Ap5101, New prod uc t applications information – Diodes AP5101 User Manual
Page 10
AP5101
1.5A Step-Down Converter with 1.4MHz Switching
Frequency
AP5101
Document number: DS32258 Rev. 1 - 2
10 of 15
July 2010
© Diodes Incorporated
NEW PROD
UC
T
Applications Information
(Continued)
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-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.
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
Ч
Ч
−
Ч
=
Where
L
ΔI
is the inductor ripple current.
And
f
sw
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
+
=
Peak current determines the required saturation current rating, which influences the size of the inductor. Saturating the
inductor decreases the converter efficiency while increasing the temperatures of the inductor, the MOSFET and the diode.
Hence choosing an inductor with appropriate saturation current rating is important.
A 1µH to 10µH inductor with a DC current rating of at least 25% percent higher than the maximum load current is
recommended for most applications.
For highest efficiency, the inductor’s DC resistance should be less than 200m
Ω. Use a larger inductance for improved
efficiency under light load conditions.