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L4902A
DUAL 5V REGULATOR WITH RESET AND DISABLE
L4902ADUAL 5V REGULAT OR WITH RESET AND DISABLE
June 2000
PIN CONNECTION DOUBLE BA TTERY OPERATING. OUTPUT CURRENTS : I01 = 300 mA
I02 = 300 mA. FIXED PRECISION OUTPUT VOLTAGE
5 V ± 2 %. RESET FUNCTION CONTROLLED BY INPUT
VOLTAGE AND OUTPUT 1 VOLTAGE. RESET FUNCTION EXTERNALLY PRO-
GRAMMABLE TIMING . RESET OUTPUT LEVEL RELATED TO OUT-
PUT 2. OUTPUT 2 INTERNALL Y SWITCHED WITH
ACTIVE DISCHARGING. OUTPUT 2 DISABLE LOGICAL INPUT . LOW LEAKAGE CURRENT, LESS THAN 1μA T OUTPUT 1. RESET OUTPUT NORMALL Y HIGH. INPUT OVERVOLT AGE PROTECTION UP TO
60V. OUTPUT TRANSISTORS SOA PROTECTION. SHORT CIRCUIT AND THERMAL OVER-
LOAD PROTECTION
DESCRIPTIONThe L4902A is a monolithic low drop dual 5V
regulator designed mainly for supplying microproc-
essor systems.
Reset and data save functions and remote switch
on/off control can be realized.
1/9
BLOCK DIAGRAM
SCHEMA TIC DIAGRAM
PIN FUNCTIONS
L4902A2/9
ABSOLUTE MAXIMUM RATINGS
THERMAL DATA
ELECTRICAL CHARACTERISTICS (VIN = 14.4V , Tamb = 25o C unless otherwise specified))
L4902A3/9
TEST CIRCUIT
APPLICATION INFORMATIONIn 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
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 together at 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
L4902A4/9
This output may therefore be used to supply circuits
continuously , such as volatile RAMs, allowing the
use of a back-up battery.
The V02 output can supply other non essential 5 V
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 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
control of circuits connected to the V02 output.
APPLICATION SUGGESTIONSFigure 2 illustrate how the L4902A’s disable input
may be used in a CMOS μComputer application.
The V01 regulator (low consumption) supply perma-
nently a CMOS time of day clock and a CMOS
μcomputer chip with volatile memory. V02 output,
supplying non-essential circuits, is turned OFF un-
der control of a μP unit.
Figure 2
Figure 3 : P .C. Board Component Layout of Figure 2
L4902A5/9
Configurations of this type are used in products
where the OFF switch is part of a keyboard
scanned by a micro which operates continuously
even in the OFF state.
Another application for the L4902A is supplying a
shadow-ram microcomputer chip (SGS M38SH72
for example) where a fast NV memory is backed up
on chip by a EEPROM when a low level on the reset
output occurs.
By adding two CMOS-SCHMIDT -TRIGGER and
few external components, also a watch dog func-
tion may be realized (see Figure 5). During normal
operation the microsystem supplies a periodical
pulse waveform ; if an anomalous condition occours
(in the program or in the system), the pulses will be
absent and the disable input will be activated after
a settling time determined by R1 C1. In this condi-
tion all the circuitry connected to V02 will be dis-
abled, the system will be restarted with a new reset
front.
The disable of V02 prevent spurious operation dur-
ing microprocessor malfunctioning.
Figure 4
Figure 5
L4902A6/9
