CS51221 Ver la hoja de datos (PDF) - Cherry semiconductor

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componentes Descripción

Fabricante

CS51221

Enhanced Voltage Mode PWM Controller

Cherry semiconductor 

Application Information: continued

800000

700000

600000

500000 10K

400000

RT = 5K

300000

200000

50K

100000

0

0.0001

0.001

0.01

CT (µF)

Figure 8: Typical Performance Characteristics: Oscillator frequency vs

CT

( ) VIN × TON ≈

VOUT × TS

n

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,

VCOMP doesn’t change for input voltage variations. This

intuitively explains why FF voltage mode control has supe-

rior line regulation and line transient response. Knowing

the nominal value of VIN and TON, one can also select the

value of RC to place VCOMP at the center of its dynamic

range.

Select Feedback Voltage Divider

1

0.95

0.9

0.85

0.8

0.75

0.7

0.65

0.6

0.55

0.5

1000

10000

100000

1000000

Figure 9: Typical Performance Characteristics: Oscillator duty cycle vs

RT

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

( ) 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

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

is equal to VIN/R. Therefore, the volt-second value is deter-

mined by:

Ri = DC resistance between the FB pin and the voltage

divider output.

Ier = VFB input current, 1.3µA typical.

VIN × TON = (VCOMP − VFF(d)) × R × C

where VCOMP = COMP pin voltage

VFF(d) = FF pin discharge voltage.

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

the VCOMP clamp voltage which is equal to 1.8V. In

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

10

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