The UC3845 is a classic fixed-frequency, current-mode PWM controller from HeroMicro designed specifically for off-line and DC-to-DC converter applications. Known for its high integration, minimal external component count, and built-in protections, it is widely used in low-to-medium power switch-mode power supplies (SMPS), such as AC/DC adapters, industrial auxiliary supplies, and LED drivers. Unlike high-duty-cycle variants like the UC3843, the UC3845 features a maximum duty cycle clamped near 50%, making it ideal for flyback topologies that rely on natural transformer reset.
1. Key Features Overview
Based on the UC3845 datasheet:
Control Architecture: Peak current-mode control
Maximum Duty Cycle: 48% (typical) — prevents transformer saturation, ideal for DCM/QR flyback
Start-up Current: 0.3 mA typ. @ Vi = 6.5V, suitable for low standby power designs
Operating Frequency: Up to 500 kHz (set by external RT/CT)
Supply Voltage Range:
Start Threshold (VTH): 8.8 V (typ.)
UVLO Turn-off (VOPR): 7.6 V (typ.)
UVLO Hysteresis: 1.2 V for noise immunity
Output Drive: ±1 A peak current, capable of directly driving power MOSFETs
Reference Voltage (VREF): 5.0 V ±2% (@ Io = 1 mA), powers RT and feedback network
Operating Temperature: 0°C to +70°C (commercial grade)
Packages: DIP8 / SOP8
2. Pin Functions (8-Pin)
| Pin | Name | Description |
|---|---|---|
| 1 | COMP | Error amplifier output; connect compensation network (RC) |
| 2 | VFB | Inverting input of error amp; connect output voltage divider |
| 3 | ISENSE | Current-sense input; connect to sense resistor across switch |
| 4 | RT/CT | Connect timing resistor RT (to VREF) and capacitor CT (to GND) |
| 5 | GROUND | Common ground for control and power stages |
| 6 | OUTPUT | Totem-pole output; drives MOSFET gate directly (±1A peak) |
| 7 | Vi | IC supply pin (decouple with ≥1 μF capacitor) |
| 8 | VREF | Internal 5V reference output (up to 20mA) |
3. Critical Design Guidelines
(1) 50% Duty Cycle Clamp — Ideal for Flyback
The max duty cycle is internally limited to ~48%, ensuring sufficient off-time for natural transformer reset and preventing core saturation.
Makes UC3845 the go-to choice for discontinuous conduction mode (DCM) or quasi-resonant (QR) flyback converters (5–100 W range).
(2) Start-up & UVLO Circuit
Turns on at Vi > 8.8V and shuts down below 7.6V, with 1.2V hysteresis to prevent chatter.
Use a high-voltage start-up resistor and place a ≥1 μF low-ESR ceramic capacitor close between Vi and GND.
(3) RT/CT Frequency Setting
Oscillator frequency: f ≈ 1.72 / (RT × CT) (RT in kΩ, CT in nF).
Example: RT = 10 kΩ, CT = 3.3 nF → f ≈ 52 kHz
The duty cycle clamp is determined by the CT discharge slope and cannot be easily exceeded.
(4) Current-Mode Control & Protection
PWM pulse terminates when ISENSE voltage ≥ 1 V (typ.), providing fast cycle-by-cycle current limiting.
Keep the sense resistor and ISENSE trace short and isolated from noise.
(5) No Slope Compensation Needed
Since max duty cycle < 50%, subharmonic oscillation cannot occur.
✅ No external slope compensation required, simplifying design.
(6) PCB Layout Recommendations
Keep ISENSE trace away from switching nodes (e.g., MOSFET drain) and OUTPUT pin.
Place VFB divider resistors close to the IC with a dedicated local ground return.
Decoupling capacitor on Vi must be placed as close as possible to pins 7 and 5.
4. Typical Applications
5–60 W flyback AC/DC adapters (phone chargers, laptop supplies)
Industrial equipment auxiliary power
LED drivers (CV or primary-side CC)
Internal power modules in home appliances, printers, set-top boxes
5. Summary
The UC3845 remains a benchmark for cost-effective, reliable flyback designs thanks to its 50% duty cycle limit, built-in UVLO, strong drive capability, and elimination of slope compensation. Its inherent protection against transformer saturation and simplified control loop make it an excellent choice for commercial-grade, medium-power SMPS applications. With careful attention to start-up circuitry, RT/CT selection, and PCB layout, engineers can achieve high efficiency and robust performance in a wide range of consumer and industrial products.