參數(shù)資料
型號： L4964HT
廠商： 意法半導體
元件分類： 基準電壓源/電流源
英文描述： Isolated Flyback Switching Regulator with 9V Output
中文描述： 隔離反激式開關穩(wěn)壓9V輸出
文件頁數(shù)： 3/13頁
文件大?。?/td> 208K
代理商： L4964HT
CIRCUIT OPERATION
(refer to the block diagram)
TheL4964isamonolithicstepdownswitchingregu-
lator providing output voltages from 5.1 V to 28 V
anddelivering 4A.
Theregulationloopconsistsofasawtoothoscillator,
erroramplifier,comparatorandtheoutputstage.An
error signal is produced by comparing the output
voltage with a precise 5.1 V on-chip reference
(zener zap trimmed to
±
3 %). This error signal is
thencomparedwith thesawtoothsignalto generate
the fixed frequency pulse width modulated pulses
which drive the output stage. The gain and fre-
quencystabilityof theloop canbeajustedbyan ex-
ternal RC network connectedto pin 9. Closing the
loopdirectlygivesan outputvoltageof5.1V.Higher
voltagesare obtainedby inserting a voltagedivider.
Outputovercurrents at switch on are preventedby
the soft start function.The error amplifier output is
initiallyclampedbytheexternalcapacitorC
ss
andal-
lowed to rise, linearly, as this capacitor is charged
bya constantcurrent source.
Outputoverloadprotectionisprovidedintheformof
a current limiter. The load current is sensed by an
internal metal resistor connectedto a comparator.
When the load current exceedsa preset threshold
this comparator sets a flip flop which disables the
outputstageanddischargesthesoftstartcapacitor.
A second comparator resets the flip flop when the
voltageacross the soft start capacitorhas fallento
0.4 V. The output stage is thus re-enable and the
outputvoltagerisesundercontroofthesoftstartnet-
work. If the overload condition is still present the
limiter will trigger againwhen the thersholdcurrent
is reached. The average shortcircuit current is lim-
ited to a safe value by thedead time introducedby
the soft start network.
The reset circuit generates an output signal when
the supply voltage exceeds a threshold pro-
grammedbyan externaldivider. The reset signal is
generatedwith adelaytime programmed byan ex-
ternal capacitor. When the supply falls below the
threshold the reset output goes low immediately.
The reset outputis an opencollector.
A TTL - level input is provided for applications such
as remote on/off control.This input is activated by
highlevel and disablescircuit operation.Afteran in-
hibittheL4964restartsundercontrolofthesoftstart
network.
The thermal overload circuit disables circuit opera-
tion when the junction temperaturereaches about
150 and has hysteresis to prevent unstablecondi-
tions.
Figure 1 :
Reset Output Waveforms
L4964
3/13
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