UC3845B

Manufacturer Part NumberUC3845B
DescriptionHIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
ManufacturerSTMicroelectronics
UC3845B datasheet
 


1
Page 1
2
Page 2
3
Page 3
4
Page 4
5
Page 5
6
Page 6
7
Page 7
8
Page 8
9
Page 9
10
Page 10
11
12
13
14
15
Page 1/15

Download datasheet (140Kb)Embed
Next
HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER
.
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
DESCRIPTION
TheUC384xB family ofcontrolICs providesthe nec-
essary features to implement off-line or DC to DC
fixed frequency current mode control schemes with
a minimal external parts count. Internally imple-
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
BLOCK DIAGRAM (toggle flip flop used only in UC3844B and UC3845B)
7
Vi
UVLO
34V
5
GROUND
2.50V
4
RT/CT
OSC
ERROR AMP.
+
2
VFB
-
1
COMP
3
CURRENT
SENSE
March 1999
UC2842B/3B/4B/5B
UC3842B/3B/4B/5B
Minidip
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 UC3842B and UC3844B have
UVLO thresholds of 16V (on) and 10V (off), ideally
suitedoff-lineapplicationsThecorrespondingthresh-
oldsforthe UC3843BandUC3845Bare8.5 V and7.9
V. The UC3842B and UC3843B can operate to duty
cycles approaching 100%. A range of the zero to <
50 % is obtained by the UC3844B and UC3845B by
the addition of an internal toggle flip flop which blanks
the output off every otherclock cycle.
5V
S/R
REF
INTERNAL
BIAS
VREF GOOD
LOGIC
T
2R
S
R
PWM
R
1V
LATCH
CURRENT
SENSE
UC3842B
COMPARATOR
SO8
8
VREF
5V 50mA
6
OUTPUT
D95IN331
1/15

UC3845B Summary of contents