CIRCUIT OPERATION
(refer to the block diagram)
The L296 and L296P are monolithic stepdown
switching regulatorsprovidingoutputvoltagesfrom
5.1Vto 40Vand delivering 4A.
Theregulationloopconsistsofasawtoothoscillator,
erroramplifier,comparatorandtheoutputstage.An
error signal is produced by comparing the output
voltagewithaprecise5.1Von-chipreference(zener
zaptrimmedto
±
2%).Thiserrorsignalisthencom-
paredwiththesawtoothsignalto generatethefixed
frequencypulsewidthmodulatedpulseswhichdrive
theoutputstage.Thegainandfrequencystabilityof
theloopcanbeadjustedby anexternalRCnetwork
connectedtopin9.Closingtheloopdirectlygivesan
outputvoltageof5.1V.Highervoltagesareobtained
by insertinga voltagedivider.
Outputovercurrentsat switch on are preventedby
the soft start function.The error amplifier output is
initially clamped by the externalcapacitor Css and
allowedto rise, linearly,asthiscapacitorischarged
by a constantcurrent source.
Outputoverloadprotectionisprovidedintheformof
a current limiter. The load current is sensed by an
internal metal resistor connected to a comparator.
When the load current exceeds a presetthreshold
this comparator sets a flip flop which disables the
outputstageanddischargesthesoftstartcapacitor.
A second comparator resets the flip flop when the
voltage across the soft start capacitor has fallen to
0.4V. The output stage is thus re-enabled and the
output voltage rises under control of the soft start
network.If theoverloadconditionis still presentthe
limiter will trigger againwhen the thresholdcurrent
is reached.The averageshort circuit current is lim-
ited to a safevalue bythe dead time introducedby
the soft startnetwork.
The reset circuit generatesan output signal when
the supply voltage exceeds a threshold pro-
grammedbyanexternaldivider.Theresetsignalis
generatedwith a delaytime programmedbyan ex-
ternal capacitor. When the supply falls below the
threshold the reset output goes low immediately.
The reset outputis an open collector.
The scrowbarcircuit sensestheoutputvoltageand
the crowbaroutputcan providea currentof 100mA
to switchonan externalSCR.ThisSCRistriggered
when the output voltage exceeds the nominal by
20%. There is no internal connectionbetween the
outputand crowbarsenseinputthereforethecrow-
barcan monitor eitherthe inputor theoutput.
ATTL-levelinhibitinputis providedforapplications
suchasremoteon/offcontrol.Thisinputisactivated
byhighlogiclevelanddisablescircuitoperation.Af-
ter an inhibit the L296 restarts under control of the
soft start network.
The thermaloverloadcircuit disablescircuit opera-
tion when the junction temperature reaches about
150
°
C andhas hysteresisto preventunstablecon-
ditions.
Figure1 :
Reset OutputWaveforms
L296 - L296P
3/22