6

LT1944

APPLICATIO S I FOR ATIO

limited to 350mA, the power switch of the LT1944 can

handle larger currents without problem, but the overall

efficiency will suffer. Best results will be obtained when

Capacitor Selection

Low ESR (Equivalent Series Resistance) capacitors should

be used at the output to minimize the output ripple voltage.

Multilayer ceramic capacitors are the best choice, as they

have a very low ESR and are available in very small

packages. Their small size makes them a good companion

to the LT1944’s MS10 package. Solid tantalum capacitors

(like the AVX TPS, Sprague 593D families) or OS-CON

capacitors can be used, but they will occupy more board

area than a ceramic and will have a higher ESR. Always use

a capacitor with a sufficient voltage rating.

Ceramic capacitors also make a good choice for the input

decoupling capacitor, which should be placed as close as

possible to the LT1944. A 4.7

µF input capacitor is suffi-

cient for most applications. Table 2 shows a list of several

capacitor manufacturers. Consult the manufacturers for

more detailed information and for their entire selection of

related parts.

Table 2. Recommended Capacitors

CAPACITOR TYPE

VENDOR

Ceramic

Taiyo Yuden

(408) 573-4150

www.t-yuden.com

Ceramic

AVX

(803) 448-9411

www.avxcorp.com

Ceramic

Murata

(714) 852-2001

www.murata.com

Setting the Output Voltage

Set the output voltage for each switching regulator by

choosing the appropriate values for feedback resistors R1

and R2 (see Figure 1).

RR

V

V

OUT

12

123

1

=−









.

Diode Selection

For most LT1944 applications, the Motorola MBR0520

surface mount Schottky diode (0.5A, 20V) is an ideal

choice. Schottky diodes, with their low forward voltage

drop and fast switching speed, are the best match for the

LT1944. For higher output voltage applications the 30V

MBR0530 or 40V MBR0540 can be used. Many different

manufacturers make equivalent parts, but make sure that

the component is rated to handle at least 0.35A.

Lowering Output Voltage Ripple

Using low ESR capacitors will help minimize the output

ripple voltage, but proper selection of the inductor and the

output capacitor also plays a big role. The LT1944 pro-

vides energy to the load in bursts by ramping up the

inductor current, then delivering that current to the load.

If too large of an inductor value or too small of a capacitor

value is used, the output ripple voltage will increase

because the capacitor will be slightly overcharged each

burst cycle. To reduce the output ripple, increase the

output capacitor value or add a 4.7pF feed-forward capaci-

tor in the feedback network of the LT1944 (see the circuits

in the Typical Applications section). Adding this small,

inexpensive 4.7pF capacitor will greatly reduce the output

voltage ripple.