LT1943 datasheet(13/20 Pages) LINER

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LT1943

1943fa

Output Capacitor Selection

For 5V and 3.3V outputs, a 10

µF 6.3V ceramic capacitor

(X5R or X7R) at the output results in very low output

voltage ripple and good transient response. Other types

and values will also work; the following discussion ex-

plores tradeoffs in output ripple and transient perfor-

mance.

The output capacitor filters the inductor current to gener-

ate an output with low voltage ripple. It also stores energy

in order satisfy transient loads and stabilizes the LT1943’s

control loop. Because the LT1943 operates at a high

frequency, minimal output capacitance is necessary. In

addition, the control loop operates well with or without the

presence of output capacitor series resistance (ESR).

Ceramic capacitors, which achieve very low output ripple

and small circuit size, are therefore an option.

You can estimate output ripple with the following

equations:

VRIPPLE = ∆IL/(8 • f • COUT) for ceramic capacitors, and

VRIPPLE = ∆IL • ESR for electrolytic capacitors (tantalum

and aluminum);

where

∆IL is the peak-to-peak ripple current in the induc-

tor. The RMS content of this ripple is very low so the RMS

current rating of the output capacitor is usually not of

concern. It can be estimated with the formula:

IC(RMS)= ∆IL/√12

Another constraint on the output capacitor is that it must

have greater energy storage than the inductor; if the stored

energy in the inductor transfers to the output, the resulting

voltage step should be small compared to the regulation

voltage. For a 5% overshoot, this requirement indicates:

COUT > 10 • L • (ILIM/VOUT)2

The low ESR and small size of ceramic capacitors make

them the preferred type for LT1943 applications. Not all

ceramic capacitors are the same, however. Many of the

higher value capacitors use poor dielectrics with high

temperature and voltage coefficients. In particular, Y5V

and Z5U types lose a large fraction of their capacitance

with applied voltage and at temperature extremes.

Because loop stability and transient response depend on

the value of COUT, this loss may be unacceptable. Use X7R

and X5R types.

Electrolytic capacitors are also an option. The ESRs of

most aluminum electrolytic capacitors are too large to

deliver low output ripple. Tantalum and newer, lower ESR

organic electrolytic capacitors intended for power supply

use are suitable, and the manufacturers will specify the

ESR. Chose a capacitor with a low enough ESR for the

required output ripple. Because the volume of the capaci-

tor determines its ESR, both the size and the value will be

larger than a ceramic capacitor that would give similar

ripple performance. One benefit is that the larger capaci-

tance may give better transient response for large changes

in load current. Table 2 lists several capacitor vendors.

Table 2. Low ESR Surface Mount Capacitors

Vendor

Type

Series

Taiyo Yuden

Ceramic

X5R, X7R

AVX

Ceramic

X5R, X7R

Tantalum

TPS

Kemet

Tantalum

T491, T494, T495

Ta Organic

T520

Al Organic

A700

Sanyo

Ta or Al Organic

POSCAP

Panasonic

Al Organic

SP CAP

TDK

Ceramic

X5R, X7R

Diode Selection

The catch diode (D1 from Figure 1) conducts current only

during switch off time. Average forward current in normal

operation can be calculated from:

ID(AVG) = IOUT (VIN – VOUT)/VIN

The only reason to consider a diode with a larger current

rating than necessary for nominal operation is for the

worst-case condition of shorted output. The diode current

will then increase to the typical peak switch current.

Peak reverse voltage is equal to the regulator input volt-

age. Use a diode with a reverse voltage rating greater than

the input voltage. Table 3 lists several Schottky diodes and

their manufacturers.

OPERATIO

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