ADP2311ACPZ-2-R7 datasheet(17/20 Pages) AD

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Data Sheet

ADP2311

Rev. B | Page 17 of 20

APPLICATIONS INFORMATION

INPUT CAPACITOR SELECTION

The input capacitor reduces the input voltage ripple caused by

the switch current on PVINx. Place the input capacitor as close

as possible to the PVINx pin. A ceramic capacitor in the 10 μF

to 47 μF range is recommended. The loop composed of the input

capacitor, the high-side MOSFET, and the low-side MOSFET

must be kept as small as possible.

The voltage rating of the input capacitor must be greater than

the maximum input voltage. Ensure that the rms current rating

of the input capacitor is larger than the value calculated from

the following equation:

ICIN_RMS = IOUT ×

(

)

−

× 1

where D is the duty cycle (D = VOUT/VIN).

OUTPUT VOLTAGE SETTING

The output voltage of the ADP2311 can be set by an external

resistor divider using the following equation:

VOUT = 0.6 ×













BOT

TOP

To limit the output voltage accuracy degradation due to the FB

bias current (0.1 µA maximum) to less than 0.5% (maximum),

ensure that RBOT < 30 kΩ.

Table 5 lists the recommended resistor divider values for various

output voltages.

Table 5. Resistor Divider Values for Various Output Voltages

VOUT (V)

RTOP ± 1% (kΩ)

RBOT ± 1% (kΩ)

1.0

1.2

1.5

1.8

2.5

47.5

3.3

2.21

5.0

INDUCTOR SELECTION

The inductor value is determined by the operating frequency,

input voltage, output voltage, and inductor ripple current. Using

a small inductor value leads to a faster transient response, but

degrades efficiency due to a larger inductor ripple current. Using

a large inductor value leads to smaller ripple current and better

efficiency, but results in a slower transient response.

As a guideline, the inductor ripple current, ΔIL, is typically set

to one-third of the maximum load current. The inductor value

is calculated using the following equation:

L =

OUT

∆

−

)

(

where:

VIN is the input voltage.

VOUT is the output voltage.

D is the duty cycle (D = VOUT/VIN).

ΔIL is the inductor current ripple.

fSW is the switching frequency.

The peak inductor current is calculated by

IPEAK = IOUT +

∆

The saturation current of the inductor must be larger than the

peak inductor current. For ferrite core inductors with a quick

saturation characteristic, the saturation current rating of the

inductor must be higher than the current-limit threshold of the

switch to prevent the inductor from reaching saturation.

The rms current of the inductor is calculated using the follow-

ing equation:

IRMS =

OUT

∆

Shielded ferrite core materials are recommended for low core

loss and low EMI. Table 6 lists some recommended inductors.

Table 6. Recommended Inductors

Vendor

Part No.

Value

(µH)

ISAT (A)

IRMS (A)

DCR

(mΩ)

Sumida

CDRH8D58/

LDNP-100NC

2.2

4.5

20.5

CDRH8D58/

LDNP-150NC

1.9

3.6

CDRH8D58/

LDNP-220NC

1.4

3.3

36.2

Coilcraft

XAL6060-103ME

7.6

XAL6060-153ME

5.8

39.7

XAL6060-223ME

5.6

55.1

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