IRU3037(2003) View Datasheet(PDF) - International Rectifier
Part Name
Description
Manufacturer
IRU3037 Datasheet PDF : 21 Pages
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IRU3037 / IRU3037A
For a general solution for unconditionally stability for any
type of output capacitors, in a wide range of ESR values
we should implement local feedback with a compensa-
tion network. The typically used compensation network
for voltage-mode controller is shown in Figure 7.
ZIN
VOUT
C12
C10
R8
R6
R7
C11
Zf
Gain(dB)
H(s) dB
Fb
R5
VREF
E/A
Ve
Comp
FZ1 FZ2
FP2
FP3 Frequency
Figure 7 - Compensation network with local
feedback and its asymptotic gain plot.
In such configuration, the transfer function is given by:
Ve
1 - gmZf
VOUT = 1 + gmZIN
The error amplifier gain is independent of the transcon-
ductance under the following condition:
gmZf >> 1 and gmZIN >>1
---(14)
By replacing ZIN and Zf according to Figure 7, the trans-
former function can be expressed as:
[ ( )] H(s)=
1
3
sR6(C12+C11)
(1+sR7C11)3[1+sC10(R6+R8)]
1+sR7 C123C11 3(1+sR8C10)
C12+C11
As known, transconductance amplifier has high imped-
ance (current source) output, therefore, consider should
be taken when loading the E/A output. It may exceed its
source/sink output current capability, so that the ampli-
fier will not be able to swing its output voltage over the
necessary range.
The compensation network has three poles and two ze-
ros and they are expressed as follows:
FP1 = 0
FP2
=
1
2p3R83C10
( ) 1
FP3 =
2p3R73 C123C11
C12+C11
≅
1
2p3R73C12
FZ1 =
1
2p3R73C11
1
1
FZ2 = 2p3C103(R6 + R8) ≅ 2p3C103R6
Cross Over Frequency:
FO =
R73C103
VIN
VOSC
3
1
2p3Lo3Co
Where:
VIN = Maximum Input Voltage
VOSC = Oscillator Ramp Voltage
Lo = Output Inductor
Co = Total Output Capacitors
---(15)
The stability requirement will be satisfied by placing the
poles and zeros of the compensation network according
to following design rules. The consideration has been
taken to satisfy condition (14) regarding transconduc-
tance error amplifier.
1) Select the crossover frequency:
Fo < FESR and Fo [ (1/10 ~ 1/6)3 fS
2
2) Select R7, so that R7 >> gm
3) Place first zero before LC’s resonant frequency pole.
FZ1 ≅ 75% FLC
C11 =
1
2p 3 FZ1 3 R7
4) Place third pole at the half of the switching frequency.
FP3 =
fS
2
C12
=
2p
3
1
R7
3
FP3
C12 > 50pF
If not, change R7 selection.
5) Place R7 in (15) and calculate C10:
C10 [
2p 3 Lo 3 Fo 3 Co
R7
3
VOSC
VIN
Rev. 2.8
03/10/03
www.irf.com
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