Application Information: continued

10

Figure 8: Typical Performance Characteristics: Oscillator frequency vs

Figure 9: Typical Performance Characteristics: Oscillator duty cycle vs

Select RC for Feed Forward Ramp

If the line voltage is much greater than the FF pin Peak

Voltage, the charge current can be treated as a constant and

mined by:

As shown in the equation, the volt-second clamp is set by

Forward or Flyback circuits, the volt-second clamp value is

designed to prevent transformers from saturation.

In a buck or forward converter, volt-second is equal to

n = transformer turns ratio

which is a constant determined by the regulated output

voltage, switching period and transformer turns ration (use

1 for buck converter). It is interesting to notice from the

aforementioned two equations that during steady state,

intuitively explains why FF voltage mode control has supe-

rior line regulation and line transient response. Knowing

range.

Select Feedback Voltage Divider

As shown in Fig.10, 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 1.27V (typ) reference voltage. The FB pin

has an input current which has to be considered for accu-

rate DC outputs. The following equation can be used to

calculate the R1 and R2 value

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.

)

R2

R1 + R2

(

)

n

(

1000000

100000

10000

1000

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

800000

700000

600000

500000

400000

300000

200000

100000

0

0.0001

0.001

0.01

CT (µF)

10K

RT = 5K

50K