LT1934IDCB-1 查看數據表（PDF） - Linear Technology

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LT1934IDCB-1

Micropower Step-Down Switching Regulators in ThinSOT and DFN

Linear Technology 

LT1934/LT1934-1

APPLICATIONS INFORMATION

capacitor is subject to large surge currents if the LT1934

circuit is connected to a low impedance supply, and that

some electrolytic capacitors (in particular tantalum) must

be specified for such use.

Output Capacitor and Output Ripple

The output capacitor filters the inductor’s ripple current and

stores energy to satisfy the load current when the LT1934

is quiescent. In order to keep output voltage ripple low, the

impedance of the capacitor must be low at the LT1934’s

switching frequency. The capacitor’s equivalent series

resistance (ESR) determines this impedance. Choose one

with low ESR intended for use in switching regulators. The

contribution to ripple voltage due to the ESR is approxi-

mately ILIM • ESR. ESR should be less than ~150mΩ for

the LT1934 and less than ~500mΩ for the LT1934-1.

The value of the output capacitor must be large enough

to accept the energy stored in the inductor without a large

change in output voltage. Setting this voltage step equal to

1% of the output voltage, the output capacitor must be:

COUT > 50 • L • (ILIM/VOUT)2

For example, an LT1934 producing 3.3V with L = 47μH

requires 33μF. This value can be relaxed if small circuit

size is more important than low output ripple.

Sanyo’s POSCAP series in B-case and C-case sizes

provides very good performance in a small package for

the LT1934. Similar performance in traditional tantalum

capacitors requires a larger package (C- or D-case).

The LT1934-1, with its lower switch current, can use a

B-case tantalum capacitor.

With a high quality capacitor filtering the ripple current

from the inductor, the output voltage ripple is determined

by the hysteresis and delay in the LT1934’s feedback

comparator. This ripple can be reduced further by adding

a small (typically 10pF) phase lead capacitor between the

output and the feedback pin.

Ceramic Capacitors

Ceramic capacitors are small, robust and have very low

ESR. However, ceramic capacitors can cause problems

when used with the LT1934.

Not all ceramic capacitors are suitable. X5R and X7R

types are stable over temperature and applied voltage

and give dependable service. Other types (Y5V and Z5U)

have very large temperature and voltage coefficients of

capacitance. In the application circuit they may have only

a small fraction of their nominal capacitance and voltage

ripple may be much larger than expected.

Ceramic capacitors are piezoelectric. The LT1934’s switch-

ing frequency depends on the load current, and at light

loads the LT1934 can excite the ceramic capacitor at audio

frequencies, generating audible noise. If this is unaccept-

able, use a high performance electrolytic capacitor at the

output. The input capacitor can be a parallel combination

of a 2.2μF ceramic capacitor and a low cost electrolytic

capacitor. The level of noise produced by the LT1934-1

10

Table 3. Capacitor Vendors

VENDOR PHONE

Panasonic (714) 373-7366

Kemet

Sanyo

(864) 963-6300

(408) 749-9714

Murata

AVX

(404) 436-1300

Taiyo Yuden (864) 963-6300

URL

PART SERIES

www.panasonic.com

Ceramic,

Polymer,

Tantalum

www.kemet.com

Ceramic,

Tantalum

www.sanyovideo.com Ceramic,

Polymer,

Tantalum

www.murata.com

Ceramic,

www.avxcorp.com

Ceramic,

Tantalum

www.taiyo-yuden.com Ceramic

COMMENTS

EEF Series

T494, T495

POSCAP

TPS Series

1934fe

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