0

LOAD CURRENT (A)

1

2

STABLE REGION

.001

.01

0.1

1.0

10

LP3892

SNVS236D – SEPTEMBER 2003 – REVISED APRIL 2013

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Application Hints

EXTERNAL CAPACITORS

To assure regulator stability, input and output capacitors are required as shown in the Typical Application Circuit.

OUTPUT CAPACITOR

At least 10µF of output capacitance is required for stability (the amount of capacitance can be increased without

limit). The output capacitor must be located less than 1 cm from the output pin of the IC and returned to a clean

analog ground. The ESR (equivalent series resistance) of the output capacitor must be within the "stable" range

as shown in Figure 19 over the full operating temperature range for stable operation.

Figure 19. Minimum ESR vs Output Load Current

Tantalum capacitors are recommended for the output as their ESR is ideally suited to the part's requirements

and the ESR is very stable over temperature. Aluminum electrolytics are not recommended because their ESR

increases very rapidly at temperatures below 10°C. Aluminum caps can only be used in applications where lower

temperature operation is not required.

A second problem with Al caps is that many have ESR's which are only specified at low frequencies. The typical

loop bandwidth of a linear regulator is a few hundred kHz to several MHz. If an Al cap is used for the output cap,

it must be one whose ESR is specified at a frequency of 100 kHz or more.

Because the ESR of ceramic capacitors is only a few milliohms, they are not suitable for use as output capacitors

on LP389X devices. The regulator output can tolerate ceramic capacitance totaling up to 15% of the amount of

Tantalum capacitance connected from the output to ground.

INPUT CAPACITOR

The input capacitor must be at least 10 µF, but can be increased without limit. It's purpose is to provide a low

source impedance for the regulator input. Ceramic capacitors work best for this, but Tantalums are also very

good. There is no ESR limitation on the input capacitor (the lower, the better). Aluminum electrolytics can be

used, but their ESR increase very quickly at cold temperatures. They are not recommended for any application

where temperatures go below about 10°C.

BIAS CAPACITOR

The 0.1µF capacitor on the bias line can be any good quality capacitor (ceramic is recommended).

BIAS VOLTAGE

The bias voltage is an external voltage rail required to get gate drive for the N-FET pass transistor. Bias voltage

must be in the range of 4.5 - 6V to assure proper operation of the part.

UNDER VOLTAGE LOCKOUT

The bias voltage is monitored by a circuit which prevents the regulator output from turning on if the bias voltage

is below approximately 4V.

8

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