CS51221 datasheet(11/12 Pages) CHERRY

Part #	CS51221

Description	Enhanced Voltage Mode PWM Controller

PDF	12 Pages

Scroll/Zoom	100%

Manufacturer	CHERRY [Cherry Semiconductor Corporation]

Direct Link	http://www.cherrycorp.com/

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Application Information: continued

Figure 10. The design of feedback voltage divider has to consider the

error amplifier input current.

Design voltage dividers for OV and UV detection

In Fig.11, the voltage divider uses three resistors in series

to set OV and UV threshold seen from the input voltage.

The values of the resistors can be calculated from the fol-

lowing three equations, where the third equation is

derived from OV hysteresis requirement.

VIN(LOW) ×

= 1V

(A)

VIN(HIGH) ×

= 2V

(B)

12.5µA × (R1 + R2) = VHYST

(C)

where

VLINE(LOW), VLINE(HIGH) = input voltage OV and UV threshold

VHYST = OV hysteresis seen at VIN

It is self-evident from equation A and B that to use this

design, VIN(HIGH) has to be two times greater than

VIN(LOW). Otherwise, two voltage dividers have to be used

to program OV and UV separately.

Figure 11. OV/UV Monitor Divider.

VIN

VUV

VOV

)

R2 + R3 + R1

(

)

R2 + R3

R2 + R3 + R1

(

COMP

Ier

1.27

VOUT

CS51221 Datasheet(PDF) 11 Page - Cherry Semiconductor Corporation