Low Dropout Regulator with Delayed Reset

LM2925

HTC

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8 −

APPLICATION HINT

EXTERNAL CAPACITORS

The LM2925 output capacitoris required for stability. Without it, the regulator output will oscillate,

sometimes by many volts. Though the 10µF shown is the minimum recom-mended value,actual size

and type may vary depending upon the application load and temperature range. Capacitor effective

series resistance (ESR) also effects the IC stability.

Since ESR varies from one brand to the next, some bench work may be required to determine the

minimum capacitor value to use in production. Worst-case is usually determined at the minimum

junction and ambient temperature and maxi- mum load expected.

Output capacitors can be increased in size to any desired value above the minimum. One possible

purpose of this would be to maintain the output voltages during brief condi-tions of negative input

transients that might be characteristic of a particular system.

Capacitors must also be rated at all ambient temperatures expected in the system. Many aluminum

type electrolytics will freeze at temperatures less than −30°C, reducing their effective capacitance to

zero. To maintain regulator stability down to −40°C, capacitors rated at that temperature (such as

tantalums) must be used.

RESET OUTPUT

The range of values for the delay capacitor is limited only by stray capacitances on the lower extreme

and capacitance leakage on the other. Thus, delay times from microseconds to seconds are possible.

The low charging current, typically 2.0 microamps, allows the use of small, inexpensive disc ca-

pacitors for the nominal range of 100 to 500 milliseconds. This is the time required in many

microprocessor systems for the clock oscillator to stabilize when initially powered up.

during this part of op-eration The same delay is incorporated after any other fault condition in the

regulator output is corrected.