參數(shù)資料
型號: IR3621MTR
廠商: International Rectifier
英文描述: 2-PHASE / DUAL SYNCHRONOUS PWM CONTROLLER WITH OSCILLATOR SYNCHRONIZATION AND PRE-BIAS STARTUP
中文描述: 2相/雙同步PWM控制器,帶有振蕩器同步操作和預偏置啟動
文件頁數(shù): 14/29頁
文件大?。?/td> 389K
代理商: IR3621MTR
14
IR3621 & (PbF)
www.irf.com
The ESR zero of the output capacitor is expressed as
follows:
Figure 15 - Compensation network without local
feedback and its asymptotic gain plot.
The transfer function (Ve / V
OUT
) is given by:
The (s) indicates that the transfer function varies as a
function of frequency. This configuration introduces a gain
and zero, expressed by:
|H(s)| is the gain at zero cross frequency.
First select the desired zero-crossover frequency (F
O1
):
V
OUT
Vp=V
REF
R
5
R
9
R
4
C
9
Ve
E/A
F
Z
H(s) dB
Frequency
Gain(dB)
Fb
Comp
C
POLE
F
ESR
= 1
2
π×
ESR
×
Co
H(s) = g
m
× ×
( )
R
1 ---(11)
4
C
9
sC
9
F
Z
= 1
2
π×
R
4
×
C
9
|H(s=j
×
2
π×
F
O
)| = g
m
×
×
R
4
---(12)
R
5
R
9
+R
5
Feedback Compensation
The IR3621 is a voltage mode controller; the control loop
is a single voltage feedback path including error ampli-
fier and error comparator. To achieve fast transient re-
sponse and accurate output regulation, a compensation
circuit is necessary. The goal of the compensation net-
work is to provide a closed loop transfer function with
the highest 0dB crossing frequency and adequate phase
margin (greater than 45
!
).
The output LC filter introduces a double pole, –40dB/
decade gain slope above its corner resonant frequency,
and a total phase lag of 180
!
(see Figure 14). The Reso-
nant frequency of the LC filter is expressed as follows:
1
2
π×
L
O
×
C
O
Where: Lo is the output inductor
For 2-phase application, the effective output
inductance should be used
Co is the total output capacitor
Figure 14 shows gain and phase of the LC filter. Since
we already have 180
!
phase shift just from the output
filter, the system risks being unstable.
F
LC
= ---(10)
Gain
F
LC
0dB
Phase
0
!
F
LC
-180
!
Frequency
Frequency
-40dB/decade
Figure14 - Gain and phase of LC filter
The IR3621’s error amplifier is a differential-input transcon-
ductance amplifier. The output is available for DC gain
control or AC phase compensation.
The E/A can be compensated with or without the use of
local feedback. When operated without local feedback,
the transconductance properties of the E/A become evi-
dent and can be used to cancel one of the output filter
poles. This will be accomplished with a series RC circuit
from Comp pin to ground as shown in Figure 15.
Note that this method requires the output capacitor to
have enough ESR to satisfy stability requirements. In
general, the output capacitor’s ESR generates a zero
typically at 5kHz to 50kHz which is essential for an ac-
ceptable phase margin.
F
O1
> F
ESR
and F
O1
(1/5 ~ 1/10)
×
f
S
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