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參數(shù)資料
| 型號(hào)： | CS51221EN16 |
| 廠商： | ZF Electronics Corporation |
| 英文描述： | Enhanced Voltage Mode PWM Controller |
| 中文描述： | 增強(qiáng)的電壓模式PWM控制器 |
| 文件頁(yè)數(shù)： | 10/12頁(yè) |
| 文件大小： | 164K |
| 代理商： | CS51221EN16 |
Application Information: continued
10
C
Figure 8: Typical Performance Characteristics: Oscillator frequency vs
C
T
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
V
IN
×
T
ON
≈
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 V oltage 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
)
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.
R2
(
)
V
OUT
×
T
S
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
CT (
μ
F)
0.01
F
10K
RT = 5K
50K
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