derived from the following equation:

Keep the inductor current ripple percentage LIR

between 20% and 40% of the maximum load current for

best compromise of cost, size, and performance. The

maximum inductor current is:

Input Capacitor

The input filter capacitor reduces peak currents drawn

from the power source and reduces noise and voltage

ripple on the input caused by the circuit’s switching.

The input capacitor must meet the ripple current

defined by the following equation:

A ceramic capacitor is recommended due to its low

equivalent series resistance (ESR), equivalent series

inductance (ESL), and lower cost. Choose a capacitor

that exhibits less than a 10°C temperature rise at the

maximum operating RMS current for optimum long-term

reliability.

Output Capacitor

The key selection parameters for the output capacitor

are its capacitance, ESR, ESL, and the voltage rating

requirements. These affect the overall stability, output

ripple voltage, and transient response of the DC-DC

converter.

The output ripple is due to variations in the charge

stored in the output capacitor, the voltage drop due to

the capacitor’s ESR, and the voltage drop due to the

capacitor’s ESL.

The output voltage ripple due to the output capaci-

tance, ESR, and ESL is:

ESL, whichever is greater.

These equations are suitable for initial capacitor selec-

tion, but final values should be set by testing a proto-

type or evaluation circuit. As a rule, a smaller ripple

current results in less output voltage ripple. Since the

inductor ripple current is a factor of the inductor value,

the output voltage ripple decreases with larger induc-

tance. Ceramic capacitors are recommended due to

their low ESR and ESL at the switching frequency of the

converter. For ceramic capacitors, the ripple voltage

due to ESL is negligible.

Load transient response depends on the selected out-

put capacitor. During a load transient, the output

I

VV

fL

V

V

PP

IN

OUT

SW

OUT

IN

−

×

×

V

I

ESR

RIPPLE ESR

P P

−

V

I

Cf

RIPPLE C

PP

OUT

SW

××

−

8

VV

V

V

RIPPLE

RIPPLE C

RIPPLE ESR

RIPPLE ESL

=+

+

()

(

)

(

)

I

V

IV

V

V

IV

V

V

RMS

IN

OUT

OUT

IN

OUT

OUT

OUT

IN

OUT

=

×−

×−

()

1

11

1

22

2

2

2

I

LIR

I

L MAX

OUT MAX

()

()

=+









1

2

L

VV

V

V

LIR

I

f

INIT

OUT

IN

OUT

IN

OUT MAX

OSC

=

−

()

××

×

()

Dual, 180° Out-of-Phase, 1.4MHz, 750mA Step-

Down Regulator with POR and RSI/PFO

______________________________________________________________________________________

17

Table 1. Output Voltage Settings

FBSEL1

OUTPUT 1

FBSEL2

OUTPUT 2

3.3V

2.5V

GND

1.8V

GND

1.5V

Open

Ext Divider

Open

Ext Divider

Table 2. Suggested Inductors

MANUFACTURER

PART

INDUCTANCE

(µH)

ESR

(m

Ω)

SATURATION

CURRENT (A)

DIMENSIONS (mm)

Coilcraft

DO1606

4.7

120

1.2

Sumida

CR43-4R7

4.7

108.7

1.15

Sumida

CDRH3D16-4R7

4.7

80

0.9