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LT1933

1933f

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 resis-

tance (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 Compensa-

tion 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.

Figure 1 shows the transient response of the LT1933 with

several output capacitor choices. The output is 3.3V. The

load current is stepped from 100mA to 400mA and back

to 100mA, and the oscilloscope traces show the output

voltage. The upper photo shows the recommended value.

The second photo shows the improved response (less

voltage drop) resulting from a larger output capacitor and

a phase lead capacitor. The last photo shows the response

to a high performance electrolytic capacitor. Transient

performance is improved due to the large output capaci-

tance, but output ripple (as shown by the broad trace) has

increased because of the higher ESR of this capacitor.

Table 2. Capacitor Vendors

Vendor

Phone

URL

Part Series

Comments

Panasonic

(714) 373-7366

www.panasonic.com

Ceramic,

Polymer,

EEF Series

Tantalum

Kemet

(864) 963-6300

www.kemet.com

Ceramic,

Tantalum

T494, T495

Sanyo

(408) 749-9714

www.sanyovideo.com

Ceramic,

Polymer,

POSCAP

Tantalum

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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