ADP1864

Rev. 0 | Page 11 of 16

MOSFET

Choose the external P-channel MOSFET based on the following:

Vt (threshold voltage), maximum voltage and current ratings,

The minimum operating voltage of the ADP1864 is 3.15 V.

Choose a MOSFET with a Vt that is at least 1 V lower than the

minimum input supply voltage used in the application.

a few volts greater than the maximum input voltage used with

the ADP1864.

Estimate the rms current in the MOSFET under continuous

conduction mode by

()

(

)

()

LOAD

D

IN

D

OUT

RMS

MOSFET

I

V

V

V

V

I

×

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

+

=

Derate the MOSFET current by at least 20% to account for

inductor ripple and changes in the diode voltage.

The MOSFET power dissipation is the sum of the conducted

and the switching losses:

()

(

)

(

2

)

(

1

ON

DS

RMS

MOSFET

COND

MOSFET

R

T

I

PD

×

+

×

=

where T is 0.005/˚C × (MOSFET Junction Temperature − 25˚C).

Ensure the maximum power dissipation calculated is signif-

icantly less than the maximum rating of the MOSFET.

DIODE

The diode carries the inductor current during the off time of

the external FET. The average current of the diode is, therefore,

dependent on the duty cycle of the controller as well as the

output load current.

()

()

()

LOAD

D

IN

D

OUT

AV

DIODE

I

V

V

V

V

I

×

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

+

−

= 1

has a 0.5 V forward drop.

A Schottky diode is recommended for best efficiency because

it has a low forward drop and faster switching speed than

junction diodes. If a junction diode is used it must be an

ultrafast recovery diode. The low forward drop reduces the

power losses during the FET off time, and fast switching speed

reduces the switching losses during PFET transitions.

INPUT CAPACITOR

The input capacitor provides a low impedance path for the

pulsed current drawn by the external P-channel FET. Choose an

input capacitor whose impedance at the switching frequency is

preferred input capacitor is a 10μF ceramic capacitor due to its

low ESR and low impedance.

For all types of capacitors, make sure the ripple current rating of

the capacitor is greater than half of the maximum output load

current.

Where space is limited, multiple capacitors can be placed in

parallel to meet the rms current requirement. Place the input

capacitor as close as possible to the IN pin of the ADP1864.

OUTPUT CAPACITOR

The ESR and capacitance value of the output capacitor

determine the amount of output voltage ripple:

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

×

×

×

Δ

≅

Δ

OUT

C

OUT

ESR

C

f

I

V

8

1

where f = oscillator frequency (typically 580 kHz).

Because the output capacitance is typically >40 μF, the ESR

dominates the voltage ripple. Ensure the output capacitor ripple

rating is greater than the maximum inductor ripple.

() (

)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

×

×

−

×

+

×

×

≅

IN

OUT

IN

D

OUT

rms

V

f

L

V

V

V

V

I

3

2

1

POSCAP capacitors from Sanyo offer a good size, ESR, ripple,

and current capability trade-off.