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APU3048O View Datasheet(PDF) - Advanced Power Electronics Corp

Part Name
Description
Manufacturer
APU3048O
A-POWER
Advanced Power Electronics Corp A-POWER
APU3048O Datasheet PDF : 17 Pages
1 2 3 4 5 6 7 8 9 10 Next Last
APU3048
Feedback Compensation
The APU3048 is a voltage mode controller; the control
loop is a single voltage feedback path including error
amplifier and error comparator. To achieve fast transient
response and accurate output regulation, a compensa-
tion circuit is necessary. The goal of the compensation
network is to provide a closed loop transfer function with
the highest 0dB crossing frequency and adequate phase
margin (greater than 458).
VOUT
R6 Fb
R8
Comp
E/A
Ve
C18
VREF
R9
Gain(dB)
The output LC filter introduces a double pole, –40dB/
decade gain slope above its corner resonant frequency,
and a total phase lag of 1808 (see Figure 5). The Reso-
nant frequency of the LC filter is expressed as follows:
1
FLC =
2p Lo3Co
---(7)
H(s) dB
FZ Frequency
Figure 6 - Compensation network without local
feedback and its asymptotic gain plot.
Figure 5 shows gain and phase of the LC filter. Since we
already have 1808 phase shift just from the output filter,
the system risks being unstable.
Gain
0dB
Phase
08
-40dB/decade
The transfer function (Ve / VOUT) is given by:
( ) H(s) = gm 3 R8 3 1 + sR9C18
R6 + R8
sC18
---(9)
The (s) indicates that the transfer function varies as a
function of frequency. This configuration introduces a gain
and zero, expressed by:
-1808
FLC Frequency
FLC Frequency
Figure 5 - Gain and phase of LC filter.
The APU3048's error amplifier is a differential-input
transconductance 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 Comp1 pin to ground as shown in Figure 6.
The ESR zero of the LC filter expressed as follows:
FESR
=
2p
1
3 ESR
3 Co
---(8)
|H(s)| = gm 3
R6
R8
3
R8
3
R9
FZ
=
2p
3
1
R9
3
C18
---(11)
---(10)
The gain is determined by the voltage divider and E/A's
transconductance gain.
First select the desired zero-crossover frequency (Fo):
FO1 > FESR and FO1 [ (1/5 ~ 1/10)3 fS
Use the following equation to calculate R4:
g R9 =
VOSC
VIN1
3FO1F3LCF1E2SR1
3
R8 +
R8
R6
3
1
m
---(12)
Where:
VIN1 = Maximum Input Voltage
VOSC = Oscillator Ramp Voltage
FO1 = Crossover Frequency for the master E/A
FESR1 = Zero Frequency of the Output Capacitor
FLC1 = Resonant Frequency of Output Filter
gm = Error Amplifier Transconductance
R8 and R6 = Resistor Dividers for Output Voltage
Programming
8
 

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