UC3843AN

The UC3843AN is a high-performance, fixed-frequency current-mode PWM controller IC designed for isolated and non-isolated switch-mode power supplies.

It improves on the UC3842 by allowing:

Lower minimum supply voltage

Higher oscillator frequency

Better performance in low-power or high-frequency SMPS

Commonly found in: adapters, LED drivers, industrial power supplies, and battery chargers.

✨ Key Features

✅ Current-mode PWM control
✅ Fixed-frequency oscillator
✅ Undervoltage lockout (UVLO)
✅ High-speed output driver (1 A source/sink)
✅ Low startup current (~0.5 mA)
✅ Thermal shutdown protection
✅ Duty cycle limited to ~50%
✅ 8-pin DIP/SOIC package

📊 Electrical Specifications
Parameter Typical / Max Value
Supply voltage (VCC) 7.6 V – 30 V
Startup voltage 16 V typical
Undervoltage lockout (UVLO) 8.5 V rising / 7.6 V falling
Oscillator frequency 47 kHz – 500 kHz
Output current (peak) ±1 A
Duty cycle 50% max
Input bias ~0.5 mA startup
Package 8-pin PDIP / SOIC
Operating temp 0 °C – 70 °C
🔌 Pin Configuration (DIP-8 / AN Version)
UC3843AN
┌───────────┐
1 COMP VCC 8
2 VFB CS 7
3 RT/CT GND 6
4 OUT VREF 5
└───────────┘

Pin functions (simplified):

VCC (8) → IC supply

GND (6) → ground

OUT (4) → PWM output to MOSFET/IGBT

CS (7) → current sense input for overcurrent protection

VFB (2) → feedback input from output voltage

COMP (1) → error amplifier output (duty cycle control)

RT/CT (3) → sets oscillator frequency

VREF (5) → 5 V reference for feedback and compensation

⚙️ How It Works (Overview)

Voltage feedback → compares output to internal reference

Error amplifier (COMP) → adjusts PWM duty cycle

Current sense (CS) → limits MOSFET peak current

PWM drives power switch → regulates output

Fixed-frequency operation → predictable switching and easier filtering

It’s current-mode PWM, which gives fast response and inherent overcurrent protection.

🛠️ Typical Applications

Switch-mode power supplies (SMPS)

Flyback converters

Buck / boost converters

LED drivers

Battery chargers

Industrial power supply modules

💡 Design Tips
✅ Start-up

Provide VCC from auxiliary winding or resistor until UVLO allows IC to start.

✅ Oscillator

Choose RT/CT for desired switching frequency; typical 50–100 kHz for small SMPS.

✅ Current limiting

CS pin protects MOSFET; ensure correct sensing resistor.

✅ Thermal management

The IC itself handles little heat; focus on MOSFET and transformer.

✅ PWM output

Compatible with power MOSFET or IGBT, capable of 1 A peak drive.

✅ Quick Summary

UC3843AN = high-performance current-mode PWM controller IC

Perfect for:
✔ high-efficiency switch-mode power supplies
✔ overcurrent protection
✔ high-frequency operation
✔ industrial and consumer SMPS designs