Tool/software:
Hi experts,
I'm KIM
I am working on a buck converter design using **LMR14050SQDDAR (non-spread spectrum)**
to charge a **3-cell Li-ion battery** with these target specs:
- **Input Voltage**: 30V (bench supply, tested from 12V to 30V)
- **Output Voltage**: 12.6V
- **Output Current**: 5A
I designed and built a PCB following the **datasheet layout guidelines**.
Here’s what I observed in testing:
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### Observations
1. **No-load condition**
- Output is stable at **12.6V**, as expected.
2. **Loaded with 5Ω resistor (~2.5A load)**
- Around **14V input**, the output **drops slightly to 12.4V** .
- However, **increasing VIN above 14V causes VOUT to drop further**, down to **about 12.0V at 20V input**.
3. **Higher current load (lower resistance)**
- The **output collapses** to near **0V**, and the **device fails**, which I suspect is due to **thermal overstress**.
4. **Additional findings**
- **VSW waveform** shows an unexpected **high switching frequency around 680kHz**, even though my **RT resistor is 133kΩ**,
which according to the datasheet **should set the frequency to 200kHz**.
- **FB pin noise increases** as VIN increases.
- I tried adding **extra ceramic capacitors (0.47uF + 1uF) on VIN** and **duplicating output same capacitors**, but these changes had **little effect**.
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### Design Summary
- **RT**: 133kΩ (targeting 200kHz, measured 132.8kΩ on board)
- **Inductor**: 22uH (calculated for ~30% ripple at 200kHz, matching datasheet recommendation)
- **Input capacitors**: 39uF Aluminum Organic Polymer Capacitor+ 1uF + 0.47uF X7R ceramic (placed near VIN pin)
- **Output capacitors**: **180uF*2** (Aluminum Organic Polymer Capacitor)
- **Schottky diode**: MBR10100 (100V, 10A)
- **FB resistors**: 10kΩ / 158kΩ (targeting 12.6V)
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### Key Questions
1. **Why does VOUT collapse when increasing VIN under load?**
- Shouldn't VOUT remain regulated at 12.6V even with load?
2. **Why is the measured switching frequency so high (~680kHz) despite RT being set for 200kHz with 133kΩ?**
- Is this a measurement artifact (oscilloscope showing sub-harmonics)?
- Or is the device unintentionally shifting to a higher frequency due to noise or instability?
3. I noticed that the **VSW waveform has significant overshoot** (about +7V above VIN).
- **Could this overshoot cause instability or damage?**
- **If so, what mitigation techniques do you recommend?**
(e.g., snubber, layout changes, etc.)
4. **Is there anything I missed in the component selection or layout, even though I followed the datasheet recommendations?**
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I will attach my **schematic**, **PCB layout snapshot**, and **SW node waveform** for reference.
Thank you very much for your insights.
