CS51221
Application Information: continued
800000
700000
Frequency
600000
500000
400000
300000
200000
100000
0
0.0001
50K
10K
RT = 5K
V
IN
×
T
ON
≈
(
V
OUT
×
T
S
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,
V
COMP
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 V
IN
and T
ON
, one can also select the
value of RC to place V
COMP
at the center of its dynamic
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
0.001
CT (µF)
0.01
Figure 8: Typical Performance Characteristics: Oscillator frequency vs
C
T
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
(
R2
R1 + R2
)
V
OUT
= 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.
Ier = V
FB
input current, 1.3µA typical.
Figure 9: Typical Performance Characteristics: Oscillator duty cycle vs
R
T
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 V
IN
/R. Therefore, the volt-second value is deter-
mined by:
V
IN
×
T
ON =
(
V
COMP
−
V
FF(d)
)
×
R
×
C
where V
COMP
= COMP pin voltage
V
FF(d)
= FF pin discharge voltage.
As shown in the equation, the volt-second clamp is set by
the V
COMP
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 ]
