UC3844, UC3845, UC2844, UC2845
VCC
VCC 7(12)
Vref
8(14)
R
2.5V
RT
R
Internal
Bias
4(7)
CT
Voltage Feedback
Input
2(3)
Output
Compensation
1(1)
Oscillator
+
1.0mA
+
−
Error
Amplifier
2R
R
Reference
+
36V
Regulator
+
VCC − +
UVLO −
3.6V + − Vref
− UVLO
TQ
−
+
1.0V
Current Sense
Comparator
S
Q
R PWM
Latch
VC
7(11)
Output
6(10)
Power Ground
5(8)
Current Sense Input
3(5)
GND 5(9)
+
−
Pin numbers in parenthesis are for the D suffix SOIC−14 package.
=
Sink Only
Positive True Logic
Figure 16. Representative Block Diagram
Vin
Q1
RS
Capacitor CT
Latch
‘‘Set’’ Input
Output/
Compensation
Current Sense
Input
Latch
‘‘Reset’’ Input
Output
Large RT/Small CT
Figure 17. Timing Diagram
Small RT/Large CT
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