L6564 STMicroelectronics, L6564 Datasheet - Page 18

• Manufacturer Part Number: L6564
• Description: 10 pin transition-mode PFC controller
• Manufacturer: STMicroelectronics

Application information
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6.1
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Application information
Overvoltage protection
Normally, the voltage control loop keeps the output voltage Vo of the PFC pre-regulator
close to its nominal value, set by the ratio of the resistors R1 and R2 of the output divider. A
pin of the device (PFC_OK) has been dedicated to monitor the output voltage with a
separate resistor divider (R3 high, R4 low, see
the voltage at the pin reaches 2.5 V if the output voltage exceeds a preset value, usually
larger than the maximum Vo that can be expected.
Example: V
51 kΩ.
When this function is triggered, the gate drive activity is immediately stopped until the
voltage on the pin PFC_OK drops below 2.4 V. Notice that R1, R2, R3 and R4 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 bias current of
both INV and PFC_OK pins.
Figure 27. Output voltage setting, OVP and FFP functions: internal block diagram
O
= 400 V, V
OX
= 434 V. Select: R3 = 8.8 MΩ; then: R4 = 8.8 MΩ ·2.5/(434-2.5) =
Doc ID 16202 Rev 4
Figure
27). This divider is selected so that
L6564