AN-8027 데이터 시트보기 (PDF) - Fairchild Semiconductor
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AN-8027 Datasheet PDF : 17 Pages
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AN-8027
[STEP-12] PWM Ramp Circuit Design
For voltage-mode operation, the RAMP pin can be
connected to a DC voltage through a resistor. When it is
connected to the input of forward converter, ramp signal
slope is automatically adjusted according to the input
voltage providing line feed-forward operation. However, it
can cause more power dissipation in the resistor. For better
efficiency and lower standby power consumption, it is
recommended to connect the RAMP pin to the VREF pin.
[STEP-13] Feedback Compensation Design
for PWM Stage
Figure 21 shows the typical cross regulation compensation
circuit configuration for multi-output converters. The small
signal characteristics of the compensation network is given as:
v)FBPWM
=
−
1+
RB
s / ωCP
⋅ ( 1+ s / ωCZ1
ROS1RDCF s
v)O1
+
1 + s / ωCZ 2
ROS 2 RDCF s
v)O2 )
(53)
where:
ωCP
=
1
(RB1 // RB2 )CB
ωCZ 1
=
1
RF CF
ωCZ 2
=
(RF
+
1
ROS 2 )CF
(54)
Figure 22. Ramp Generation Circuit for PWM
It is typical to use 470pF~1nF capacitor on the RAMP pin
and to have the peak of the ramp signal around 2~3V.
The peak of the ram voltage is given as:
V PK
RAMP
=
1
CRAMP
⋅ VREF
RRAMP
⋅1
2 fSW
(52)
VREF
RB1
FBPWM
RB
CB
VO2
VO1
ROS1
RD
ROS2
RF CF
KA431
ROS3
(Design Example) Selecting CRAMP and RRAMP as
1nF and 22k, the PWM ramp voltage is obtained as:
V PK
RAMP
=
1
CRAMP
⋅ VREF
RRAMP
⋅1
2 fSW
=
1
1×10−9
⋅ 7.5
22 ×103
⋅1
2 ⋅ 65×103
= 2.6V
Figure 23. Feedback Compensation Circuit for
PWM Stage
The small signal equivalent circuit for control-to-output
transfer function of the PWM power stage can be simplified
as shown in Figure 24. The transfer function is fourth-order
system because additional LC filters are used to meet the
output voltage ripple specification. Therefore, it is
recommended to use engineering software, such as PSPICE
or Mathlab, to design the feedback loop.
VBOUT
⋅
N S1
NS 2
LM
LLK
LL12
VO1
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 8/26/09
CO11
CO12
RL1
1
VRAMP
VFBPWM
LLK
CO21N
L22N
VO2N
VO2
CO22N
RL2N
NS1:NS2
Figure 24. Simplified Small Signal Equivalent Circuit
for Control-to-Output Transfer Function
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13
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