ADP2370ACPZ-3.3-R7 datasheet(28/32 Pages) AD

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ADP2370/ADP2371

Data Sheet

Rev. A | 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

10.40%

CH1 500mA Ω BW

CH3 500mA

Ω BW

CH2 50.0mV

LOAD CURRENT

VOUT

INDUCTOR CURRENT

Figure 84. Output Transient Response, VOUT2 = 1.8 V, COUT = 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.

DC BIAS VOLTAGE (V)

10µF/25V/1210

10µF/35V/1210

10µF/16V/0805

10µF/16V/1206

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.

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 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 8.53 μF at 12 V

for the 10 μF, 35 V capacitor in a 1210 package (see Figure 85).

Substituting these values in Equation 1 yields

CEFF = 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.

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