divider

PFC

REF

Ω

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

PFC

REF

MAX

OUT

Ω

⎟

⎠

⎞

⎜

⎝

⎛

−

⋅

Ω

721

430

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