Datasheet

ADP222/ADP223/ADP224/ADP225 Data Sheet
Rev. D | Page 18 of 24
APPLICATIONS INFORMATION
CAPACITOR SELECTION
Output Capacitor
The ADP222/ADP223/ADP224/ADP225 are designed for
operation with small, space-saving ceramic capacitors but
function with most commonly used capacitors as long as care is
taken with regard to the effective series resistance (ESR) value.
The ESR of the output capacitor affects the stability of the LDO
control loop. A minimum of 0.7 µF capacitance with an ESR of
1 Ω or less is recommended to ensure the stability of the ADP222/
ADP223/ADP224/ADP225. 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 ADP222/ADP223/ADP224/ADP225 to large changes in
load current. Figure 65 shows the transient responses for an
output capacitance value of 1 µF.
A CH1 200mA
1
M10µs
T 10.20%
3
2
CH1 200mA
CH3 10mV
CH2 50mV
B
W
B
W
Ω
B
W
09376-043
V
OUT2
V
OUT1
LOAD CURRENT
ON V
OUT1
Figure 65. Output Transient Response, C
OUT
= 1 µF
Input Bypass Capacitor
Connecting a 1 µF capacitor from VIN to GND reduces the
circuit sensitivity to the printed circuit board (PCB) layout,
especially when long input traces or high source impedance
are encountered. If greater than 1 µF of output capacitance is
required, the input capacitor should be increased to match it.
Input and Output Capacitor Properties
Any good quality ceramic capacitors can be used with the
ADP222/ADP223/ADP224/ADP225, as long as they meet the
minimum capacitance and maximum ESR requirements.
Ceramic capacitors are manufactured 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 dielectrics with a
voltage rating of 6.3 V or 10 V are recommended, but Y5V and
Z5U dielectrics are not recommended, due to their poor
temperature and dc bias characteristics.
Figure 66 depicts the capacitance vs. voltage bias characteristic
of an 0402, 1 µF, 10 V, X5R capacitor. 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 ~±15% over the −40°C to +85°C temperature
range and is not a function of package or voltage rating.
1.2
1.0
0.8
0.6
0.4
0.2
0
0 2 4 6 8 10
CAPACITANCE (µF)
VOLTAGE (V)
09376-044
Figure 66. Capacitance vs. Voltage Bias Characteristic
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 temperature coefficient (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 0.94 µF at 1.8 V, as shown in Figure 66.
Substituting these values in Equation 1 yields
C
EFF
= 0.94 µF × (1 − 0.15) × (1 − 0.1) = 0.719 µF
Therefore, the capacitor chosen in this example meets the
minimum capacitance requirement of the LDO over temperature
and tolerance at the chosen output voltage.
To guarantee the performance of the ADP222/ADP223/
ADP224/ADP225, it is imperative that the effects of dc bias,
temperature, and tolerances on the behavior of the capacitors
be evaluated for each application.