L296 - L296P
Figure 43 : 5.1V/2APower Supply using External
Limiting Current Resistor and Crow-
bar Protectionon the Supply Voltage
(L296Ponly)
sistor may be added, as shown in Figure 45 ; with
this circuit discharge times of a few microseconds
may be obtained.
Figure 45
SOFT-START AND REPETITIVE POWER-ON
Whenthedeviceisrepetitivelypowered-on,thesoft-
start capacitor, CSS, must be discharged rapidly to
ensurethateachstartis ”soft”.Thiscanbeachieved
economicallyusingtheresetcircuit,asshowninFig-
ure 44.
HOW TO OBTAIN BOTH RESET AND
POWER FAIL
Inthis circuit the dividerR1, R2 connectedto pin12
determines the minimum supply voltage, below
whichthe opencollectortransistorat thepin14 out-
put discharges CSS.
Figure46 illustrateshowit ispossibleto obtainat the
same time both the power fail and reset functions
simply byaddingonediode(D)andoneresistor(R).
In this case the Reset delay time (pin 13) can only
start when the outputvoltageis VO ≥ VREF - 100mV
and the voltageaccross R2 is higher than 4.5V.
Figure 44
Withthehysteresisresistorit is possibletofixthein-
put pin 12 hysteresis in order to increase immunity
to the 100Hzripple present on the supply voltage.
Moreover, the power fail and reset delay time are
automatically locked tothe soft-start.Soft-startand
delayed reset are thus two sequential functions.
The hysteresis resistor should be In the range of
aboit100kΩ and the pull-up resistor of 1 to 2.2kΩ.
Figure 46
Theapproximatedischargetimesobtainedwiththis
circuit are :
CSS ( F)
tDIS ( s)
µ
µ
2.2
4.7
10
200
300
600
Ifthesetimesarestilltoolong,an externalPNPtran-
19/22
STMICROELECTRONICS [ ST ]
