Part Number: TPS709
Application: For an 8-V GaN half‑bridge buck converter, serve as bootstrap supply for the high‑side driver,circuit shown as below:
I am using an LDO to pre‑regulate the bootstrap rail for the high‑side gate driver of a GaN buck converter.
- Input: 12 V (from main supply)
- Output: 5 V (to the driver’s bootstrap pre‑regulator)
- LDO input capacitor: 1 × 10 µF 0805 +1 × 4.7 µF 0402 MLCC (close to the IN pin)
- LDO output capacitor: 3 × 4.7 µF 0402 MLCC in parallel → effective total capacitance ≈ 4.9 µF (derated by DC bias)
In this application, I've run into the following issue that I don't quite understand.
Issue 1 – Triangular ripple on the output
Even at light/no load, the 5 V output shows a clear triangular ripple (see attached screenshot). Is this normal behavior for this LDO, or does it indicate instability?
Issue 2 – Intermittent voltage dips under increased load
When the load current increases, I observe intermittent drops of the 5 V rail. These dips are deep enough to trigger the UVLO of the downstream gate driver, causing the system to malfunction. Shown as below:
zoomed in:
My main suspicion:
Could this be a self‑oscillation / instability of the LDO, possibly caused by the output capacitance ESR being too low (all‑ceramic output) or by an unsuitable layout or any other reasons?
