CIRCUI T OPERATION
(refer to the block diagram)
The L296 and L296P are monolithic stepdown
sw it c hing re gulators pr ov iding out pu t v oltages fr om
5. 1V to 40V and delive ri ng 4A .
The regulation loop consists of a sawtooth oscillator,
error amplifier, comparator and the output stage. An
error signal is produced by comparing the output
voltage with a precise 5.1V on-chip reference (zener
zap trimmed to ± 2 %). This error signal is then com-
pared with the sawtooth signal to gen erate the fixed
frequency pulse width modulated pulses which drive
the output stage. The gain and frequency stability of
the loop can be adjusted by an external RC network
connected to pin 9. Closing the loop directly gives an
output voltage of 5.1V. Higher voltages are obtained
by ins er ti ng a v ol ta ge di vi der .
Output overcur rents at s witch on are prevented by
the sof t st art function. The error amplifier output is
initially clamped by the external capacitor Css and
allo wed to ris e, line arly , as t his capacit or i s char ged
by a constant current source.
Output overload protection is provided in the form of
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
output stage and discharges the soft start capacitor.
A second comparator resets the flip flop when the
volta ge acro ss th e soft st art capaci to r has fall en to
0.4V. The output stage is thus re-enabled and the
output voltage rises under control of the soft start
netw ork. If the o verload c onditio n is sti ll p resent the
limiter will trigger again when the threshold current
is reached. The average s hort circuit current is lim-
ited to a safe v alu e by the dead time int ro duc ed by
the sof t star t network.
The reset circuit generates an output signal when
the supply voltage exceeds a threshold pro-
gram med by an exte rnal d ivide r. The re set si gnal is
gener ated wi th a delay tim e prog rammed by an ex -
ternal capacitor. When the supply falls below the
threshold the reset output goes low immediately.
The res et output is an open colle ctor.
The scr owbar ci rcui t s enses t he output vol tage an d
the cr owbar output can prov ide a current of 100mA
to switch on an external SCR. This SCR is triggered
when the output voltage exceeds the nominal by
20%. There is no internal connect ion between t he
outpu t and cr owbar sense i npu t th erefore the crow -
bar c an m oni to r ei ther th e inp ut or the out put.
A TT L - level in hibit input is provided for ap plications
such as remote on/off control. This input is activated
by hig h logic le vel 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 hys teres is to pre vent un stable c on -
ditions.
F ig ure 1 : Rese t Output Waveforms
L296 - L296P
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