TPS56339 Operating Conditions and Inductor Design
Thought for 1 min 13 sec
Use this workflow for an LMR51430 buck converter:
1. Define the requirements
Choose:
- Minimum and maximum input voltage
- Output voltage
- Maximum load current
- 500 kHz or 1.1 MHz variant
- PFM for better light-load efficiency or FPWM for fixed-frequency operation
The LMR51430 supports 4.5–36 V input and up to 3 A output. TI offers 500 kHz and 1.1 MHz variants.
2. Build the schematic
Connect:
- VIN to the input supply
- SW to the inductor
- Inductor output to VOUT
- Input capacitors between VIN and GND
- Output capacitors between VOUT and GND
- Feedback divider: VOUT → RFBT → FB → RFBB → GND
- EN to VIN if the converter should always turn on
The feedback reference is typically 0.6 V.
Use 1% resistors and keep RFBT between 10 kΩ and 100 kΩ.
3. Example: 6–36 V input, 5 V/3 A output, 500 kHz
A good datasheet-based starting point is:
- L: 6.8 µH
- Inductor rating: at least 4 A RMS and 5 A saturation
- CIN: 2 × 4.7 µF, 50 V, X7R plus 100 nF
- COUT: 2 × 22 µF, 25 V, X7R
- RFBT: 100 kΩ
- RFBB: 13.7 kΩ
TI’s example specifies the 6.8 µH inductor with 4 A RMS and 5 A saturation ratings. The input capacitor voltage rating must exceed maximum VIN; TI recommends additional voltage-rating margin because ceramic capacitance falls under DC bias.
4. PCB placement order
Place components in this order:
- LMR51430
- Input capacitors immediately beside VIN and GND
- Inductor immediately beside SW
- Output capacitors beside the inductor output
- Feedback resistors immediately beside FB
- Input/output connectors
The input capacitor forms the most critical high-frequency current loop, so it must be extremely close to the IC.
5. Routing rules
- Make VIN, VOUT and GND power paths short and wide.
- Keep the SW copper compact; do not make it a large plane.
- Keep SW-to-inductor routing as short as possible.
- Use a continuous ground plane underneath the circuit.
- Connect input- and output-capacitor grounds directly into the IC’s local ground region.
- Route FB away from SW and the inductor.
- Sense VOUT at the output-capacitor/load node, not at the inductor pad.
- If possible, place FB routing on another layer with ground shielding it from SW.
The exact component values should be recalculated if your VIN range, VOUT or switching-frequency variant differs from the 5 V example.
