The UC3842 is a reliable current-mode PWM controller IC widely used in DC-DC converters and off-line power supplies. With an 8-pin design and minimal external components, it helps build compact switching circuits with stable regulation. Its built-in current limiting and strong totem-pole output stage make it effective for driving N-channel MOSFETs while improving protection and switching performance.
What is UC3842?
The UC3842 is a current-mode PWM controller IC used in DC-DC converters and off-line power supplies. It works with only a few external parts, making it a good choice for compact switching power designs.
It includes built-in current limiting to protect the circuit during overloads. The totem-pole output stage can source or sink high peak current, so it can drive an N-channel MOSFET gate effectively. When the output is OFF, it stays low, helping the power switch turn off cleanly.
UC3842 Pinout Configuration
| Pin Number | Pin Name | Description |
|---|---|---|
| 1 | COMP (Compensation) | Error amplifier output. Usually connected to an RC network to stabilize the feedback loop and control response. |
| 2 | VFB (Voltage Feedback) | Feedback input for the error amplifier. Monitors output voltage through a feedback network and adjusts PWM to correct errors. |
| 3 | CS (Current Sense) | Current sense input. A shunt resistor creates a voltage signal that is fed here for current-mode control and current limiting. |
| 4 | RT/CT (Timing Resistor/Capacitor) | Sets the switching frequency using an external resistor and capacitor connected to the internal oscillator. |
| 5 | GND (Ground) | Ground reference for the IC and control circuit. |
| 6 | OUT (Output) | PWM output for driving a gate driver or directly switching a MOSFET, depending on the circuit design. |
| 7 | VCC | Power supply input for the IC. Typical operating range is 12V to 28V, depending on the design. |
| 8 | VREF | Reference voltage output used for internal stability and biasing. Also supports timing functions and should be bypassed properly. |
UC3842 Features and Specifications
Main Features
| Feature | Description |
|---|---|
| Current-mode PWM control | Provides stable regulation and fast response |
| Compact 8-pin package | Helps reduce circuit size and part count |
| DC-DC and off-line converter support | Works well in many switching power designs |
| Built-in current limiting | Adds protection during overload conditions |
| Totem-pole output driver | Enables fast switching of external MOSFETs |
Key Specifications
| Specification | Value / Details |
|---|---|
| Operating voltage (VCC) | 12V to 28V |
| Switching frequency | Up to 500 kHz |
| Maximum duty cycle | Up to 100% (depends on operating conditions) |
| Typical supply current | 25 mA |
| Output type | Transistor driver |
| Peak output drive current | Up to 1A (depends on design conditions) |
| Rise time / Fall time | ~50 ns |
| Mounting type | Through-hole |
| Package | 8-DIP |
| Operating temperature range | 0°C to 70°C |
| Technology | BiCMOS |
| Supported converter types | Buck, Boost, Flyback, Buck-Boost, Forward |
Equivalent and Alternative of UC3842
Equivalent
• UC3843 – A close alternative to the UC3842 with a similar current-mode PWM control function and the same 8-pin pinout. It is often used in similar SMPS designs, but it typically has a different undervoltage lockout (UVLO) start/stop level, so the startup and operating behavior may change slightly depending on the circuit.
Alternative
• UC2842 – Similar current-mode PWM controller to the UC3842 family, often used in off-line and DC-DC SMPS designs with stable regulation and strong gate drive support.
• UC3844 – A current-mode PWM controller with a similar function to the UC3842, but with different UVLO thresholds, which can affect startup and operating behavior in some circuits.
• SG2524 – A classic PWM control IC with a built-in oscillator, error amplifier, and output drive stage, commonly used in push-pull and forward-type power supply designs rather than direct UC3842 drop-in replacements.
UC3842 Applications
• SMPS (Switch Mode Power Supply) circuits – Regulates output voltage efficiently using PWM control, while maintaining stable performance under changing input and load conditions.
• DC-DC converter circuits (Buck, Boost, Flyback) – Controls MOSFET switching to convert voltage levels smoothly, using current-mode regulation for faster response and better stability.
• Off-line flyback power supplies – Commonly used in AC to DC flyback designs because it supports stable switching control, feedback regulation, and cycle-by-cycle current limiting for protection.
• Battery-powered conversion and regulation – Helps convert battery voltage into stable regulated outputs while limiting peak current during load surges for safer operation.
• Current-limited power control stages – Supports safer power delivery by using current-sense feedback to reduce excessive current during overload or fault conditions.
• Load switching and control systems – Drives switching devices to turn loads ON and OFF cleanly, while supporting controlled current flow and reducing stress on power components.
How to Use the UC3842 IC in a Circuit
The UC3842 is simple to use because it only needs a few external parts to build a stable current-mode PWM control loop. A regulated supply is applied to VCC (Pin 7), and switching begins after the IC passes its undervoltage lockout (UVLO) start threshold.
Basic Connection
VCC (Pin 7) powers the controller, while GND (Pin 5) is the reference point for the control and sensing circuits. The OUT pin (Pin 6) generates the PWM gate-drive signal, which is commonly used to switch an N-channel MOSFET in SMPS and DC-DC converter designs.
Feedback Regulation Path (VFB → COMP → PWM)
Output voltage regulation is controlled through the feedback loop:
• VFB (Pin 2) receives a scaled version of the output voltage through a resistor divider.
• The internal error amplifier compares this feedback level to the internal reference.
• COMP (Pin 1) provides the compensation output and is usually connected to an RC network.
• This compensation signal adjusts PWM behavior to keep the output voltage steady during input and load changes.
Current Sense and Cycle-by-Cycle Protection
Current control and protection are handled using:
• CS (Pin 3) for current-sense input
• A low-value shunt resistor to produce a small voltage proportional to switch current
When the voltage at CS reaches the internal limit level, the UC3842 reduces or terminates the switching pulse for that cycle. This supports cycle-by-cycle current limiting, which helps protect the power stage during overload or fault conditions.
To reduce false triggering, the current-sense trace should be kept short and clean, and the sense resistor should have low inductance.
Switching Frequency Setup
The switching frequency is set using external timing parts on:
• RT/CT (Pin 4)
A resistor and capacitor connected here control the oscillator timing, allowing the switching frequency to be tuned based on the converter design requirements.
VREF Bypassing and Layout Stability
VREF (Pin 8) provides a stable reference output that supports internal biasing and timing functions. For stable operation and lower noise, bypass VREF to GND with a ceramic capacitor placed close to the IC.
Good PCB layout strongly affects performance. Keeping power and sense grounding clean and placing bypass parts close to the IC helps improve regulation accuracy and prevents noise from interfering with current sensing.
Conclusion
UC3842 remains a practical choice for many switching power designs because it combines simple setup with strong control features. Its pin functions support feedback regulation, frequency setting, and cycle-by-cycle current limiting for safer operation. With proper compensation, clean current sensing, and good PCB layout, it can deliver stable PWM control in SMPS and DC-DC converter circuits with dependable performance.
Frequently Asked Questions [FAQ]
What is the typical startup voltage (UVLO) for UC3842?
The UC3842 starts switching only after its VCC rises above the undervoltage lockout (UVLO) start threshold. Once running, it can continue operating at a lower VCC until it reaches the UVLO stop level. This feature prevents unstable switching during low supply conditions and improves reliable startup behavior.
Why does the UC3842 output stay LOW when the controller is OFF?
The UC3842 uses a totem-pole output stage that forces the OUT-pin LOW when the controller is not switching. This helps keep the MOSFET gate discharged, reduces accidental turn-on, and improves switching safety during startup, shutdown, and fault conditions.
How do I choose the current-sense resistor value for the UC3842?
The current-sense resistor is selected so the voltage at the CS pin reaches the internal current-limit threshold at the desired peak switch current. A lower resistor value allows higher peak current, while a higher value limits current sooner. Using a low-inductance resistor and a clean CS trace helps avoid false triggering.
What causes UC3842 PWM jitter or unstable switching frequency?
PWM jitter is often caused by noise coupling into the CS pin, poor grounding, long sense traces, or improper compensation around COMP/VFB. Weak VREF bypassing can also increase instability. Keeping current-sense wiring short and placing bypass capacitors close to the IC helps stabilize switching behavior.
Can UC3842 directly drive a MOSFET gate without a separate driver?
In many low-to-medium power designs, the UC3842 can drive an N-channel MOSFET gate directly using its high peak output current. However, large MOSFET gate charge, high switching frequency, or tight rise/fall time targets may require an external gate driver to reduce losses and improve switching performance.