The UC3846 is HeroMicro a high-performance current-mode pulse-width modulation (PWM) controller designed for high-frequency switching power supplies. It is widely used in DC-DC converters, inverters, motor drives, and industrial power systems. Its current-mode control architecture enables fast transient response, cycle-by-cycle current limiting, and excellent load regulation. By integrating an oscillator, error amplifier, current-sense comparator, latch, and totem-pole output stage, the UC3846 simplifies power supply design while enhancing system reliability.
1. Device Overview
Type: Current-Mode PWM Controller
Package: 16-pin DIP or SOIC
Operating Supply Voltage (VIN): 8 V to 40 V
Start-Up Threshold: Typical 7.7 V (with 0.75 V hysteresis)
Output Drive Capability:
High-Level Output: ≥13 V (@ ISOURCE = 20 mA)
Low-Level Output: ≤0.4 V (@ ISINK = 20 mA)
Peak Sink Current: 100 mA
Oscillator Frequency: Up to 500 kHz
Shutdown Threshold: Typical 350 mV (Pin 1)
2. Key Functional Blocks & Pin Descriptions
Error Amplifier
Regulates output voltage with high gain (80–105 dB PSRR). Its output drives external compensation networks for stable loop control.
Current Sense Amplifier
Typical gain: 2.75 V/V. Input common-mode range: 0 to VIN–3 V. Max differential input: 1.2 V. Enables cycle-by-cycle current limiting with 200 ns typical response delay.
Oscillator & Sync Input
Frequency set by external RT/CT. Pin 8 accepts external sync signal (low level ≤2.5 V) for multi-module synchronization or system clock alignment.
Shutdown Function
When voltage at Pin 1 exceeds 350 mV (typical), outputs shut down immediately (delay ≤600 ns). Ideal for overcurrent, overtemperature, or external fault protection.
Totem-Pole Outputs (Pins 11 & 14)
Complementary outputs capable of directly driving MOSFET/IGBT gates. Rise/fall times ≤300 ns (CL = 1 nF).
3. Key Usage Guidelines
(1) Power Supply & Start-Up Design
VIN must exceed 8 V for normal operation. Place a 0.1 μF ceramic capacitor and a 10 μF electrolytic capacitor close to VIN to suppress noise.
Ensure reliable start-up when VIN ≥7.7 V, leveraging the 0.75 V hysteresis to prevent chatter.
(2) Current Sensing & Slope Compensation
Connect current sense signal to Pin 3 (CS–) and Pin 4 (CS+). Differential input must not exceed 1.2 V.
For duty cycles >50%, add slope compensation (inject ramp signal at Pin 4) to avoid subharmonic oscillation.
(3) Loop Compensation
Connect an RC compensation network from Pin 7 (error amp output) to ground. Use Type II or Type III compensation to ensure phase margin >45°.
(4) Output Drive Capability
The internal driver can directly drive small-to-medium MOSFETs. For high-power devices, use an external gate driver to improve switching speed and reduce losses.
(5) PCB Layout & Noise Immunity
Keep current-sense traces short and away from high-noise nodes (e.g., switch node, output diode).
Use single-point grounding to separate analog and power grounds.
4. Typical Applications
Forward / Flyback / Push-Pull DC-DC Converters
Half-Bridge / Full-Bridge Power Supplies
Motor Drive PWM Control
Industrial UPS and Inverters
Battery Charging Systems
5. Summary
The UC3846 remains a classic choice for medium-to-high-power switching supplies due to its robust current-mode architecture, high integration, and comprehensive protection features. By carefully designing the start-up circuit, current sensing, loop compensation, and PCB layout, engineers can build efficient, stable, and reliable power systems with this versatile controller.
