CS51221

http://onsemi.com

11

0.0001

0.001

0.01

0

800

400

700

600

500

300

200

100

1000

10000

1000000

0.50

1.00

0.80

0.95

0.90

0.85

0.75

0.70

0.65

0.60

0.55

100000

Figure 10. Typical Performance Characteristics,

Figure 11. Typical Performance Characteristics,

voltage variations. This intuitively explains why FF voltage

mode control has superior line regulation and line transient

of its dynamic range.

Select Feedback Voltage Divider

As shown in Figure 12, the voltage divider output feeds to

the FB pin, which connects to the inverting input of the error

amplifier. The non−inverting input of the error amplifier is

connected to a 1.27 V (typ) reference voltage. The FB pin

has an input current which has to be considered for accurate

DC outputs. The following equation can be used to calculate

the R1 and R2 value

R2

R1

) R2

VOUT + 1.27 *

where

∇ is the correction factor due to the existence of the

FB pin input current Ier.

+ (Ri ) R1 R2)Ier

Ri = DC resistance between the FB pin and the voltage

divider output.

mA typical.

Design Voltage Dividers for OV and UV Detection

In Figure 13, 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 following three equations, where the third equation is

derived from OV hysteresis requirement.

VIN(LOW)

R2

) R3

R2

) R3 ) R1 +

1.0 V

(A)

VIN(HIGH)

R3

R2

) R3 ) R1 +

2.0 V

(B)

12.5

mA

(R1

) R2) + VHYST

(C)

where:

threshold;

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

to program OV and UV separately.

Figure 12. The Design of Feedback Voltage Divider

Has to Consider the Error Amplifier Input Current

+

−

−

+

R1

COMP

FB

R2

Ri

1.27

Ier

Figure 13. OV/UV Monitor Divider

R1

R2

R3