13

LTC1530

1530fa

where L is the inductor value in

µH. With proper frequency

compensation, the combination of the inductor and output

capacitor values determine the transient recovery time. In

general, a smaller value inductor improves transient

response at the expense of ripple and inductor core

saturation rating. A 2

µH inductor has a 0.9A/µs rise time

in this application, resulting in a 5.5

µs delay in responding

to a 5A load current step. During this 5.5

µs, the difference

between the inductor current and the output current is

made up by the output capacitor. This action causes a

temporary voltage droop at the output. To minimize this

effect, the inductor value should usually be in the 1

µH to

5

µH range for most 5V input LTC1530 circuits. Different

combinations of input and output voltages and expected

loads may require different values.

Once the required inductor value is selected, choose the

inductor core type based on peak current and efficiency

requirements. Peak current in the inductor is equal to the

maximum output load current plus half of the peak-to-

peak inductor ripple current. Inductor ripple current is set

by the inductor’s value, the input voltage, the output

voltage and the operating frequency. If the efficiency is

high, ripple current is approximately equal to:

I

VV

V

fL

V

RIPPLE

IN

OUT

OUT

OSC

O

IN

=

−

()( )

()( )( )

where

Solving this equation for a typical 5V to 2.8V application

with a 2

µH inductor, ripple current is:

22

056

300

2

2

..

V

kHz

H

A

()( )

()( )

=

µ

P-P

Peak inductor current at 11.2A load:

11 2

2

2

12 2

..

A

A

A

+=

The ripple current should generally fall between 10% and

40% of the output current. The inductor must be able to

withstand this peak current without saturating, and the

copper resistance in the winding should be kept as low as

possible to minimize resistive power loss. Note that in

APPLICATIO S I FOR ATIO

RDS(ON)

TYPICAL INPUT

AT 25

RATED CURRENT

CAPACITANCE

MANUFACTURER

PART NO.

PACKAGE

(

(A)

Ciss (pF)

(

(

Siliconix

SUD50N03-10

TO-252

0.019

15A at 25

°C

3200

1.8

175

10A at 100

°C

Siliconix

Si4410DY

SO-8

0.020

10A at 25

°C

2700

—

150

8A at 75

°C

ON Semiconductor

MTD20N03HDL

DPAK

0.035

20A at 25

°C

880

1.67

150

16A at 100

°C

Fairchild

FDS6680

SO-8

0.01

11.5A at 25

°C

2070

25

150

ON Semiconductor

MTB75N03HDL*

0.0075

75A at 25

°C

4025

1.0

150

59A at 100

°C

IR

IRL3103S

0.014

56A at 25

°C

1600

1.8

175

40A at 100

°C

IR

IRLZ44

TO-220

0.028

50A at 25

°C

3300

1.0

175

36A at 100

°C

Fuji

2SK1388

TO-220

0.037

35A at 25

°C

1750

2.08

150

Note: Please refer to the manufacturer’s data sheet for testing conditions and detailed information.

*Users must consider the power dissipation and thermal effects in the LTC1530 if driving external MOSFETs with high values of input capacitance.

Table 1. Recommended MOSFETs for LTC1530 Applications