Datasheet

Data Sheet ADP1607
Rev. C | Page 13 of 16
CHOOSING THE INPUT CAPACITOR
The ADP1607 requires a 10 µF or greater input bypass capacitor
(C
IN
) between VIN and GND to supply transient currents while
maintaining a constant input voltage. The value of the input
capacitor can be increased without any limit for smaller input
voltage ripple and better input voltage filtering. The capacitor must
have a 4 V or higher voltage rating to support the maximum
input operating voltage. It is recommended that C
IN
be placed as
close to the ADP1607 as possible.
Different types of capacitors can be considered, but for battery-
powered applications, the best choice is the multilayer ceramic
capacitor, due to its small size, low equivalent series resistance
(ESR), and low equivalent series inductance (ESL). X5R or X7R
dielectrics are recommended. Y5V capacitors should not be used
due to their variation in capacitance over temperature. Alterna-
tively, use a high value, medium ESR capacitor in parallel with a
0.1 µF low ESR capacitor.
CHOOSING THE OUTPUT CAPACITOR
The ADP1607 also requires a 10 µF output capacitor (C
OUT
) to
maintain the output voltage and supply current to the load. The
output capacitor supplies the current to the load when the N-
channel switch is turned on. Similar to C
IN
, a 4 V or greater, low
ESR, X5R or X7R ceramic capacitor is recommended for C
OUT
.
When choosing the output capacitor, it is also important to account
for the loss of capacitance due to output voltage dc bias. This
may result in using a capacitor with a higher rated voltage to
achieve the desired capacitance value. See Figure 25 for an
example of how the capacitance of a 10 µF ceramic capacitor
changes with the dc bias voltage.
0
2
4
6
8
10
12
0 1 2 3 4 5 6
10276-034
DC BIAS VOLTAGE (V)
CAPACITANCE (µF)
Figure 25. Typical Ceramic Capacitor Performance
The value and characteristics of the output capacitor greatly
affect the output voltage ripple, transient performance, and
stability of the regulator. The output voltage ripple (∆V
OUT
) in
continuous operation is calculated as follows:
OUT
ON
OUT
OUT
C
OUT
C
t
I
C
Q
V
×
==
∆
(4)
where:
Q
C
is the charge removed from the capacitor.
t
ON
is the on time of the N-channel switch.
C
OUT
is the effective output capacitance.
I
OUT
is the output load current.
SW
ON
f
D
t =
(5)
and,
OUT
IN
OUT
V
VV
D
−
=
(6)
As shown in the duty cycle and output ripple voltage equations,
the output voltage ripple increases with the load current.