L296 - L296P
CIRCUIT OPERATION
(refer to the block diagram)
0.4V. The output stage is thus re-enabled and the
output voltage rises under control of the soft start
network.If the overload conditionis still presentthe
limiter will trigger again when the thresholdcurrent
is reached. The averageshort circuit current is lim-
ited to a safevalue by the dead time introduced by
the soft start network.
The L296 and L296P are monolithic stepdown
switching regulatorsproviding outputvoltages from
5.1V to 40V and delivering 4A.
Theregulationloopconsistsofasawtoothoscillator,
erroramplifier,comparatorandtheoutputstage.An
error signal is produced by comparing the output
voltagewithaprecise5.1Von-chipreference(zener
zaptrimmedto ± 2%).Thiserror signalis thencom-
paredwith the sawtoothsignalto generatethefixed
frequencypulsewidthmodulatedpulseswhichdrive
theoutputstage.Thegainandfrequencystabilityof
theloopcan beadjustedby anexternalRCnetwork
connectedtopin9. Closingtheloopdirectlygivesan
outputvoltageof5.1V.Highervoltagesareobtained
by inserting a voltagedivider.
The reset circuit generates an output signal when
the supply voltage exceeds a threshold pro-
grammed by an externaldivider.Thereset signal is
generatedwith a delay time programmed by an ex-
ternal capacitor. When the supply falls below the
threshold the reset output goes low immediately.
The reset outputis an open collector.
The scrowbar circuit sensesthe outputvoltage and
the crowbar outputcan provide a currentof 100mA
to switch onan externalSCR. This SCR is triggered
when the output voltage exceeds the nominal by
20%. There is no internal connection between the
outputand crowbarsenseinput thereforethe crow-
bar can monitor eitherthe inputor the output.
Output overcurrents at switch on are prevented by
the soft start function. The error amplifier output is
initially clamped by the externalcapacitor Css and
allowed to rise, linearly, as thiscapacitoris charged
by a constantcurrent source.
A TTL-levelinhibitinputis providedforapplications
suchasremoteon/offcontrol.Thisinputis activated
by highlogic levelanddisablescircuit operation.Af-
ter an inhibit the L296 restarts under control of the
soft start network.
Outputoverloadprotectionis providedintheformof
a current limiter. The load current is sensed by an
internal metal resistor connected to a comparator.
When the load current exceeds a preset threshold
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
The thermal overload circuit disables circuit opera-
tion when the junction temperature reaches about
150 °C andhas hysteresisto preventunstablecon-
ditions.
Figure 1 : Reset OutputWaveforms
3/22
STMICROELECTRONICS [ ST ]
