Overview
The UC3845ANG4 is a fixed-frequency current-mode PWM (Pulse Width Modulation) controller produced by Unitrode, now part of Texas Instruments. This integrated circuit is specifically designed for off-line and DC to DC converter applications, requiring minimal external components. It is part of the UC3842/UC3843/UC3844/UC3845 family, which offers a range of features suitable for various power supply designs.
Key Specifications
| Parameter | Symbol | Value | Unit |
|---|---|---|---|
| Supply Voltage | VCC | 7.6 to 18 | V |
| Output Current | IO | ±1 | A |
| Operating Frequency | f | Up to 500 kHz | kHz |
| Maximum Duty Cycle | DMAX | 50% (clamped by internal flip-flop) | % |
| Reference Output Voltage | VREF | 4.90 to 5.10 | V |
| UVLO Thresholds | VCC(on), VCC(off) | 8.5V, 7.9V | V |
| Storage Temperature Range | TSTG | -65 to +150 | °C |
Key Features
- Low Start-Up Current: Less than 1 mA for efficient bootstrapping from the rectified input of an off-line converter.
- UVLO With Hysteresis: Under-voltage lockout with thresholds of 8.5V (on) and 7.9V (off) to ensure stable operation.
- Current Sensing and Limiting: Pulse-by-pulse current limiting with an internal comparator to prevent overcurrent conditions.
- High Gain Error Amplifier: For precise control and feedback in the power supply loop.
- High Current Totem Pole Output: Capable of sourcing or sinking high peak currents to drive power MOSFETs.
- Internal Oscillator: Programmable up to 500 kHz with a timing capacitor and resistor.
- Automatic Feed Forward Compensation: Enhances load response characteristics.
Applications
The UC3845ANG4 is suitable for a variety of power supply applications, including:
- Off-Line Converters: For converting AC to DC with high efficiency and reliability.
- DC to DC Converters: For step-up, step-down, and step-up/step-down configurations such as boost, buck, flyback, and forward converters.
- Power Supplies: For use in various electronic devices requiring stable and efficient power supply solutions.
Q & A
- What is the UC3845ANG4 used for?
The UC3845ANG4 is a fixed-frequency current-mode PWM controller used in off-line and DC to DC converter applications.
- What are the UVLO thresholds for the UC3845ANG4?
The under-voltage lockout (UVLO) thresholds are 8.5V (on) and 7.9V (off).
- What is the maximum operating frequency of the UC3845ANG4?
The maximum operating frequency is up to 500 kHz.
- What is the maximum duty cycle for the UC3845ANG4?
The maximum duty cycle is clamped to 50% by an internal flip-flop.
- How does the current sensing and limiting work in the UC3845ANG4?
The current sensing is done externally with a ground-referenced resistor, and current limiting occurs if the voltage at the current sense pin reaches a threshold value.
- What is the purpose of the error amplifier in the UC3845ANG4?
The error amplifier provides precise control and feedback in the power supply loop.
- Can the UC3845ANG4 be used in high-temperature environments?
The storage temperature range is -65°C to +150°C, but operational temperature should be within specified limits for reliable performance.
- How is the oscillator frequency set in the UC3845ANG4?
The oscillator frequency is set using a timing capacitor and resistor, with the formula Fosc (kHz) = 1.72 / (RT (k) * CT (pf)).
- What are the benefits of using current-mode control in the UC3845ANG4?
Current-mode control provides better transient response and easier implementation of current limiting.
- What types of converters can the UC3845ANG4 be used in?
The UC3845ANG4 can be used in boost, buck, flyback, and forward converters.
