ADP7102ACPZ-5.0-R7 datasheet(18/28 Pages) AD

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ADP7102

Data Sheet

Rev. A | Page 18 of 28

APPLICATIONS INFORMATION

CAPACITOR SELECTION

Output Capacitor

The ADP7102 is designed for operation with small, space-

saving ceramic capacitors but functions with most commonly

used capacitors as long as care is taken with regard to the

effective series resistance (ESR) value. The ESR of the out-

put capacitor affects the stability of the LDO control loop. A

minimum of 1 µF capacitance with an ESR of 0.2 Ω or less is

recommended to ensure the stability of the ADP7102. 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 ADP7102 to large changes in load

current. Figure 63 shows the transient responses for an output

capacitance value of 1 µF.

CH2 50mV

CH1 200mA

M 20µs

A CH1

76mA

T 10.4%

Ω

LOAD CURRENT

OUTPUT VOLTAGE

Figure 63. Output Transient Response, VOUT = 1.8 V, COUT = 1 µF

Input Bypass Capacitor

Connecting a 1 µF capacitor from VIN to GND reduces

the circuit sensitivity to 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

ADP7102, as long as they meet the minimum capacitance and

maximum ESR requirements. Ceramic capacitors are manufac-

tured 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 to 50 V are

recommended. Y5V and Z5U dielectrics are not recommended,

due to their poor temperature and dc bias characteristics.

Figure 64 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

VOLTAGE (V)

Figure 64. Capacitance vs. Voltage Characteristic

Use Equation 1 to determine the worst-case capacitance

accounting for capacitor variation over temperature,

component tolerance, and voltage.

CEFF = CBIAS × (1 − TEMPCO) × (1 − TOL)

(1)

where:

CBIAS 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 CBIAS is 0.94 μF at 1.8 V, as shown

in Figure 64.

Substituting these values in Equation 1 yields

CEFF = 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 temper-

ature and tolerance at the chosen output voltage.

To guarantee the performance of the ADP7102, it is imperative

that the effects of dc bias, temperature, and tolerances on the

behavior of the capacitors be evaluated for each application.

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