L4902A datasheet(4/9 Pages) STMICROELECTRONICS

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TEST CIRCUIT

APPLICATION INFORMATION

In power supplies for

µP systems it is necessaryto

provide power continuously to avoid loss of infor-

mation in memories and in time of day clocks, or to

save datawhen the primary supply is removed.The

L4902A makes it very easy to supply such equip-

ments ; it provides two voltage regulators (both 5V

high precision) with common inputs plus a reset

output for the data save function and a Reg. 2

disable input.

CIRCUIT OPERATION (see Figure 1)

After switch on Reg. 1 saturates until V01 rises to

the nominal value.

When the input reaches VIT and the output 1 is

higher than VRT the output 2 (V02) switches on and

the reset output (VR) also goes high after a pro-

grammable time TRD (timing capacitor).

V02 and VR are switched togetherat low level when

one of the following conditions occurs :

- a high level ( VDT) is applied on pin 3 ;

- an input overvoltage ;

- an overload on the output 1 (V01 VRT);

- a switch off (VIN VIT -VITH);

and they start again as before when the condition

is removed.

An overload on output 2 does not switch Reg. 2,

and does not influence Reg. 1.

The V01 output features :

- 5V internal reference without voltage divider be-

tween the output and the error comparator

- very low drop series regulator element utilizing

current mirrors

permit high output impedance and then very low

leakage current even in power down condition.

Figure 1

L4902A

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