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L4947PD

5V-0.5A VERY LOW DROP REGULATOR WITH RESET

STMicroelectronics 

L4947PD

current) is maximum 13mA - over full T - when no

load current is required.

The internal antisaturation circuit allows a drastic

reduction in the current peak which takes place

during the start up.

The reset function supervises the regulator output

voltage inhibiting the microprocessor when the

device is out of regulation and resetting it at the

power-on after a settable delay. The reset is LOW

when the output voltage value is lower than the

reset threshold voltage. At the power-on phase

the output voltage increases (see Fig. 2) and -

when it reaches the power-on VO threshold VRT

(On) - the reset output becomes HIGH after a de-

lay time set by the external capacitor Cd. At the

power-off the output voltage decreases : at the

VRT(Off) threshold value (VO-0.15V typ. value) the

reset output instantaneously goes down (LOW

status) inhibiting the microprocessor. The typical

Figure 1: Typical Dropout Voltage vs. Tj

(Io = 500mA).

Figure 2: Reset Waveforms:

(1) Without External Capacitor Cd.

(2) With External Capacitor Cd.

VO

VRT(on)

VRT(off)

Vrthys

power on-off hysteresis is 50mV.

The three gain stages (operational amplifier,

driver and power PNP) require the external ca-

pacitor (Comin = 20µF) to guarantee the global

stability of the system.

Load dump and field decay protections (± 80V),

reverse voltage (– 18V) and short circuit protec-

tion, thermal shutdown are the main features that

make the L4947PD specially suitable for applica-

tions in the automotive enviroment.

EXTERNAL COMPENSATION

Since the purpose of a voltage regulator is to sup-

ply and load variations, the open loop gain of the

regulator must be very high at low frequencies.

This may cause instability as a result of the vari-

ous poles present in the loop. To avoid this insta-

bility dominant pole compensation is used to re-

duce phase shift due to other poles at the unity

gain frequency. The lower the frequency of these

others poles at the unity gain frequency. The

lower the frequency of these other poles, the

greater must be capacitor esed to create the

dominant pole for the same DC gain.

Where the output transistor is a lateral PNP type

there is a pole in the regulation loop at a fre-

quencybtoo low to be compensated by a capaci-

tor which can be integrated. An external compen-

sation is therefore necessary so a very high value

capacitor must be connected from the output to

ground.

The paeassitic equivalent series resistance of the

capacitor used adds a zero to the regulation loop.

This zero may compromise the stability of the

system since its effect tends to cancel the effect

of the pole added. In regulators this ESR must be

less than 3Ω and the minimum capacitor value is

47µF.

VR

Vdthh

Vdthl

Vrpeak

VD

(1)

Td

(2)

(2)

D94AT109

Vrsat

Vdisch

4/6

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