UC3844 Current-Mode PWM Controller

The UC3844 is a fixed frequency current-mode PWM controller. This IC is specially designed for Off-Line and DC to DC converter applications with minimum external components.

 

Note: This IC also comes in 14-Pin, 16-Pin, and 20-Pin packages. This article covers only the 8-Pin version of the IC.

 

UC3844 Pinout Configuration

Pin Number	Pin Name	Description
1	COMP (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.
2	VFB (Voltage Feedback)	It is an input to the error amplifier inside the IC. The difference in voltage level is supplied to this pin.
3	Current Sense	A 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
4	RT/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.
5	Ground	Connected to the ground of the circuit
6	Output	This pin outputs the PWM signal based on the feedback provided and we can use this to switch the power electronic device.
7	Vcc	Supply voltage for the IC (Nominal 11V)
8	VREF	Reference voltage based on which the PWM signal is produced.

 

UC3844 Specifications & Features

• Current-Mode PWM controller
• Operating Voltage: 12V to 28V
• Output pin current: 200 mA
• Switching Frequency: 500 kHz
• Duty Cycle - Max: 100 %
• Operating Supply Current:25 mA
• Optimized for Off-line and DC-to-DC Converters
• High-Current Totem-Pole Output
• Output Deadtime Adjustable from 50% t

 

Note: Complete Technical Details can be found at the UC3844 datasheet given at the end of this page.

 

Equivalent/Substitute for UC3844: UC3843, UC3842

Alternatives PWM controller IC:  UC2842, SG2524

 

Where to use the UC3844 PWM Controller IC:

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 SMPS, RPS, DC-DC Converters, Line 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.

 

How to use UC3844 IC

Using UC3844 in a circuit is very simple as it requires a minimum number of components. A sample application circuit from the UC3844 datasheet is shown below.

The input voltage to the VCC pin should be 12V to 28v. The output pin of the IC is connected to the gate driver circuit of the Power switch which is to be switched. The VFB (Voltage feedback) pin acts as feedback based on which the PWM signal is controlled. A shunt resistor is used to monitor the change in current in the circuit, and then this difference voltage across the shunt is provided to the feedback pin. VREF is used to provide charging current to the oscillator timing capacitor through the timing resistor. It is important for reference stability that VREF is bypassed to GROUND with a ceramic capacitor connected as close to the pin as possible.

 

Applications of UC3844 IC

• SMPS (Switch Mode Power Supplies) circuits
• DC-DC converter circuits
• Electronics power supply
• Battery drain circuit
• Load machines

 

2D-Model of UC3844 IC

Dimensions for the UC3844 IC is given below. These dimensions are for the 8-Pin GDIP package. If you are using a different package IC, please refer to the UC3844 datasheet.