ADP170/ADP171

Rev. B | Page 12 of 20

APPLICATIONS INFORMATION

CAPACITOR SELECTION

Output Capacitor

The ADP170/ADP171 are designed for operation with small,

space-saving ceramic capacitors but will 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 1 μF capacitance with an ESR of 1 Ω or less is

recommended to ensure stability of the ADP170/ADP171. 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 ADP170/ADP171 to

large changes in load current. Figure 27 and Figure 28 show the

transient responses for output capacitance values of 1 μF and

4.7 μF, respectively.

B

W

M200ns

A CH1

112mA

1

2

T

500.000ns

1mA TO 300mA LOAD STEP,

2.5A/µs

B

W

M200ns

A CH1

108mA

1

2

T

500.000ns

1mA TO 300mA 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

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 capacitor can be used with the

ADP170/ADP171, as long as it meets 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. A X5R or X7R

dielectric with a voltage rating of 6.3 V or 10 V is recommended.

The Y5V and Z5U dielectrics are not recommended, due to their

poor temperature and dc bias characteristics.

Figure 29 depicts the capacitance vs. bias voltage characteristics

of a 0402, 1 μF, 10 V X5R capacitor. The variance 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

will exhibit less capacitance variance over bias voltage. 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.

1.2

1.0

0.8

0.6

0.4

0.2

0

02

46

8

BIAS VOLTAGE (V)

10

Figure 29. Capacitance vs. Bias Voltage Characteristics

Use Equation 1 to determine the worst-case capacitance

accounting for capacitor variation over temperature,

component tolerance, and voltage.

(1)

where:

TEMPCO is the worst-case capacitor temperature coefficient.

TOL is the worst-case component tolerance.