SM72238

SM72238

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SNVS694C – JANUARY 2011 – REVISED APRIL 2013

APPLICATION HINTS

EXTERNAL CAPACITORS

A 1.0µF (or greater) capacitor is required between the output and ground for stability. Without this capacitor the

part will oscillate. Most types of tantalum or aluminum electrolytics work fine here; even film types work but are

not recommended for reasons of cost. Many aluminum electrolytics have electrolytes that freeze at about

−30°C,

so solid tantalums are recommended for operation below

−25°C. The important parameters of the capacitor are

an ESR of about 5

Ω or less and a resonant frequency above 500kHz. The value of this capacitor may be

increased without limit.

Ceramic capacitors whose value is greater than 1000pF should not be connected directly from the SM72238

output to ground. Ceramic capacitors typically have ESR values in the range of 5 to 10m

Ω, a value below the

lower limit for stable operation (see Figure 25).

The reason for the lower ESR limit is that the loop compensation of the part relies on the ESR of the output

capacitor to provide the zero that gives added phase lead. The ESR of ceramic capacitors is so low that this

phase lead does not occur, significantly reducing phase margin. A ceramic output capacitor can be used if a

series resistance is added (recommended value of resistance about 0.1

Ω to 2Ω).

At lower values of output current, less output capacitance is required for stability. The capacitor can be reduced

to 0.33µF for currents below 10mA or 0.1µF for currents below 1mA.

Unlike many other regulators, the SM72238 will remain stable and in regulation with no load in addition to the

internal voltage divider. This is especially important in CMOS RAM keep-alive applications.

A 1µF tantalum, ceramic or aluminum electrolytic capacitor should be placed from the SM72238 input to ground

if there is more than 10 inches of wire between the input and the AC filter capacitor or if a battery is used as the

input.

REDUCING OUTPUT NOISE

In reference applications it may be advantageous to reduce the AC noise present at the output. One method is to

reduce the regulator bandwidth by increasing the size of the output capacitor. This is the only way noise can be

reduced but is relatively inefficient, as increasing the capacitor from 1µF to 220µF only decreases the noise from

430µV to 160µV rms for a 100kHz bandwidth at 5V output.

Typical Applications

*Minimum input-output voltage ranges from 40mV to 400mV, depending on load current. Current limit is typically

160mA.

Figure 26. 5 Volt Current Limiter

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