TEST CIRCUIT

Figure 1 : P.C. Board and Components Layout of the Test Circuit

APPLICATION INFORMATION

In power supplies for

µP systems it is necessary to

provide power continuously to avoid loss of infor-

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

save data when the primary supply is removed. The

L4903 makes it very easy to supply such equip-

ments ; it provides two voltage regulators (both 5 V

high precision) with separate inputs plus a reset

output for the data save function and Reg. 2 disable

input.

CIRCUIT OPERATION (see Figure 2)

the nominal value.

when one of the following conditions occurs ;

- an input overvoltage ;

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.

- 5 V internal reference without voltage divider

between 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 conditions.

This output may thereforebe used to supply circuits

continuously, such as volatile RAMs, allowing the

use of a back-up battery.

which may be powered down when the system is

inactive, or that must be powered down to prevent

uncorrect operation for supply voltages below the

minimum value.

The reset output can be used as a ”POWER DOWN

INTERRUPT”, permitting RAM access only in cor-

rect power conditions, or as a ”BACK-UP ENABLE”

to transfer data into in a NV SHADOW MEMORY

when the supply is interrupted.

The disable function can be used for remote on/off

L4903

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