ADP1851

Data Sheet

Rev. B | Page 16 of 24

VIN PIN FILTER

It is recommended that a low-pass filter be connected to the

VIN pin. Connecting a resistor, between 2 Ω and 10 Ω, in series

with VIN and a 1 µF ceramic capacitor between VIN and AGND

creates a low-pass filter that effectively filters out any unwanted

glitches caused by the switching regulator. Keep in mind that the

input current may be larger than 100 mA when driving large

MOSFETs. A 100 mA current across a 10 Ω resistor creates a

1 V drop, which is the same voltage drop in VCCO. In this

case, a lower resistor value is desirable.

VIN

AGND

2

Ω TO 10Ω

1µF

ADP1851

Figure 25. Input Filter Configuration

BOOST CAPACITOR SELECTION

Connect a boost capacitor between the SW and BST pins to

provide the current for the high-side driver during switching.

Choose a ceramic capacitor with a value between 0.1 µF and

0.22 µF.

INDUCTOR SELECTION

For most applications, choose an inductor value such that

the inductor ripple current is between 20% and 40% of the

maximum dc output load current.

Choose the inductor value using the following equation:

IN

OUT

L

SW

OUT

IN

V

V

I

f

V

V

L

×

∆

×

−

=

where:

L is the inductor value.

Check the inductor data sheet to make sure that the saturation

current of the inductor is well above the peak inductor current

of a particular design.

OUTPUT CAPACITOR SELECTION

For maximum allowed switching ripple at the output, choose an

output capacitor that is larger than

)

)

4

(

(

1

8

2

2

2

2

ESL

SW

ESR

L

OUT

SW

L

OUT

L

f

R

I

V

f

I

C

×

−

×

∆

−

∆

×

∆

≅

where:

(or the parallel combined ESR of all output capacitors).

(or the parallel combined ESL of all capacitors).

The impedance of the output capacitor at the switching

frequency multiplied by the ripple current gives the output

voltage ripple. The impedance is made up of the capacitive

impedance plus the nonideal parasitic characteristics, the

equivalent series resistance (ESR), and the equivalent series

inductance (ESL).

Usually the capacitor impedance is dominated by ESR. The

maximum ESR rating of the capacitor, such as in electrolytic

or polymer capacitors, is provided in the manufacturer’s data

sheet; therefore, the output ripple reduces to

ESR

L

OUT

R

I

V

×

∆

≅

∆

Electrolytic capacitors also have significant ESL, on the order

of 5 nH to 20 nH, depending on type, size, and geometry. PCB

traces contribute some ESR and ESL as well. However, using the

maximum ESR rating from the capacitor data sheet usually

provides some margin such that measuring the ESL may not be

required.

In the case of output capacitors where the impedances of the

ESR and ESL are small at the switching frequency, for example,

where the output capacitor is a bank of parallel MLCC capaci-

tors, the capacitive impedance dominates; therefore, the output

capacitance must be larger than

SW

OUT

L

OUT

f

V

I

C

×

∆

∆

≅

8

(1)

Make sure that the ripple current rating of the output capacitors

is greater than the maximum inductor ripple current.

To meet the requirement of the output voltage overshoot during

load release, the output capacitance should be larger than

2

2

2

)

(

OUT

OVERSHOOT

OUT

STEP

OUT

V

V

V

L

I

C

−

∆

+

∆

≅

(2)

where:

Select the largest output capacitance given by either Equation 1

or Equation 2.