LT1933 datasheet(9/20 Pages) LINER

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LT1933

1933fe

APPLICATIONS INFORMATION

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

voltage rating. This situation is easily avoided; see the Hot

Plugging Safely section.

Output Capacitor

The output capacitor has two essential functions. Along

with the inductor, it ﬁlters the square wave generated

by the LT1933 to produce the DC output. In this role it

determines the output ripple, and low impedance at the

switching frequency is important. The second function

is to store energy in order to satisfy transient loads and

stabilize the LT1933’s control loop.

Ceramic capacitors have very low equivalent series re-

sistance (ESR) and provide the best ripple performance.

A good value is

COUT = 60/VOUT

where COUT is in μF. Use X5R or X7R types, and keep

in mind that a ceramic capacitor biased with VOUT will

have less than its nominal capacitance. This choice will

provide low output ripple and good transient response.

Transient performance can be improved with a high value

capacitor, but a phase lead capacitor across the feedback

resistor R1 may be required to get the full beneﬁt (see the

Compensation section).

High performance electrolytic capacitors can be used for

the output capacitor. Low ESR is important, so choose one

that is intended for use in switching regulators. The ESR

should be speciﬁed by the supplier, and should be 0.1Ω

or less. Such a capacitor will be larger than a ceramic

capacitor and will have a larger capacitance, because the

capacitor must be large to achieve low ESR. Table 2 lists

several capacitor vendors.

Table 2.Inductor Vendors

Vendor

Phone

URL

Part Series

Comments

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

Tantalum

EEF Series

Kemet

(864) 963-6300

www.kemet.com

Ceramic,

Tantalum

T494, T495

Sanyo

(408) 749-9714

www.sanyovideo.com

Ceramic,

Polymer,

Tantalum

POSCAP

Murata

(404)436-1300

www.murata.com

Ceramic

AVX

www.avxcorp.com

Ceramic,

Tantalum

TPS Series

Taiyo Yuden

(864)963-6300

www.taiyo-yuden.com

Ceramic

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