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UC3844 Controller Introduction

UC3844 is a fixed frequency current mode PWM controller. The IC is designed for off-line and DC-DC converter applications with minimal external components.

What is UC3844 Controller

The UCx84x series of control integrated circuits provide the features that are necessary to implement off-line or DC-to-DC fixed-frequency current-mode control schemes, with a minimum number of external components. The internally implemented circuits include an undervoltage lockout (UVLO), featuring a start-up current of less than 1 mA, and a precision reference trimmed for accuracy at the error amplifier input. Other internal circuits include logic to ensure latched operation, a pulse-width modulation (PWM) comparator that also provides current-limit control, and a totem-pole output stage that is designed to source or sink high-peak current. The output stage, suitable for driving N-channel MOSFETs, is low when it is in the off state.

The UCx84x family offers a variety of package options, temperature range options, choice of maximum duty cycle, and choice of turnon and turnoff thresholds and hysteresis ranges. Devices with higher turnon or turnoff hysteresis are ideal choices for off-line power supplies, while the devices with a narrower hysteresis range are suited for DC-DC applications. The UC184x devices are specified for operation from –55°C to 125°C, the UC284x series is specified for operation from –40°C to 85°C, and the UC384x series is specified for operation from 0°C to 70°C.

UC3844 datasheet

UC3844 Pinout &Configuration

UC3844 has eight terminals, as shown in the table.

Pin NumberPin NameDescription
1COMP (Comparator)It is an output pin that outputs low impedance 1MHz signal based on the difference between the set and current-voltage. It is normally connected to the voltage feedback pin of the IC through a resistor and capacitor.
2VFB (Voltage Feedback)It is an input to the error amplifier inside the IC. The difference in voltage level is supplied to this pin.
3Current SenseA shunt resistor is used to monitor the current through the circuit, and the voltage across it is provided as a feedback to the current sense pin
4RT/CT (Timing Resistor/ Timing Capacitor)The IC has an internal oscillator which can be set using an external resistor and capacitor connected to this pin.
5GroundConnected to the ground of the circuit
6OutputThis pin outputs the PWM signal based on the feedback provided and we can use this to switch the power electronic device.
7VccSupply voltage for the IC (Nominal 11V)
8VREFReference voltage based on which the PWM signal is produced.
UC3844 Pinout &Configuration
UC3844 pinout
UC3844 pinout

UC3844 Features

  • Optimized for Off-Line and DC-to-DC Converters
  • Low Start-Up Current (< 1 mA)
  • Automatic Feedforward Compensation
  • Pulse-by-Pulse Current Limiting
  • Enhanced Load-Response Characteristics
  • Undervoltage Lockout With Hysteresis
  • Double-Pulse Suppression
  • High-Current Totem-Pole Output
  • Internally Trimmed Bandgap Reference
  • Up to 500-kHz Operation
  • Error Amplifier With Low Output Resistance

UC3844 Equivalent IC

  • The UC3844 Equivalent are: uc3844: UC3843, UC3842
  • The UC3844 Alternative PWM Controller IC: uc2842, sg2524

How does a UC3844 Controller work?

Using uc3844 in a circuit is very simple because it requires a minimum number of components. The example application circuit in uc3844 data table is shown in the UC3844 circuit block below. The input voltage of VCC pin should be 12V to 28V. The output pin of the IC is connected to the grid driver circuit of the power switch to be switched. The VFB (voltage feedback) pin is used as a feedback based on which the PWM signal can be controlled. The shunt resistor is used to monitor the change of current in the circuit, and then supply the differential voltage at both ends of the shunt to the feedback pin. VREF is used to provide charging current for oscillator timing capacitor through timing resistor. For reference stability, it is important that the bypass of VREF be connected to ground through a ceramic capacitor as close to the pin as possible.

UC3844 Circuit


Where to use the UC3844 Controller

The UC3844 IC is a current Mode PWM Controller, meaning it can be used to provide a constant current by varying the output voltage to the load. This IC features a trimmed oscillator for precise duty cycle control, a temperature compensated reference, a high gain error amplifier, a current sensing comparator, and a high current totem pole output for driving a Power MOSFET. The UC3842 and UC3844 have the same UVLO thresholds of 16V (on) and 10V (off).

The UC3844 can be used to regulate or limit current in applications like SMPSRPSDC-DC ConvertersLine voltage regulators, etc. So if you are looking for an IC to produce PWM signals for controlling a power switch based on the current flowing through the circuit, then this IC might be the right choice for you.

UC3844 Applications

  • SMPS (switch mode power supply) circuit
  • DC-DC converter circuit
  • Electronic power supply
  • Battery consumption circuit
  • Loader

That’s all for today. Hope you enjoyed this article. However, if you have doubts or have any questions about this, you can contact me in the comments section below. I’ll be happy to help you as best I can. Let us know your feedback and suggestions at any time, they allow us to provide you with high-quality work that responds to your needs and expectations, and helps you continue to give back to our products. Thank you for reading this article.


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