Application Information: continued
V
OUT × TS
800000
700000
VIN × TON ≈
(
)
n
600000
500000
400000
n = transformer turns ratio
R = 5K
T
10K
50K
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.
300000
200000
100000
0
0.0001
0.01
0.001
CT (µF)
Figure 8: Typical Performance Characteristics: Oscillator frequency vs
CT
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
1
0.95
0.9
0.85
0.8
0.75
0.7
R2
R1 + R2
VOUT = 1.27 − ∇
(
)
0.65
0.6
0.55
0.5
where ∇ is the correction factor due to the existence of the
1000
10000
100000
1000000
FB pin input current Ier.
Figure 9: Typical Performance Characteristics: Oscillator duty cycle vs
RT
∇ = (Ri + R1//R2)Ier
Select RC for Feed Forward Ramp
Ri = DC resistance between the FB pin and the voltage
divider output.
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:
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
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
