Ap6503, Application information – Diodes AP6503 User Manual
Page 10
AP6503
Document number: DS35077 Rev. 5 - 2
10 of 15
January 2013
© Diodes Incorporated
AP6503
Application Information
(cont.)
Compensation Components (cont.)
The control loop transfer function incorporates two poles one is due to the compensation capacitor (C3) and the output resistor of error amplifier,
and the other is due to the output capacitor and the load resistor. These poles are located at:
VEA
A
3
C
2
EA
G
P1
f
Ч
Ч
π
=
LOAD
R
2
C
2
1
P2
f
Ч
Ч
π
=
Where G
EA
is the error amplifier trans-conductance.
One zero is present due to the compensation capacitor (C3) and the compensation resistor (R3). This zero is located at:
3
R
3
C
2
1
Z1
f
Ч
Ч
π
=
The goal of compensation design is to shape the converter transfer function to get a desired loop gain. The system crossover frequency where
the feedback loop has the unity gain is crucial.
A rule of thumb is to set the crossover frequency to below one-tenth of the switching frequency. Use the following procedure to optimize the
compensation components:
1. Choose the compensation resistor (R3) to set the desired crossover frequency. Determine the R3 value by the following equation:
FB
V
OUT
V
CS
G
EA
G
fs
1
.
0
2
C
2
FB
V
OUT
V
CS
G
EA
G
fc
2
C
2
3
R
Ч
Ч
Ч
Ч
Ч
π
<
Ч
Ч
Ч
Ч
π
=
Where f
C
is the crossover frequency, which is typically less than one tenth of the switching frequency.
2. Choose the compensation capacitor (C3) to achieve the desired phase margin set the compensation zero, f
Z1
, to below one forth of the
crossover frequency to provide sufficient phase margin. Determine the C3 value by the following equation:
fc
3
R
2
3
C
Ч
Ч
π
>
Where R3 is the compensation resistor value.
V
OUT
(V)
C
IN
/C1
(µF)
C
OUT
/C2
(µF)
R
C
/R3
(kΩ)
C
C
/C3
(nF)
L1
(µH)
1.2 22 47 3.24 6.8 3.3
1.8 22 47 6.8 6.8 3.3
2.5 22 47 6.8 6.8 10
3.3 22 47 6.8 6.8 10
5 22 47 6.8 6.8 10
12 22 47 6.8 6.8 15
Table 2 – Recommended Component Selection