Onsemi KA3525A: Datasheet, Pinout, and Application Circuit Design Guide
The Onsemi KA3525A is a highly integrated pulse width modulation (PWM) controller IC that has become a cornerstone in the design of switch-mode power supplies (SMPS), DC-DC converters, and motor control circuits. Its robust design, flexibility, and protective features make it a preferred choice for engineers. This article provides a comprehensive overview of its datasheet specifications, pinout configuration, and a practical application circuit design guide.
Datasheet Overview and Key Features
The KA3525A is designed to improve upon the industry-standard SG3525, offering enhanced performance and reliability. Its key features, as outlined in the datasheet, make it exceptionally versatile.
PWM Control Core: It operates at a variable frequency up to 500 kHz, allowing for the design of both high-power and compact, high-frequency power supplies.
Undervoltage Lockout (UVLO): This feature ensures the IC remains inactive until the supply voltage (Vcc) reaches a sufficient level (typically above 8V), preventing erratic operation during power-up and power-down sequences.
Soft-Start Functionality: A dedicated soft-start pin allows for a gradual increase in duty cycle at startup, minimizing inrush currents and reducing stress on the power switching components.
Dual Source/Sink Outputs: The IC features two totem-pole output drivers (OUT A, OUT B) capable of sourcing and sinking high peak currents (over 500mA), making them ideal for directly driving power MOSFETs or bipolar transistors via coupling transformers.
Internal Error Amplifier: A built-in high-gain error amplifier simplifies the feedback loop design for voltage regulation.
Shutdown Capability: A separate shutdown pin allows for immediate termination of the output pulses, providing a fast method for implementing overcurrent or overtemperature protection.
Pinout Configuration and Function
Understanding the pinout is crucial for effective circuit design. The KA3525A is typically available in a 16-pin DIP or SOIC package.
1. Inverting Input (Inv. Input): The inverting input of the internal error amplifier.
2. Non-Inverting Input (Non-Inv. Input): The non-inverting input of the error amplifier.
3. Sync: Pin for synchronizing the oscillator of multiple KA3525A ICs.
4. Oscillator Output (OSC Output): Provides a discharge waveform.
5. CT: Connects to the timing capacitor for oscillator frequency setting.
6. RT: Connects to the timing resistor for oscillator frequency setting.
7. Discharge: Used in conjunction with RT and CT to set dead time.
8. Soft-Start: A capacitor connected here programs the soft-start duration.
9. Compensation: Pin for connecting frequency compensation components to stabilize the feedback loop.
10. Shutdown: A high signal (> 2.4V) on this pin immediately disables the outputs.
11. Output A (Out A): The first of the two complementary PWM output signals.
12. Ground (Gnd): Circuit ground.
13. Vc: The supply voltage for the output stage, which can be separate from the main Vcc for improved noise immunity.
14. Output B (Out B): The second complementary PWM output.
15. Vcc: Main supply voltage for the IC (typically 12V to 30V).
16. Vref: A stable 5.1V reference output, capable of sourcing up to 50mA, useful for powering external circuitry.
Application Circuit Design Guide: A Step-by-Step Example
Designing a basic push-pull converter showcases the KA3525A's capabilities.
Step 1: Set the Oscillator Frequency
The operating frequency (f) is determined by the timing resistor (RT) and capacitor (CT). Use the formula from the datasheet:
f (kHz) ≈ 1.15 / (RT (kΩ) × CT (μF))
For a 100 kHz design, choose RT = 7.5 kΩ and CT = 1.5 nF.
Step 2: Configure the Soft-Start
A capacitor (C_ss) on Pin 8 defines the soft-start time. A value of 1μF to 10μF is common. A 4.7μF capacitor provides a smooth, controlled startup over several milliseconds.
Step 3: Implement the Feedback Loop
Connect the output voltage sense through a voltage divider to the Inv. Input (Pin 1). The Non-Inv. Input (Pin 2) is connected to the internal Vref (Pin 16) through another divider to set the desired output voltage. A compensation network (typically a series RC circuit) is connected between Pin 9 and ground to ensure loop stability and prevent oscillations.
Step 4: Drive the Power Stage
The outputs (Pins 11 and 14) are used to drive the gates of the power MOSFETs in a push-pull configuration. For isolation and enhanced driving capability, it is highly recommended to use a gate drive transformer. Small resistors (e.g., 10Ω) in series with each output and MOSFET gate are essential to dampen ringing and prevent parasitic oscillations.
Step 5: Add Protection
Utilize the Shutdown pin (Pin 10) for protection. A current sense resistor in the primary ground path can feed a signal to a comparator. The comparator's output is then connected to the Shutdown pin, instantly disabling the IC if an overcurrent condition is detected.
ICGOODFIND: The Onsemi KA3525A stands as a testament to robust and intelligent power management design. Its integration of critical features like undervoltage lockout, soft-start, and precise PWM generation into a single package simplifies the development of reliable and efficient power electronics, making it an enduringly popular solution for both novice and experienced designers.
Keywords: PWM Controller, Switch-Mode Power Supply, Undervoltage Lockout, Soft-Start, Oscillator Frequency.
