Introduction to the UC3844 PWM Controller
The UC3844 is a fixed-frequency pulse width modulation (PWM) controller integrated circuit (IC) commonly used in switch mode power supplies (SMPS). It is designed to regulate output voltage and provide power supply protection features in off-line and DC-to-DC converter applications.
The UC3844 offers several key benefits:
– Simplified design compared to discrete component controllers
– Precision reference voltage for accurate regulation
– Built-in protection features like under-voltage lockout and current limiting
– Wide operating voltage range from 8V to 40V
– Adjustable deadtime control and maximum duty cycle limit
This guide will cover everything you need to know about the UC3844, including pinout and functionality, key specifications, typical application circuits, design considerations, and frequently asked questions. By the end, you’ll have a solid understanding of how to use this versatile PWM controller IC in your power supply designs.
UC3844 Pinout and Pin Functions
The UC3844 comes in a 14-pin dual inline package (DIP-14) or 16-pin small outline package (SOIC-16). The pinout is as follows:
| Pin | Name | Function |
|---|---|---|
| 1 | COMP | Error amplifier output and PWM comparator non-inverting input. A RC network from this pin to ground compensates the feedback loop. |
| 2 | FB | Feedback signal input connected to an external voltage divider from the output. |
| 3 | CS | Current sense input for current-mode control and limiting. |
| 4 | RC | Oscillator timing resistor connection. A resistor from this pin to ground sets the oscillator frequency. |
| 5 | CT | Oscillator timing capacitor connection. A capacitor from this pin to ground sets the oscillator frequency. |
| 6 | RT | Deadtime control input. A resistor from this pin to ground sets the power MOSFET deadtime. |
| 7 | GND | Ground reference. |
| 8 | OUT | Totem-pole output driver for direct driving a power MOSFET gate. |
| 9 | VCC | Positive supply voltage input, internally clamped to 12V. |
| 10 | N/C or SYNC | Not internally connected on DIP-14. SYNC input on SOIC-16 for synchronization to external clock. |
| 11 | VC | PWM control signal input. Used for feed-forward control or external shutdown. |
| 12 | REF | 5V precision reference voltage output. |
| 13 | OVP | Overvoltage protection input. |
| 14 | N/C | Not connected. |
| 15, 16 | N/C | Not connected on SOIC-16 package. |
The COMP, FB, CS, and VC pins are critical for controlling the PWM duty cycle and managing voltage and current feedback. The timing resistor and capacitor on RC and CT set the operating frequency. RT allows deadtime adjustment. REF provides a stable voltage reference, while OVP implements overvoltage protection.
Key Specifications and Ratings
Some of the key electrical characteristics of the UC3844:
| Parameter | Min | Typ | Max | Unit |
|---|---|---|---|---|
| Operating voltage (VCC) | 8 | — | 40 | V |
| Reference voltage (REF) | 4.85 | 5.0 | 5.15 | V |
| Oscillator frequency range | 1 | — | 500 | kHz |
| PWM duty cycle range | 0 | — | 49 | % |
| Deadtime control range | 1 | — | 5 | % |
| Shutdown threshold (VC) | 0.35 | 0.65 | 0.95 | V |
| OVP threshold | 1.5 | 1.75 | 2.0 | V |
| FB & CS input bias currents | — | — | 500 | nA |
| Operating temperature range | -40 | — | +85 | °C |
The UC3844 has impressive specs, with a wide operating voltage and frequency range, precision voltage reference, and low bias currents. The deadtime and overvoltage thresholds are user-adjustable.
Typical Application Circuits
The UC3844 can be used in a variety of SMPS topologies like flyback, forward, and push-pull converters. Here is a typical application schematic for a UC3844-based Flyback Converter:
[Flyback converter schematic]
The key components are:
– T1: Flyback transformer. Provides isolation and steps down voltage.
– Q1: Power MOSFET switch, driven by UC3844 OUT pin.
– D1: Output rectifier diode.
– C2, C3: Input and output Filter capacitors.
– R3, R4: Resistive voltage divider to sense output voltage for FB pin.
– R7, C4: RC compensation network on COMP pin.
– R5, R6, C1: RC timing components to set switching frequency.
– R2: Current sense resistor for CS pin.
The UC3844 maintains a constant output voltage by modulating the switching MOSFET Q1’s on-time based on the CS and FB voltage feedback signals. R7 and C4 compensate the feedback loop for stability. Switching frequency is set by R5, R6, C1 according to:
f = 1.72 / (R5 + R6) * C1
The turns ratio and primary inductance of T1 must also be properly selected based on input voltage range, duty cycle, and desired output voltage.
UC3844 Design Considerations and Tips
When designing with the UC3844, keep these points in mind:
-
Component selection: Use capacitors with low ESR and sufficient ripple current ratings. MOSFETs should have low gate charge and appropriate current/voltage ratings for your application.
-
Snubber design: Implement an RC or RCD snubber across the MOSFET or transformer primary to reduce voltage spikes and losses during switching transitions.
-
PCB layout: Minimize high current loops and keep the MOSFET gate drive and UC3844 Bypass Capacitor traces short. Provide sufficient copper pours for heat dissipation.
-
Cross-regulation: For multiple outputs, optimize the flyback transformer turns ratios and consider post-regulation using linear regulators or secondary-side PWM controllers.
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Feedback loop compensation: Measure the loop gain and phase response, then adjust the COMP pin RC values to ensure adequate phase margin and gain bandwidth.
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Protection features: Set the OVP divider and peak current limit based on your output requirements. You can implement overcurrent protection using the CS pin and soft-start by ramping up the VC pin on startup.
By following these guidelines and referring to application notes and reference designs, you’ll be well on your way to a robust UC3844-based power supply. Always build a prototype and verify performance over line, load, and temperature variations before finalizing your design.
Frequently Asked Questions (FAQ)
- What are the differences between the UC3844 and UC3845?
The UC3844 and UC3845 are functionally similar, but the UC3845 has a higher operating voltage range up to 45V and includes an on-chip error amplifier output clamp. The UC3844 is typically sufficient for most applications.
- Can the UC3844 be used for current-mode control?
Yes, the UC3844 supports current-mode control by sensing the switch current through the CS pin and using that signal as the PWM ramp. This provides cycle-by-cycle current limiting and improved transient response compared to voltage-mode control.
- How do I synchronize multiple UC3844 controllers?
The SOIC-16 package variant of the UC3844 includes a SYNC pin that allows synchronization to an external clock source. Multiple UC3844s can be daisy-chained by connecting the SYNC pin to the RT pin of the previous device in the chain.
- What is the maximum switching frequency of the UC3844?
The UC3844 can operate at frequencies up to 500 kHz, but practical limitations like MOSFET switching losses, transformer core losses, and minimum on/off times often restrict the maximum usable frequency to around 200-300 kHz in most applications.
- How do I select the feedback divider resistors for a given output voltage?
The feedback divider ratio should be chosen so that the FB pin voltage equals the 2.5V reference when the output is at its nominal voltage. For a desired output voltage Vout, the divider ratio is:
(R3 + R4) / R4 = Vout / 2.5
Where R3 and R4 are the upper and lower feedback divider resistors, respectively. Choose high enough values to avoid loading the output while maintaining low divider bias current.
Conclusion
The UC3844 is a versatile and widely-used PWM controller IC for off-line and DC-DC converters. Its combination of features, protection mechanisms, and adjustable parameters make it suitable for a broad range of SMPS applications.
By understanding the UC3844’s functionality, key specifications, and design considerations outlined in this guide, you’ll be able to effectively implement it in your power supply designs. As with any power electronics design, careful component selection, PCB layout, and thorough testing are essential for optimal performance and reliability.
For further information, consult the UC3844 datasheet, application notes, and reference designs from the manufacturer and power supply design resources. With the UC3844 in your toolkit, you’ll be well-equipped to tackle a variety of SMPS design challenges.
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