ADP121

Rev. 0 | Page 12 of 20

APPLICATIONS INFORMATION

CAPACITOR SELECTION

Output Capacitor

The ADP121 is designed for operation with small, space-saving

ceramic capacitors, but functions with most commonly used

capacitors as long as care is taken with the effective series resistance

(ESR) value. The ESR of the output capacitor affects stability of the

LDO control loop. A minimum of 0.70 μF capacitance with an

ESR of 1 Ω or less is recommended to ensure stability of the

ADP121. The 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 ADP121 to

large changes in the load current. Figure 28 and Figure 29 show

the transient responses for output capacitance values of 1 μF and

4.7 μF, respectively.

CH1 MEAN

115.7mA

(400ns/DIV)

1mA TO 150mA LOAD STEP,

2.5A/µs

(400ns/DIV)

1mA TO 150mA LOAD STEP,

2.5A/µs

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

is encountered. If output capacitance greater than 1 μF is

required, the input capacitor should be increased to match it.

Input and Output Capacitor Properties

Any good quality ceramic capacitor can be used with the

ADP121, as long as it meets the minimum capacitance and

maximum ESR requirements. Ceramic capacitors are manufac-

tured with a variety of dielectrics, each with a different behavior

over temperature and applied voltage. Capacitors must have an

adequate dielectric 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. Y5V and Z5U dielectrics are not recommended,

due to their poor temperature and dc bias characteristics.

Figure 30 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 about ±15% over the −40°C to +85°C tempera-

ture 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

468

VOLTAGE (V)

10

Figure 30. Capacitance vs. Voltage Bias Characteristic