15

LTC1553

Table 5. Recommended MOSFETs for LTC1553 Applications

TYPICAL INPUT

CAPACITANCE

PARTS

AT 25

°C (mΩ)

RATED CURRENT (A)

Siliconix SUD50N03-10

19

15 at 25

°C

3200

1.8

175

TO-252

10 at 75

°C

Siliconix Si4410DY

20

10 at 25

°C

2700

—

150

SO-8

8 at 75

°C

Motorola MTD20N03HDL

35

20 at 25

°C

880

1.67

150

D PAK

16 at 100

°C

SGS-Thomson STD20N03L

23

20 at 25

°C

2300

2.5

175

D PAK

14 at 100

°C

Motorola MTB75N03HDL

7.5

75 at 25

°C

4025

1.0

150

DD PAK

59 at 100

°C

IRF IRL3103S

14

56 at 25

°C

1600

1.8

175

DD PAK

40 at 100

°C

IRF IRLZ44

28

50 at 25

°C

3300

1.0

175

TO-220

36 at 100

°C

Fuji 2SK1388

37

35 at 25

°C

1750

2.08

150

TO-220

Inductor Selection

The inductor is often the largest component in the LTC1553

design and should be chosen carefully. Inductor value and

type should be chosen based on output slew rate require-

ments, output ripple requirements and expected peak

current. Inductor value is primarily controlled by the

required current slew rate. The maximum rate of rise of

current in the inductor is set by its value, the input-to-

output voltage differential and the maximum duty cycle of

the LTC1553. In a typical 5V input, 2.8V output applica-

tion, the maximum current slew rate will be:

DC

VV

LL

A

s

MAX

IN

OUT

−

()

=

183

.

µ

where L is the inductor value in

µH. With proper frequency

compensation, the combination of the inductor and output

capacitor will determine the transient recovery time. In

general, a smaller value inductor will improve transient

response at the expense of increased output ripple voltage

and inductor core saturation rating. A 2

µH inductor would

have a 0.9A/

µs rise time in this application, resulting in a

5.5

µsdelayinrespondingtoa5Aloadcurrentstep.During

this 5.5

µs,thedifferencebetweentheinductorcurrentand

the output current must be made up by the output capaci-

tor, causing a temporary voltage droop at the output. To

minimize this effect, the inductor value should usually be

in the 1

µH to 5µH range for most typical 5V input LTC1553

circuits. To optimize performance, different combinations

of input and output voltages and expected loads may

require different inductor values.

Once the required value is known, the inductor core type

can be chosen based on peak current and efficiency

requirements. Peak current in the inductor will be equal to

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

peak inductor ripple current. Ripple current is set by the

inductor value, the input and output voltage and the

operating frequency. The ripple current is approximately

equal to:

I

VV

V

fL

V

RIPPLE

IN

OUT

OUT

OSC

O

IN

=

−

()( )

()( )( )

Note: Please refer to the manufacturer’s data sheet for testing conditions

and detail information.