RT8509A
9
DS8509A-00 November 2013
www.richtek.com
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Copyright 2013 Richtek Technology Corporation. All rights reserved. is a registered trademark of Richtek Technology Corporation.
where
η
is the efficiency of the converter, I
IN(MAX)
is the
maximum input current, and I
RIPPLE
is the inductor ripple
current. The input peak current can then be obtained by
adding the maximum input current with half of the inductor
ripple current as shown in the following equation :
PEAK
IN(MAX)
I
1.2 x I
Note that the saturated current of the inductor must be
greater than I
PEAK
. The inductance can eventually be
determined according to the following equation :
2
IN
OUT
IN
2
OUT
OUT(MAX)
OSC
x (V ) x(V
V )
L
0.4 x (V
) xI
x f
where f
osc
is the switching frequency. For better system
performance, a shielded inductor is preferred to avoid EMI
problems.
Diode Selection
Schottky diodes are chosen for their low forward voltage
drop and fast switching speed. When selecting a Schottky
diode, important parameters such as power dissipation,
reverse voltage rating, and pulsating peak current should
all be taken into consideration. A suitable Schottky diode's
reverse voltage rating must be greater than the maximum
output voltage and its average current rating must exceed
the average output current. Last of all, the chosen diode
should have a sufficiently low leakage current level, since
it will increase with temperature.
Output Capacitor Selection
The output ripple voltage is an important index for
estimating chip performance. This portion consists of two
parts. One is the product of the inductor current with the
ESR of the output capacitor, while the other part is formed
by the charging and discharging process of the output
OUT
OUT(MAX)
IN(MAX)
IN
RIPPLE
IN(MAX)
V
x I
I
=
x V
I
= 0.4 x I
Inductor Selection
The inductance depends on the maximum input current.
As a general rule, the inductor ripple current range is 20%
to 40% of the maximum input current. If 40% is selected
as an example, the inductor ripple current can be
calculated according to the following equations :
IN
L
OUT
IN
L
OUT
IN
OUT
OUT1
OUT
OSC
1
1
1
Q
x
I
I
I
I
I
I
2
2
2
V
1
x
x
C
x V
V
f
where f
OSC
is the switching frequency, and
Δ
I
L
is the
inductor ripple current. Bring C
OUT
to the left side to
estimate the value of
Δ
V
OUT1
according to the following
equation :
OUT
OUT1
OUT
OSC
D x I
V
x C
x f
where D is the duty cycle and
η
is the Boost converter
efficiency. Finally, taking ESR into account, the overall
output ripple voltage can be determined by the following
equation :
OUT
OUT
IN
OUT
OSC
D x I
V
I x ESR
x C
x f
The output capacitor, C
OUT
, should be selected accordingly.
Time
Time
Inductor Current
Output Current
Output Ripple
Voltage (ac)
(1-D)T
S
Δ
V
OUT1
Δ
I
L
Input Current
Figure 1. The Output Ripple Voltage without the
Contribution of ESR
Input Capacitor Selection
Low ESR ceramic capacitors are recommended for input
capacitor applications. Low ESR will effectively reduce
the input voltage ripple caused by switching operation. A
10
μ
F capacitor is sufficient for most applications.
capacitor. As shown in Figure 1,
Δ
V
OUT1
can be evaluated
based on the ideal energy equalization. According to the
definition of Q, the Q value can be calculated as the
following equation :
