Ap3440 – Diodes AP3440 User Manual
Page 11
AP3440
Document number: DS36691 Rev. 2 - 2
11 of 15
March 2014
© Diodes Incorporated
AP3440
A Product Line of
Diodes Incorporated
N
E
W
P
R
O
D
U
C
T
Application Note
(Cont.)
Output Capacitor
The output capacitor is the most critical component of a switching regulator. It is used for filtering output and keeping the loop stable. The typical
value is 44
μF.
The primary parameters for output capacitor are the voltage rating and the equivalent series resistance (ESR). A low ESR capacitor is preferred to
keep the output voltage ripple low. The output ripple is calculated as the following:
)
8
1
(
OUT
ESR
L
OUT
C
f
R
I
V
……………..(8)
Where f is the switching frequency, C
OUT
is the output capacitance and
△I
L
is the ripple current in the inductor.
Inductor
The inductor is used to supply smooth current to output when it is driven by a switching voltage. The higher the inductance, the lower the peak-to-
peak ripple current, as the higher inductance usually means the larger inductor size, so some trade-offs should be made when select an inductor.
The AP3440 is a synchronous buck converter. It always works on continuous current mode (CCM), and the inductor value can be selected as the
following:
)
(
k
I
V
f
V
V
V
L
OUT
IN
OUT
IN
OUT
…………………. ….(9)
Where V
OUT
is the output voltage, V
IN
is the input voltage, I
OUT
is the output current, k is the coefficient of ripple current, and its typical value is 20%
to 40%. Another important parameter for the inductor is the current rating. Exceeding an inductor's maximum current rating may cause the
inductor to saturate and overheat. If inductor value has been selected, the peak inductor current can be calculated as the following:
)
2
(
L
V
f
V
V
V
I
I
IN
OUT
IN
OUT
OUT
PEAK
…………..(10)
It should be ensured that the current rating of the selected inductor is 1.5 times of the I
PEAK
.
Slow Start Capacitor
The slow start capacitor determines the output voltage soft start time during power up.
The slow start capacitor value can be calculated using equation 11.
)
(
)
(
)
(
)
(
V
V
A
I
ms
t
nF
C
REF
SS
SS
SS
m
……………………… (11)
In AP3440, I
SS
is 2
μA and V
REF
is 0.803V.
Bootstrap Capacitor
A 0.1
μF ceramic capacitor must be connected between the BOOT pin and the SW pin for normal operation. It is recommended to use a ceramic
capacitor with X5R or better grade dielectric.
Feedback Resistors
It is recommended to use divider resistors with 1% tolerance or better. Start with a 10k
Ω for the R7 resistor and use the equation 12 to calculate
R6.
1
803
.
0
7
6
OUT
V
R
R
…………………………………(12)