Overview
The UC3842AD is a high-performance, fixed frequency, current-mode PWM (Pulse Width Modulation) controller produced by Fairchild Semiconductor. It is specifically designed for off-line and DC-to-DC converter applications, offering a cost-effective solution with minimal external components. This integrated circuit 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.
Key Specifications
| Parameter | Symbol | Conditions | Min. | Typ. | Max. | Unit |
|---|---|---|---|---|---|---|
| Supply Voltage | VCC | 30 | V | |||
| Output Current | IO | ±1 | A | |||
| Analog Inputs (Pin 2, 3) | V(ANA) | -0.3 | 6.3 | V | ||
| Error Amp Output Sink Current | ISINK (E.A) | 10 | mA | |||
| Power Dissipation at TA≤25°C (8DIP) | PD | 1200 | mW | |||
| Reference Output Voltage | VREF | TJ = 25°C, IREF = 1mA | 4.90 | 5.00 | 5.10 | V |
| Oscillation Frequency | f | TJ = 25°C | 47 | 52 | 57 | kHz |
| Maximum Duty Cycle | D(Max) | 95 | 97 | 100 | % | |
| Start-Up Current | IST | 1 | mA | |||
| Operating Supply Current | ICC(OPR) | Vpin3=Vpin2=ON | 14 | 17 | mA | |
| Under-Voltage Lockout Start Threshold | VTH(ST) | 14.5 | 16.0 | 17.5 | V |
Key Features
- Low Start-Up Current: The UC3842AD has a low start-up current, making it energy-efficient.
- Maximum Duty Clamp: It supports a maximum duty cycle of up to 100% for the UC3842 and UC3843 models.
- UVLO With Hysteresis: The device features under-voltage lockout (UVLO) with hysteresis to ensure stable operation.
- Operating Frequency up to 500KHz: The oscillator frequency can be programmed up to 500 KHz.
- Temperature Compensated Reference: A temperature-compensated reference voltage ensures stable operation over a range of temperatures.
- High Gain Error Amplifier: A high-gain error amplifier provides precise control over the output.
- Current Sensing Comparator: The device includes a current sensing comparator for accurate current control.
- High Current Totem Pole Output: Capable of driving a Power MOSFET with high current output.
Applications
The UC3842AD is designed for various power management applications, including:
- Off-Line Converters: Suitable for off-line power supplies where high efficiency and reliability are crucial.
- DC-to-DC Converters: Ideal for DC-to-DC conversion applications requiring precise control and minimal external components.
- Switch Mode Power Supplies (SMPS): Used in SMPS designs to manage power efficiently and reliably.
Q & A
- What is the primary function of the UC3842AD?
The UC3842AD is a fixed frequency, current-mode PWM controller designed for off-line and DC-to-DC converter applications.
- What is the maximum operating frequency of the UC3842AD?
The oscillator frequency can be programmed up to 500 KHz.
- What are the UVLO thresholds for the UC3842AD?
The UVLO thresholds are 16V (on) and 10V (off) for the UC3842 and 8.5V (on) and 7.9V (off) for the UC3843.
- What is the maximum duty cycle for the UC3842AD?
The maximum duty cycle is up to 100% for the UC3842 and UC3843 models.
- How is the oscillator frequency programmed in the UC3842AD?
The oscillator frequency is programmed by the values selected for the timing components RT and CT.
- What is the purpose of the current sensing comparator in the UC3842AD?
The current sensing comparator is used to terminate the output switch conduction when the peak inductor current reaches the threshold level established by the error amplifier output.
- What are the typical applications of the UC3842AD?
The UC3842AD is used in off-line converters, DC-to-DC converters, and switch mode power supplies (SMPS).
- How does the UC3842AD handle under-voltage lockout?
The device features under-voltage lockout (UVLO) with hysteresis, and during UVLO, the output driver is biased to a high impedance state.
- What is the significance of the temperature-compensated reference in the UC3842AD?
The temperature-compensated reference ensures stable operation over a range of temperatures.
- How can the shutdown of the UC3842AD be accomplished?
Shutdown can be accomplished by either raising pin 3 above 1V or pulling pin 1 below a voltage two diode drops above ground.
