7

LT1933

1933f

Table 1. Inductor Vendors

Vendor

URL

Part Series

Inductance Range (µH)

Size (mm)

Coilcraft

www.coi1craft.com

DO1608C

10 to 22

2.9 × 4.5 × 6.6

MSS5131

10 to 22

3.1 × 5.1 × 5.1

MSS6122

10 to 33

2.2 × 6.1 × 6.1

Sumida

www.sumida.com

CR43

10 to 22

3.5 × 4.3 × 4.8

CDRH4D28

10 to 33

3.0 × 5.0 × 5.0

CDRH5D28

22 to 47

3.0 × 5.7 × 5.7

Toko

www.toko.com

D52LC

10 to 22

2.0 × 5.0 × 5.0

D53LC

22 to 47

3.0 × 5.0 × 5.0

Würth Elektronik

www.we-online.com

WE-TPC MH

10 to 22

2.8 × 4.8 × 4.8

WE-PD4 S

10 to 22

2.9 × 4.5 × 6.6

WE-PD2 S

10 to 47

3.2 × 4.0 × 4.5

be ~1A. To keep efficiency high, the series resistance

(DCR) should be less than 0.2Ω. Table 1 lists several

vendors and types that are suitable.

Of course, such a simple design guide will not always

result in the optimum inductor for your application. A

larger value provides a slightly higher maximum load

current, and will reduce the output voltage ripple. If your

load is lower than 500mA, then you can decrease the value

of the inductor and operate with higher ripple current. This

allows you to use a physically smaller inductor, or one with

a lower DCR resulting in higher efficiency. There are

several graphs in the Typical Performance Characteristics

section of this data sheet that show the maximum load

current as a function of input voltage and inductor value

for several popular output voltages. Low inductance may

result in discontinuous mode operation, which is OK, but

further reduces maximum load current. For details of

maximum output current and discontinuous mode opera-

tion, see Linear Technology Application Note 44. Finally,

a minimum inductance required to avoid subharmonic

prevents subharmonic oscillations at all duty cycles.

Catch Diode

A 0.5A or 1A Schottky diode is recommended for the catch

diode, D1. The diode must have a reverse voltage rating

equal to or greater than the maximum input voltage. The

ON Semiconductor MBR0540 is a good choice; it is rated

for 0.5A forward current and a maximum reverse voltage

of 40V. The MBRM140 provides better efficiency, and will

handle extended overload conditions.

Input Capacitor

Bypass the input of the LT1933 circuit with a 2.2µF or

higher value ceramic capacitor of X7R or X5R type. Y5V

types have poor performance over temperature and ap-

plied voltage, and should not be used. A 2.2µF ceramic is

adequate to bypass the LT1933 and will easily handle the

ripple current. However, if the input power source has high

impedance, or there is significant inductance due to long

wires or cables, additional bulk capacitance may be nec-

essary. This can be provided with a low performance

electrolytic capacitor.

Step-down regulators draw current from the input supply

in pulses with very fast rise and fall times. The input

capacitor is required to reduce the resulting voltage ripple

at the LT1933 and to force this very high frequency

switching current into a tight local loop, minimizing EMI.

A 2.2µF capacitor is capable of this task, but only if it is

placed close to the LT1933 and the catch diode; see the

PCB Layout section. A second precaution regarding the

ceramic input capacitor concerns the maximum input

voltage rating of the LT1933. A ceramic input capacitor

combined with trace or cable inductance forms a high

quality (under damped) tank circuit. If the LT1933 circuit

is plugged into a live supply, the input voltage can ring to

twice its nominal value, possibly exceeding the LT1933’s

APPLICATIO S I FOR ATIO