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UC3845AN / UC3845BN / UC3843AN / UC3842 Current-Mode PWM Controller — DIP-8 (5-Pack)
UC3845AN / UC3845BN / UC3843AN / UC3842 Current-Mode PWM Controller DIP-8 (5-Pack)
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UC3845AN / UC3845BN / UC3843AN / UC3842 Current-Mode PWM Controller — DIP-8 (5-Pack)
The UC384x series are industry-standard current-mode PWM controller ICs in DIP-8 packages, widely used in flyback, forward, and boost switching power supply designs. They integrate an oscillator, error amplifier, current-sense comparator, and gate driver in a single package, providing all the control functions needed for a complete SMPS. The UC3842/UC3843/UC3845 variants differ in startup voltage and maximum duty cycle, making them suitable for different SMPS topologies and input voltage ranges.
Variant Comparison
| Part Number | Startup Voltage | Max Duty Cycle | Best For |
|---|---|---|---|
| UC3842 / UC3842A | 16V (UVLO on: 16V, off: 10V) | 100% | Continuous-mode flyback, forward converter |
| UC3843 / UC3843AN | 8.5V (UVLO on: 8.5V, off: 7.6V) | 100% | Low-voltage startup SMPS, battery-powered |
| UC3845 / UC3845AN / UC3845BN | 16V (UVLO on: 16V, off: 10V) | 50% | Push-pull, half-bridge, two-switch forward |
Key Specifications (All Variants)
| Parameter | Value |
|---|---|
| Package | DIP-8 (through-hole) |
| Oscillator Frequency | Up to 500kHz (external R/C) |
| Gate Drive Output | 1A peak source/sink |
| Reference Voltage | 5.0V (±1%) |
| Error Amplifier GBW | 1MHz |
| Current Sense Input | 1V threshold (cycle-by-cycle OCP) |
| Operating Temperature | 0°C to +70°C (UC38xx) / -40°C to +85°C (UC28xx) |
| Quantity | 5 pieces |
Why Choose UC384x Series?
- Cycle-by-cycle current limiting — instantaneous overcurrent protection prevents transformer saturation
- 1A gate drive — directly drives power MOSFETs up to ~100nC gate charge without external buffer
- 500kHz max frequency — allows compact transformer and filter designs
- 5V precision reference — stable reference for feedback network design
- Industry-standard pinout — compatible with UC2842/UC2843/UC2845 (industrial grade) and KA3842/KA3843 (Samsung equivalents)
Compatible With / Common Use Cases
- Flyback SMPS repair: Most common repair IC in ATX power supplies, monitor power boards, and industrial SMPS
- Forward converter design: UC3842/UC3843 for single-switch forward topology
- Push-pull / half-bridge: UC3845 (50% max duty cycle) for push-pull and half-bridge topologies
- ATX power supply repair: UC3842 or UC3843 is the primary PWM controller in most ATX PSUs
- Reference designs: Texas Instruments SLUA107, ON Semiconductor AND8016 application notes
Frequently Asked Questions
Q: What is the difference between UC3842 and UC3843?
A: Both have 100% max duty cycle and identical pinout. The key difference is UVLO (undervoltage lockout) threshold: UC3842 starts at 16V (suitable for offline SMPS with bootstrap startup), UC3843 starts at 8.5V (suitable for low-voltage and battery-powered SMPS). Use UC3843 when your VCC supply cannot reach 16V during startup.
Q: Why does my SMPS use UC3845 instead of UC3842?
A: UC3845 limits maximum duty cycle to 50%, which is required for push-pull and half-bridge topologies to prevent simultaneous conduction of both switches (shoot-through). UC3842 allows up to 100% duty cycle, suitable for single-switch flyback and forward converters only.
Q: Can I replace UC3842 with KA3842 or SG3842?
A: Yes — KA3842 (Samsung/Fairchild), SG3842 (Silicon General), and UC3842 (TI/ON Semi) are functionally identical with the same pinout and specifications. They are direct drop-in replacements for each other.
Package Contents
- 5× UC3845AN / UC3845BN / UC3843AN / UC3842 PWM Controller IC (DIP-8) — variant as selected
