SM72238TD-50 Datasheet, PDF (9/17 Pages) Texas Instruments – SM72238TD-50

SM72238
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APPLICATION HINTS
SNVS694C – JANUARY 2011 – REVISED APRIL 2013
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 5Ω 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 10mΩ, 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 0.1Ω 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
SM72238
*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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