New prod uc t ap6502, Applications information – Diodes AP6502 User Manual
Page 10
AP6502
Document Number: DS35423 Rev. 9 - 2
10 of 15
www.diodes.com
January 2013
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NEW PROD
UC
T
AP6502
Applications 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
EA
G
fs
1
.
0
2
C
2
FB
V
OUT
V
CS
G
EA
G
fc
2
C
2
3
R
CS
G
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 fourth 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
Inductor
Calculating the inductor value is a critical factor in designing a buck converter. For most designs, the following equation can be used to
calculate the inductor value;
SW
f
L
ΔI
IN
V
)
OUT
V
IN
(V
OUT
V
L
⋅
⋅
−
⋅
=
Where
L
ΔI
is the inductor ripple current.
And
SW
f
is the buck converter switching frequency.
Choose the inductor ripple current to be 30% of the maximum load current. The maximum inductor peak current is calculated from:
2
L
ΔI
LOAD
I
L(MAX)
I
+
=