Diodes AP65400 - Manual (Page 10)

AP65400
Document number: DS36497 Rev. 1 - 2
10 of 15
www.diodes.com
April 2014
© Diodes Incorporated
AP65400
Application Information
(cont.)
Compensation Components
The AP65400 has an external COMP pin through which system stability and transient response can be controlled. COMP pin is the output of the
internal trans-conductance error amplifier. A series capacitor-resistor combination sets a pole-zero combination to control the characteristics of
the control system. The DC gain of the voltage feedback loop is given by:
OUT
FB
VEA CS LOAD VDC
V
V
A G R A Ч Ч Ч =
Where V
FB
is the feedback voltage (0.800V), R
LOAD
is the load resistor value, G
CS
is the current sense trans-conductance and A
VEA
is the error
amplifier voltage gain. 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
EA
P1
A 3 C 2
G
f
Ч Ч π
=
LOAD
P2
R 2 C 2
1
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
f
Z1 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
OUT
CS G
EA
FB
OUT
CS EA
V
V
G
fs 1 . 0 2 C 2
V
V
G G
fc 2 C 2
3 R 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.
VOUT
(V)
Cin/C1
(µF)
Cout/C2
(µF)
Rc/R3
(kΩ)
Cc/C3
(nF)
L1
(µH)
1.2 44 72 3.24 6.8 3.3
1.8 44 72 6.8 6.8 3.3
2.5 44 72 6.8 6.8 10
3.3 44 72 6.8 6.8 10
5 44 72 6.8 6.8 10
12 44 72 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 L IN
OUT IN OUT
f ∆I V
) V (V V
L
⋅ ⋅
− ⋅
=
Where
L
∆I is the inductor ripple current.
And
SW
f is the buck converter switching frequency.