5

LT1935

1935f

Inductor Selection

Use inductors that are intended for high frequency power

applications. The saturation current rating should be at

least 2A. The RMS current rating, which is usually based

on heating of the inductor, should be higher than the

average current in the inductor in your application. For

best efficiency, the DC resistance should be less than

100mΩ.

A good first choice for the inductor value results in a ripple

current that is 1/3 of the maximum switch current:

switching frequency. At lower duty cycles (less than 70%),

this value can be lowered somewhat in order to use a

physically smaller inductor.

Table 1 lists several inductor manufacturers, along with

part numbers for inductors that are a good match to the

LT1935.

Table 1. Inductor Suppliers

Supplier

Model Prefix

Sumida

CDRH4D18, CDRH4D28,

CDRH5D18, CDRH5D28, CR43

Coiltronics/Cooper

SD10, SD12, SD18, SD20

Wurth Elektronik

WE-PD2S, WE-PD3S, WE-PD4S

Coilcraft

MSS5131, MSS6132, DO1608

Diode Selection

Use a Schottky rectifier with a 1A or higher current rating,

such as the On Semiconductor MBRM120. Its 20V reverse

voltage rating is adequate for most applications. Higher

output voltages may require a 30V of 40V diode.

Capacitor Selection

Use capacitors with low ESR (equivalent series resis-

tance). In most cases, multilayer ceramic capacitors are

the best choice. They offer high performance (very low

ESR) in a small package. Use only X5R or X7R types; they

maintain their capacitance over temperature and applied

voltage. Other suitable capacitor types include low-ESR

tantalum capacitors that are specified for power applica-

tions, and newer types of capacitors such as Sanyo’s

POSCAP and Panasonic’s SP CAP.

Use a 4.7µF ceramic capacitor to bypass the input of the

LT1935. Be aware that the switching regulators require a

low impedance input supply. Additional bulk capacitance

may be required if the LT1935 circuit is more than a few

inches away from the power source. If there are low ESR

capacitors nearby, the input bypass capacitor can be

reduced to 2.2µF.

The output capacitor supports the output under transient

loads and stabilizes the control loop of the LT1935. Look

at the typical application circuits as a starting point to

choose a value. Generally, a higher output capacitance is

required at higher load currents and lower input voltages.

Figure 3 shows transient response of the circuit in Fig-

ure 1. The load is stepped from 200mA to 400mA and back

to 200mA. The transient performance can be improved by

increasing the output capacitance, but may require a

phase lead capacitor between the output and the FB pin.

Figure 4 shows the transient response with the output

capacitor increased to 20µF. Figure 5 shows the additional

improvement resulting from the phase lead capacitor.

Figure 3. Transient Response of the Circuit in Figure 1,

..

VOUT

100mV/DIV

ILOAD

200mA/DIV

50µs/DIV

1935 F03

0

VOUT

100mV/DIV

ILOAD

200mA/DIV

50µs/DIV

1935 F04