Datasheet

CIRCUIT OPERATION
(refer to the block diagram)
The L296 and L296P are monolithic stepdown
switching regulators providing output voltages from
5.1V to 40V and delivering 4A.
The regulationloopconsistsof a sawtoothoscillator,
error amplifier,comparatorand theoutput stage.An
error signal is produced by comparing the output
voltagewith a precise5.1Von-chip reference(zener
zaptrimmed to± 2%).Thiserrorsignalis thencom-
paredwith the sawtoothsignalto generatethe fixed
frequencypulsewidthmodulatedpulses whichdrive
theoutput stage.The gain andfrequencystability of
theloop can beadjustedbyan externalRC network
connectedtopin9.Closingtheloopdirectlygives an
outputvoltageof5.1V.Higher voltagesareobtained
by inserting a voltage divider.
Output overcurrents at switch on are preventedby
the soft start function. The error amplifier output is
initially clamped by the external capacitor Css and
allowedto rise, linearly, as this capacitor is charged
by a constant current source.
Outputoverloadprotectionis providedin the formof
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
outputstage anddischargesthe softstartcapacitor.
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 the overload condition is still present the
limiter will trigger again when the thresholdcurrent
is reached. The average short circuit current is lim-
ited to a safe value by the dead time introduced by
the soft start network.
The reset circuit generates an output signal when
the supply voltage exceeds a threshold pro-
grammed by an externaldivider. The reset signal is
generated with 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 output voltageand
the crowbar output can provide a currentof 100mA
to switch on an externalSCR. ThisSCR is triggered
when the output voltage exceeds the nominal by
20%. There is no internal connection between the
outputand crowbar sense input thereforethe crow-
bar can monitor either the input or the output.
A TTL - level inhibit inputis providedfor applications
suchas remote on/offcontrol.This input is activated
byhigh logic level and disables circuit operation. Af-
ter an inhibit the L296 restarts under control of the
soft start network.
The thermal overload circuit disables circuit opera-
tion when the junction temperature reaches about
150 °C and has hysteresis to prevent unstablecon-
ditions.
Figure 1 : Reset Output Waveforms
L296 - L296P
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