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UC3845TN Datasheet

  • UC3845TN

  • Analog IC

  • 133.95KB

  • 15頁

  • ETC

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廬
UC3842T UC3843T
UC3844T UC3845T
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
.
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.
.
.
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.
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TRIMMED OSCILLATOR FOR PRECISE FRE-
QUENCY CONTROL
OSCILLATOR FREQUENCY GUARANTEED
AT 250kHz
CURRENT MODE OPERATION TO 500kHz
AUTOMATIC FEED FORWARD COMPENSA-
TION
LATCHING PWM FOR CYCLE-BY-CYCLE
CURRENT LIMITING
INTERNALLY TRIMMED REFERENCE WITH
UNDERVOLTAGE LOCKOUT
HIGH CURRENT TOTEM POLE OUTPUT
UNDERVOLTAGE LOCKOUT WITH HYSTER-
ESIS
LOW START-UP AND OPERATING CURRENT
Minidip
SO8
DESCRIPTION
The UC384XT family of control ICs provides the
necessary features to implement off-line or DC to
DC fixed frequency current mode control schemes
with a minimal externalparts count.Internallyimple-
mented circuits include a trimmed oscillator for pre-
cise DUTY CYCLE CONTROL under voltage lock-
outfeaturingstart-up current less than0.5mA,a pre-
cision reference trimmed for accuracy at the error
amp input, logicto insure latched operation,a PWM
comparatorwhich alsoprovidescurrent limit control,
and a totem pole output stage designed to source
or sink high peakcurrent. The outputstage, suitable
for driving N-Channel MOSFETs, is low in the off-
state.
Differences between members of this family are the
under-voltagelockout thresholds and maximum duty
cycle ranges. The UC3842T and UC3844T have
UVLO thresholds of 16V (on) and 10V (off), ideally
suited to off-line applications The corresponding
thresholds for the UC3843T and UC3845T are 8.5 V
and 7.9 V. The UC3842T and UC3843T canoperate
to duty cycles approaching 100%. A range of zero to
< 50 % is obtainedby theUC3844T and UC3845T by
the addition of an internal toggle flip flopwhich blanks
the output off every otherclock cycle.
BLOCK DIAGRAM
(toggle flip flop used only in UC3844T and UC3845T)
Vi
7
34V
GROUND
5
UVLO
S/R
5V
REF
INTERNAL
BIAS
VREF GOOD
LOGIC
RT/CT
4
OSC
ERROR AMP.
2R
R
1V
T
8
VREF
5V 50mA
2.50V
6
OUTPUT
VFB
COMP
CURRENT
SENSE
2
1
3
+
-
S
R
CURRENT
SENSE
COMPARATOR
D95IN331
PWM
LATCH
September 2001
1/15

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