The Load Transient Tool is a versatile and efficient solution for testing and analyzing voltage regulator performance. It generates fast load steps (~500nsec rise/fall times) by switching a load resistor on and off using a microcontroller and MOSFET. This tool is essential for evaluating regulator control loop behavior, detecting instability, and optimizing performance.
- Fast Load Steps: Generates load transients with ~500nsec rise/fall times.
- Wide Applicability: Works with any voltage regulator output.
- Control Loop Testing: Identifies instability and resonance effects in regulator circuits.
- Customizable: Adjustable static and pulse load resistors, duty cycle, and frequency.
- Purpose: Observing control loop stability during fast load transients.
- Operation in CCM Mode: Ensure the system operates in Continuous Current Mode (CCM) to avoid instability during PSM-CCM transitions.
- Effect of Instability: Ringing in the voltage waveform indicates control loop instability, often caused by feedback loop compensation or output capacitor value.
- Poor Response: Severe ringing in the voltage after a load transient, indicating marginal stability.
- Good Response: Minimal or no ringing, indicating a stable control loop.
- The schematic includes:
- A microcontroller driving a MOSFET switch with ~500nsec rise/fall times.
- Adjustable components such as C2 to influence switching speed.
- To achieve faster load step speeds:
- Minimize wiring inductance by using short, thick wires between the tool and the application.
- Pay extra attention when testing low voltage supplies (<2V).
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Power Setup:
- Insert 3x 1.5V AAA batteries in the holder (observe polarity).
- Connect the power jumper. The power LED lights for 1 second, then flashes once per second.
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Connections:
- Connect the Load_2 and GND terminals to the regulator output.
- Attach the adjustable 10Ω power resistor to the static load pins and adjust the slider for low-level load current (~30% of max load) in CCM mode.
- Use jumpers JP1~JP7 to select the desired pulse load resistor (suggested: VOUT/RLOAD ≈ 30% of max converter load).
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Measurement:
- Observe the regulator output voltage during load transient using an oscilloscope.
- Use the PULSE output for scope external sync, but beware of ground loops that may affect readings.
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Adjustments:
- Duty Cycle:
- Adjust using UP and DOWN buttons (range: 0.1% ~ 50%).
- Power LED flashes quickly at the adjustment range limits.
- Frequency:
- Change using FREQ + UP/DOWN buttons (options: 15Hz, 61Hz, 244Hz (default), 976Hz).
- Power LED flashes quickly at range limits.
- Ensure average power dissipation does not exceed 1W.
- Duty Cycle:
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Shutdown:
- Remove the power jumper to stop the pulse load generator. The static load will remain active.
- Re-inserting the jumper resets the duty cycle to 5% and frequency to 244Hz.
A fast load step can excite the system control loop, revealing:
- Instability: Seen as ringing in the output voltage.
- Resonance Effects: Due to parasitic trace inductance, input supply ringing, etc.
- Use short, thick wires to reduce inductance.
- Especially critical for low voltage supplies (<2V).
- Load Transient Board: Microcontroller and MOSFET-based design.
- Adjustable Power Resistor: For static load settings.
- Battery Pack: Requires 3x 1.5V AAA batteries.
- Basic Load Transient Measurement Method: Overview of the test setup.
- Practical Measurement Setup: Demonstration of the tool in a real-world application.
- Example Responses: Comparison of poor and good load step responses.
- Schematic: Detailed circuit showing the microcontroller, MOSFET switch, and other components.
- Wiring Inductance Reduction: Optimized wiring setup for low-inductance connections.
For more detailed information on converter loop stability and load transient testing, refer to Application Note AN038.
For further details or support:
- Manufacturer: Adobe Systems
- Documentation Source: Load Transient Tool