APU3073 View Datasheet(PDF) - Advanced Power Electronics Corp
Part Name
Description
Manufacturer
APU3073
Advanced Power Electronics Corp
APU3073 Datasheet PDF : 17 Pages
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APU3073
VOUT
R6 Fb
R5
Vp=VREF
E/A
Comp
Ve
C9
R4 CPOLE
Gain(dB)
H(s) dB
FZ Frequency
Figure 11 - Compensation network without local
feedback and its asymptotic gain plot.
The transfer function (Ve / VOUT) is given by:
( ) H(s) =
gm3
R6
R5
+
R5
3
1 + sR4C9
sC9
---(15)
For:
VIN = 5V
VOSC = 1.25V
Fo = 20KHz
FESR = 12KHz
FLC = 3.41KHz
R5 = 1K
R6 = 2.15K
gm = 700mmho
This results to R4=23.14K
Choose R4=24K
To cancel one of the LC filter poles, place the zero be-
fore the LC filter resonant frequency pole:
FZ ≅ 75%FLC
FZ ≅ 0.753
2p
For:
Lo = 3.3mH
Co = 660mF
1
LO 3 CO
---(19)
FZ = 2.5KHz
R4 = 24K
Using equations (17) and (19) to calculate C9, we get:
C9 ≅ 2590pF; Choose C9=2200pF
The (s) indicates that the transfer function varies as a
function of frequency. This configuration introduces a gain
and zero, expressed by:
|H(s=j32p3FO)|
=
gm3
R5
R63R5
3R4
---(16)
FZ
=
1
2p3R43C9
---(17)
One more capacitor is sometimes added in parallel with
C9 and R4. This introduces one more pole which is mainly
used to suppress the switching noise. The additional
pole is given by:
1
FP
=
2p3R43
C93CPOLE
C9 + CPOLE
|H(s)| is the gain at zero cross frequency.
First select the desired zero-crossover frequency (Fo):
Fo > FESR and FO [ (1/5 ~ 1/10)3fS
Use the following equation to calculate R4:
R4 = VOSC
VIN
3Fo3FESR 3 R5 + R63
FLC2
R5
1
gm
---(18)
Where:
VIN = Maximum Input Voltage
VOSC = Oscillator Ramp Voltage
Fo = Crossover Frequency
FESR = Zero Frequency of the Output Capacitor
FLC = Resonant Frequency of the Output Filter
R5 and R6 = Resistor Dividers for Output Voltage
Programming
gm = Error Amplifier Transconductance
The pole sets to one half of switching frequency which
results in the capacitor CPOLE:
1
CPOLE =
p3R43fS -
1
≅
1
p3R43fS
C9
for FP <<
fS
2
For a general solution for unconditionally stability for
ceramic capacitor with very low ESR and any type of
output capacitors, in a wide range of ESR values we
should implement local feedback with a compensation
network. The typically used compensation network for
voltage-mode controller is shown in Figure 12.
11/17
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