Datasheet

ADP2370/ADP2371 Data Sheet
Rev. C | Page 28 of 32
CAPACITOR SELECTION
Output Capacitor
The ADP2370/ADP2371 are designed for operation with small,
space-saving ceramic capacitors, but function with most commonly
used capacitors provided that the effective series resistance (ESR)
value is carefully considered. The ESR of the output capacitor
affects stability of the control loop. A minimum output capaci-
tance of 7 µF with an ESR of 10 mΩ or less is recommended to
ensure stability of the ADP2370/ADP2371.
Transient response to changes in load current is also affected by
output capacitance. Using a larger value of output capacitance
improves the transient response of the ADP2370/ADP2371 to
large changes in load current. Figure 84 shows the transient
response for an output capacitance value of 10 µF.
M20.0µs A CH1 560mA
1
2
3
T 10.40%
09531-084
CH1 500mA Ω
B
W
CH3 500mA Ω
B
W
CH2 50.0mV
B
W
LOAD CURRENT
V
OUT
INDUCTOR CURRENT
Figure 84. Output Transient Response, V
OUT2
= 1.8 V, C
OUT
= 10 µF,
300 mA to 800 mA, Load Current Rise Time = 200 ns
Input Bypass Capacitor
Connecting a 10 µF capacitor from VIN to GND reduces the
circuit sensitivity to the PCB layout, especially when long input
traces or high source impedance are encountered. If greater than
10 µF of output capacitance is required, increase the input
capacitor to match it to improve the transient response.
Input and Output Capacitor Properties
Use any good quality ceramic capacitors with the ADP2370/
ADP2371; however they must meet the minimum capacitance
and maximum ESR requirements. Ceramic capacitors are manu-
factured with a variety of dielectrics, each with different behavior
over temperature and applied voltage. Capacitors must have a
dielectric adequate to ensure the minimum capacitance over the
necessary temperature range and dc bias conditions. X5R or X7R
dielectric capacitors with a voltage rating of 6.3 V to 25 V are
recommended for best performance. Y5V and Z5U dielectrics
are not recommended because of their poor temperature and dc
bias characteristics.
Figure 85 depicts the capacitance vs. voltage bias characteristic
of a several 10 µF capacitors in different case sizes and voltage
ratings. The voltage stability of a capacitor is strongly influenced
by the capacitor size and voltage rating. In general, a capacitor in a
larger package or higher voltage rating exhibits better stability.
The temperature variation of the X5R dielectric is about ±15%
over the −40°C to +85°C temperature range and is not a
function of package or voltage rating.
0
1
2
3
4
5
6
7
8
9
10
11
12
0 5 10 15 20 25 30 35
CAPACITANCE (µF)
DC BIAS VOLTAGE (V)
10µF/25V/1210
10µF/35V/1210
10µF/16V/0805
10µF/16V/1206
09531-085
Figure 85. Capacitance vs. Voltage Characteristic Different Case Sizes
Use Equation 1 to determine the worst-case capacitance,
accounting for capacitor variation over temperature,
component tolerance, and voltage.
C
EFF
= C
BIAS
× (1 − TEMPCO) × (1 − TOL) (1)
where:
C
BIAS
is the effective capacitance at the operating voltage.
TEMPCO is the worst-case capacitor temperature coefficient.
TOL is the worst-case component tolerance.
In this example, the worst-case TEMPCO over −40°C to +85°C
is assumed to be 15% for an X5R dielectric. The tolerance of the
capacitor (TOL) is assumed to be 10%, and C
BIAS
is 8.53 μF at 12 V
for the 10 μ F, 35 V capacitor in a 1210 package (see Figure 85).
Substituting these values in Equation 1 yields
C
EFF
= 8.53 μF × (1 − 0.15) × (1 − 0.1) = 6.53 μF
Therefore, the capacitor chosen in this example meets the
minimum capacitance requirement of the ADP2370/ADP2371
over temperature and tolerance at the chosen output voltage.
To guarantee the performance of the ADP2370/ADP2371, it is
imperative that the effects of dc bias, temperature, and
tolerances of the capacitors are evaluated for each application.