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PDF UC3842 Fiche technique - Philips

Numéro de référence UC3842
Description Current-mode PWM controller
Fabricant Philips 
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UC3842 Datasheet, Description
Philips Semiconductors Linear Products
Current-mode PWM controller
Product specification
UC3842
DESCRIPTION
The UC3842 is available in an 8-Pin mini-DIP the necessary
features to implement off-line, fixed-frequency current-mode control
schemes with a minimal external parts count. This technique results
in improved line regulation, enhanced load response characteristics,
and a simpler, easier to design control loop. Topological advantages
include inherent pulse-by-pulse current limiting.
Protection circuitry includes built-in undervoltage lock-out and
current limiting. Other features include fully-latched operation, a 1%
trimmed bandgap reference, and start-up current less than 1mA.
These devices feature a totem-pole output designed to source and
sink high peak current from a capacitive load, such as the gate of a
power MOSFET. Consistent with N-channel power devices, the
output is low in the OFF-state.
FEATURES
Low start-up current (1mA)
Automatic feed-forward compensation
Pulse-by-pulse current limiting
Enhanced load response characteristics
Undervoltage lock-out with hysteresis
Double pulse suppression
High current totem-pole output
Internally-trimmed bandgap reference
400kHz operation, guaranteed min
PIN CONFIGURATIONS
N Package
COMP 1
VFB 2
ISENSE
RT/CT
3
4
8 VREF
7 VCC
6 OUTPUT
5 GROUND
TOP VIEW
D Package
COMP
NC
VFB
NC
ISENSE
NC
RT/CT
1
2
3
4
5
6
7
14 VREF
13 NC
12 VCC
11 VC
10 OUTPUT
9 GROUND
8 POWER GROUND
TOP VIEW
APPLICATIONS
Off-line switched mode power supplies
DC-to-DC converters UC3842
BLOCK DIAGRAM
(12)7
VCC
(9)5
GND
(7)4
RT/CT
34V UVLO
16V
2.5V
6V
OSC
S/R 5V
REF
INTERNAL
BIAS
(3)2
VFB
(1)1
COMP
CURRENT (5)3
SENSE
ERROR
AMP
+
NOTE:
Pin numbers in parentheses refer to the D package.
2R
R
S
R
1V
CURRENT
SENSE
COMPARATOR
PWM
LATCH
7(11)
8(14)
VREF
5.0V
50mA
6(10)
OUTPUT
5(8)
August 31, 1994
1100
853-0614 13721
UC3842 Fiche technique
Philips Semiconductors Linear Products
Current-mode PWM controller
Product specification
UC3842
DC AND AC ELECTRICAL CHARACTERISTICS
0TJ70°C for UC3842; VCC=15V; RT=10kW; CT=3.3nF, unless otherwise specified.
SYMBOL
PARAMETER
TEST CONDITIONS
Reference section
VOUT
Output voltage
Line regulation
VNOISE
Load regulation
Temp. stability1
Total output variation1
Output noise voltage1
Long-term stability1
Output short-circuit
Output short-circuit
Oscillator section
Initial accuracy
Voltage stability
Temp. stability1
Amplitude
Error amp section
Input voltage
IBIAS
AVOL
Input bias current
Unity gain bandwidth1
Unity gain bandwidth
PSRR Power supply rejection ratio
ISINK
Output sink current
ISOURCE Output source current
VOUT High
VOUT Low
Current sense section
PSRR
Gain 2, 3
Maximum input signal2
Power supply rejection ratio2
IBIAS
Input bias current
Delay to output1
TJ=25°C, IO=1mA
12VIN25V
1IO20mA
Line, load, temp.
10Hzf10kHz, TJ=25°C
TJ=125°C, 1000 Hrs.
TJ=25
-55<TJ0°C
TJ=25°C
12VCC25V
TMINTJTMAX
VPIN 4 peak-to-peak
V Pin 1=2.5V
2VO4V
TJ=25°C
TMIN<TJ<TMAX
12VCC25V
VPIN 2=2.7V, VPIN 1=1.1V
VPIN 2=2.3V, VPIN 1=5V
VPIN 2=2.3V, RL=15k to ground
VPIN 2=2.7V, RL=15k to Pin 8
VPIN 1=5V
12VCC25V
UC3842
Min Typ Max
UNIT
4.90 5.00 5.10
6 20
6 25
0.2 0.4
4.82 5.18
50
5 25
-30 -100 -130
-30 -100 -180
V
mV
mV
mV/°C
V
µV
mV
mA
mA
47 52 57
0.2 1
5
1.7
kHz
%
%
V
2.42 2.50 2.58
-0.3 -2
65 90
0.7 1
0.5
60 70
26
-0.5 -0.8
56
0.7 1.1
V
µA
dB
MHz
MHz
dB
mA
mA
V
V
2.85 3 3.15
0.9 1 1.1
70
-2 -10
150 300
V/V
V
dB
µA
ns
August 31, 1994
1102

3 Page

UC3842 pdf
Philips Semiconductors Linear Products
Current-mode PWM controller
Product specification
UC3842
OPEN-LOOP LABORATORY TEST FIXTURE
VREF
RT A
2N2222
4.7k
100k
1 UC3842
8
COMP
VREF
0.1µF
VCC
ERROR AMP
ADJUST
4.7k
5k
ISENSE
ADJUST
2
VFB
3
ISENSE
4
RT/CT
7
VCC
6
OUTPUT
5
GND
0.1µF
1k
1W
OUTPUT
GND
NOTE:
CT
High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected 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.
SHUTDOWN TECHNIQUES
4.7k 8
4.7k
SHUTDOWN
3
ISENSE
500
SHUTDOWN
1
COMP
TO CURRENT
SENSE RESISTOR
NOTE:
Shutdown of the UC3842 can be accomplished by two methods; either raise Pin 3 above 1V or pull Pin 1 below a voltage two diode drops above ground. Either method causes 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 Pins 1 and/or 3 is removed. In the examples shown, an externally-latched shutdown may be accomplished by adding an SCR which will be reset by cycling
VCC below the lower UVLO threshold (10V). At this point all internal bias is removed, allowing the SCR to reset.
August 31, 1994
1105

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