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L4923

5V–1A VERY LOW DROP REGULATOR WITH RESET AND INHIBIT

STMicroelectronics 

L4923

FUNCTIONAL DESCRIPTION

The operating principle of the voltage regulator is

basedon thereference,the error amplifier,the driver

and the power PNP. This stage uses an Isolated

CollectorVertical PNP transistor which allowsto ob-

tain very low dropout voltage (typ. 450 mV) and low

quiescent current (IQ = 20 mA typically at IO = 1 A).

Thanks to these features the device is particularly

suited when the power dissipation must be limited

as, for example, in automotive or industrial applica-

tions supplied by battery.

The three gain stages (operational amplifier, driver

and power PNP) require the external capacitor

(COmin = 22 µF) to guarantee the global stability of

the system.

The antisaturationcircuit allows to reducedrastically

the current peak which takes place during the start

up.

The reset function is LOW active when the output

voltage level is lower than the reset threshold volt-

age VRth (typ.value : VO - 150 mV). When the output

voltage is higher than VRth the reset becomes HIGH

aftera delay time settablewith theexternalcapacitor

Cd. Typically td = 20 ms, Cd = 0.1 µF. The reset

threshold hysteresis improves the noise immunity

allowing to avoid false switchings. The typical reset

output waveform is shown in fig. 2.

The inhibit circuit acceptsstandardTTL inputlevels :

thisblockswitchesoffthe voltageregulatorwhen the

input signal is HIGH and switches on it when the in-

put signal is LOW. Thanksto inhibit functionthe con-

sumption is drastically reduced (650 µA max) when

no load current is required.

Figure 2 : Typical Reset Output Waveform.

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