The UC1842/3/4/5 family of control devices provides the necessary features to implement off-line or dc-to-dc fixed frequency current mode control schemes with a minimal external parts count

UC2845

Manufacturer Part NumberUC2845
DescriptionThe UC1842/3/4/5 family of control devices provides the necessary features to implement off-line or dc-to-dc fixed frequency current mode control schemes with a minimal external parts count
ManufacturerTexas Instruments
UC2845 datasheet
 


Specifications of UC2845

TopologyBoost, Flyback, ForwardControl MethodCurrent
Duty Cycle(max)(%)50Frequency(max)(khz)450
Uvlo Thresholds On/off(v)8.4/7.6Pin/package14SOIC, 8PDIP, 8SOIC
Vin(min)(v)7.6Vin(max)(v)30
Regulated Outputs(#)1Operating Temperature Range(c)-40 to 85
1
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OPEN-LOOP LABORATORY FIXTURE
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypas
capacitors should be conected close to pin 5 in a single point ground. The transistor and 5k potentiometer are
used to sample the oscillator waveform and apply an adjustable ramp to pin 3.
2N2222
4.7 kW
100 kW
1 kW
ERROR AMP
ADJUST
5 kW
I
SENSE
4.7 kW
ADJUST
SHUTDOWN TECHNIQUES
Shutdown of the UC1842 can be accomplished by two methods; either raise pin 3 above 1 V or pull pin 1 below
a voltage two diode drops above ground. Either method causses the output of the PWM comparator to be high
(refer to block diagram). The PWM latch is reset dominant so that the output will remain low until the next clock
cycle after the shutdown condition at pin 1 and/or 3 is removed. In one example, an externally latched shutdown
may be accomplished by adding an SCR which will be reset by cycling V
this pint the reference turns off, allowing the SCR to reset.
1 kW
330 W
500 W
SHUTDOWN
R1
UC1842
COMP
1
V
8
REF
V
7
2
V
FB
CC
3
I
OUTPUT
6
SENSE
5
4
R
/ C
GROUND
T
T
C
T
V
8
REF
SHUTDOWN
I
3
SENSE
To Current
SENSE RESISTOR
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UC1842/3/4/5
UC2842/3/4/5
UC3842/3/4/5
SLUS223C – APRIL 1997 – REVISED JUNE 2007
V
REF
A
V
CC
0.1 mF
0.1 mF
1 kW 1 W
OUTPUT
GROUND
below the lower UVLO threshold. At
CC
COMP
1
7