TEST CIRCUIT

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

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

L4904A makes it very easy to supply such equip-

ments ; it provides two voltage regulators (booth 5V

high precision) with separate inputs plus a reset

output for the data save function.

CIRCUIT OPERATION (see Figure 2)

the nominal value.

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

mirrors ;

permit high output impedance and then very low

leakage current even in power down conditions.

This output may thereforebe used to supply circuits

continuously, such as volatile RAMs, allowing the

features low consumption (0.6 mA typ.) to minimize

is permanently connected to a battery supply.

circuits 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 usedas 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.

L4904A

4/9