The KA3525A, introduced by HeroMicro, is a highly integrated monolithic voltage-mode Pulse Width Modulation (PWM) controller. It incorporates all the essential control circuitry needed to build a Switch-Mode Power Supply (SMPS) or DC-DC converter, including a precision voltage reference, an error amplifier, an oscillator, a PWM comparator, dead-time control, soft-start, under-voltage lockout (UVLO), and totem-pole output drivers. This IC is an ideal choice for designing power converters with push-pull, half-bridge, or full-bridge topologies.
1. Key Features Overview
Based on the datasheet provided by HeroMicro (KA3525A.pdf), key features include:
High-Precision Reference: 5V ±1%, providing a stable system reference.
Dual Complementary Outputs: OUTPUT A (Pin 11) and OUTPUT B (Pin 14), capable of directly driving power MOSFETs or driving dual-ended topologies via gate-drive transformers.
Programmable Dead Time: An external resistor on the DISCHARGE (Pin 7) pin allows precise control of the dead time between the two outputs, preventing shoot-through in bridge configurations.
Internal Soft-Start: An external capacitor on the SOFT START (Pin 8) pin enables a smooth startup sequence, effectively limiting inrush current.
Under-Voltage Lockout (UVLO): The chip shuts down its outputs when the VCC (Pin 15) supply falls below a threshold (typically 7V), ensuring the system remains off during unstable power conditions.
Oscillator Synchronization: The SYNC (Pin 3) input allows an external clock signal to synchronize the internal oscillator, useful for paralleling supplies or reducing system EMI.
External Shutdown: The SHUTDOWN (Pin 10) pin provides a fast and reliable shutdown mechanism for protection logic (e.g., over-current, over-voltage).
Robust Output Drive: Capable of sourcing/sinking up to ±500mA peak current to quickly charge/discharge power transistor gates.
Packages: SOP16, DIP16
2. Pin Configuration
| Pin | Name | Function |
|---|---|---|
| 1 | EA(-) | Error Amplifier Inverting Input |
| 2 | EA(+) | Error Amplifier Non-inverting Input |
| 3 | SYNC | External Synchronization Input |
| 4 | OSC OUTPUT | Oscillator Output |
| 5 | CT | Oscillator Timing Capacitor |
| 6 | RT | Oscillator Timing Resistor |
| 7 | DISCHARGE | Dead-Time Control / Oscillator Discharge |
| 8 | SOFT START | Soft-Start Capacitor |
| 9 | EA_OUT | Error Amplifier Output / PWM Compensation |
| 10 | SHUTDOWN | External Shutdown Input |
| 11 | OUTPUT A | Output A |
| 12 | GND | Signal Ground |
| 13 | VC | Output Stage Bias Voltage (usually tied to VCC) |
| 14 | OUTPUT B | Output B |
| 15 | VCC | Chip Supply Voltage |
| 16 | VREF | 5V Reference Voltage Output |
3. Critical Design Guidelines
(1) Setting the Switching Frequency
The switching frequency is determined by RT (Pin 6) and CT (Pin 5). A typical approximation is f ≈ 1 / (CT * (0.7 * RT + 3)). Refer to the "Testing Circuit" and electrical characteristics table in the datasheet for accurate component selection.
(2) Configuring Dead Time
The DISCHARGE (Pin 7) pin is key to dead-time control. Connect a resistor Rd from this pin to ground. A larger Rd value results in a longer dead time, effectively preventing shoot-through caused by mismatched MOSFET switching speeds in push-pull or bridge circuits.
(3) Implementing Soft-Start
Connect a capacitor Css between SOFT START (Pin 8) and GND (Pin 12). On power-up, an internal constant current source (typically 51μA) charges Css, creating a voltage ramp that gradually increases the PWM duty cycle from zero, achieving a soft start. A larger Css results in a longer soft-start time.
(4) Building the Feedback Loop
The error amplifier is the heart of the feedback loop.
Connect a resistive divider from the output voltage to EA(+) (Pin 2).
Connect EA(-) (Pin 1) to the 5V reference from VREF (Pin 16).
Place a Type II or Type III compensation network (resistors and capacitors) between EA_OUT (Pin 9) and EA(-) (Pin 1) to stabilize the closed-loop response of the entire power system.
(5) Power Supply and Decoupling
VCC (Pin 15) requires a clean, stable supply, typically derived from an auxiliary winding or a start-up resistor. Low-ESR decoupling capacitors (e.g., a 0.1μF ceramic + a 10μF electrolytic) must be placed close between VCC and GND.
VC (Pin 13) is a dedicated supply pin for the output drivers. For maximum drive strength, connect it directly to VCC.
(6) External Shutdown and Protection
The SHUTDOWN (Pin 10) pin will immediately disable all outputs when its voltage exceeds 1.3V (typical). This pin can be connected to the output of comparators for over-current, over-temperature, or other protection schemes, enabling fast and reliable system shutdown.
(7) PCB Layout Considerations
Separate Power and Signal Grounds: Route the high-current power ground (from input caps, transformer, MOSFETs) separately from the IC's GND (Pin 12), joining them at a single point to minimize noise coupling.
Keep Critical Traces Short and Wide: Traces for VCC, GND, and OUTPUT A/B should be as short and wide as possible to reduce parasitic inductance and resistance.
Shield Feedback Traces: Keep error amplifier input traces (EA+, EA-) away from noisy nodes (switching nodes, inductors, transformers). Surround them with a ground guard trace for shielding.
4. Summary
HeroMicro's KA3525A is a highly integrated and powerful voltage-mode PWM controller. By properly configuring its oscillator, dead time, soft-start, and feedback compensation network, and by adhering to good PCB design practices, engineers can efficiently develop stable, reliable, and high-performance switch-mode power supplies.
