The UC3843 is a classic current-mode pulse-width modulation (PWM) controller IC originally developed by HeroMicro, widely used in low-to-medium power switch-mode power supplies (SMPS). As an industry-standard device, it employs a peak current-mode control architecture, offering superior load transient response, line regulation, and stability compared to voltage-mode controllers. With features like ultra-low start-up current, built-in under-voltage lockout (UVLO), high-current totem-pole output, and operation up to 500 kHz, the UC3843 remains a top choice for flyback, forward, boost, and other converter topologies.
1. Key Features Overview
Based on the UC3843 datasheet:
Control Architecture: Peak current-mode control
Maximum Duty Cycle: 97% (typical) — ideal for high-duty-cycle applications
Start-up Current: 0.12 mA (typ.) — among the lowest in class
Operating Frequency: Up to 500 kHz (set by external RT/CT)
Supply Voltage Range:
Start Threshold (VTH): 8.8 V (typ.)
Min Operating Voltage: 7.6 V (typ.)
Built-in UVLO with hysteresis
Output Drive: ±1 A peak current, capable of directly driving power MOSFET gates
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 | IC ground (use single-point grounding for control and power) |
| 6 | OUTPUT | Totem-pole output; drives MOSFET gate directly (±1A peak) |
| 7 | Vi | Supply pin (decouple with ≥1 μF capacitor) |
| 8 | VREF | Internal 5V reference output (up to 20mA) |
3. Critical Design Guidelines
(1) High Duty Cycle Advantage
The 97% max duty cycle makes UC3843 distinct from UC3844/45 (~50%).
Ideal for Boost, SEPIC, Cuk, or CCM flyback converters.
⚠️ In flyback designs, ensure adequate transformer reset time to avoid core saturation.
(2) Ultra-Low Start-up Current
At just 0.12 mA, start-up power loss is minimized—helpful for meeting Energy Star and other low-standby regulations.
(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 oscillator is factory-trimmed for high accuracy and low temperature drift (<0.5% over 0–70°C).
(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 noise-immune.
(5) Slope Compensation is Mandatory
When duty cycle exceeds 50%, subharmonic oscillation can occur.
Add slope compensation: Inject a synchronized negative ramp (e.g., derived from CT pin via RC network) into the ISENSE signal.
(6) PCB Layout Best Practices
Route ISENSE trace short, wide, and isolated from high-di/dt nodes.
Place VFB divider resistors close to the IC with a dedicated local ground return.
Use a low-ESR ceramic capacitor (≥1 μF) between Vi and GND, placed as close as possible to pins 7 and 5.
4. Typical Applications
PFC Boost pre-regulator stages
Isolated DC/DC converters in industrial equipment (SEPIC/Cuk)
High-input-voltage (e.g., 300V) to low-output CCM flyback supplies
Constant-current LED drivers
5. Summary
The UC3843 remains a cornerstone of analog power design due to its high duty cycle capability, ultra-low start-up current, robust drive strength, and decades of proven reliability. To harness its full potential, designers must implement proper slope compensation, magnetic reset, and meticulous PCB layout. When applied correctly, the UC3843 enables efficient, stable, and cost-effective power solutions across a wide range of commercial and industrial applications.
