APU3048M A-POWER [Advanced Power Electronics Corp.], APU3048M Datasheet - Page 8

• Manufacturer Part Number: APU3048M
• Description: DUAL SYNCHRONOUS PWM CONTROLLER CIRCUITRY AND LDO CONTROLLER
• Manufacturer: A-POWER [Advanced Power Electronics Corp.]

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).
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:
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.
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:
0dB
Gain
F
F
LC
ESR
=
Figure 5 - Gain and phase of LC filter.
=
2p
2p 3 ESR 3 Co
F
LC
Lo3Co
1
-40dB/decade
Frequency
1
-180
Phase
0
8
8
---(7)
---(8)
F
LC
Frequency
The transfer function (Ve / V
The (s) indicates that the transfer function varies as a
function of frequency. This configuration introduces a gain
and zero, expressed by:
The gain is determined by the voltage divider and E/A's
transconductance gain.
First select the desired zero-crossover frequency (Fo):
Use the following equation to calculate R4:
F
R
Where:
V
V
F
F
F
g
R
H(s) = g
|H(s)| =
F
Figure 6 - Compensation network without local
O1
O1
ESR1
LC1
Z
9
IN1
OSC
m = Error Amplifier Transconductance
8
=
and R
=
> F
= Crossover Frequency for the master E/A
feedback and its asymptotic gain plot.
= Maximum Input Voltage
= Resonant Frequency of Output Filter
2p 3 R
= Oscillator Ramp Voltage
V
= Zero Frequency of the Output Capacitor
V
OSC
(
ESR
IN1
V
g
6
OUT
m
m
H(s) dB
= Resistor Dividers for Output Voltage
3
and F
R
R
3
3
1
Programming
6
F
8
9
O1
V
R
3 C
R
F
REF
3F
Fb
Gain(dB)
6
O1
LC1
R
6
+ R
R
3 R
8
[ (1/5 ~ 1/10)3 f
18
ESR1
2
8
8
)
3
8
3
E/A
OUT
3 R
F
R
Z
) is given by:
8
1 + sR
---(11)
R
+ R
9
8
Frequency
Comp
sC
6
18
3
R
9
C
C
9
g
18
18
S
1
---(10)
m
V e
---(9)
---(12)
8