PCB layout is a critical portion of good
power supply design. There are several paths that conduct high slew-rate currents or
voltages that can interact with stray inductance or parasitic capacitance to generate noise
and EMI or degrade the power supply performance.
- To help eliminate these problems, bypass
the VIN pin to GND with a low-ESR ceramic bypass capacitor with a high-quality dielectric.
Place CIN as close as possible to the VIN and GND pins. Grounding for both the
input and output capacitors must consist of localized top-side planes that connect to the
GND pin and GND PAD.
- Minimize the loop area formed by the
input capacitor connections to the VIN and GND pins.
- Place the inductor and Schottky diode
close to the SW pin. Minimize the area of the SW trace or plane to prevent excessive
capacitive coupling.
- Place the Schottky diode anode pin in
close proximity to input capacitor ground or return.
- Tie the GND pin directly to the power pad
under the device and to a heat-sinking PCB ground plane.
- Use a ground plane in one of the middle
layers as a noise shielding and heat dissipation path.
- Have a single-point ground connection to
the plane. Route the ground connections for the feedback, soft start, and enable
components to the ground plane, which prevents any switched or load currents from flowing
in analog ground traces. If not properly handled, poor grounding results in degraded load
regulation or erratic output voltage ripple behavior.
- Make VIN, VOUT, and
ground bus connections as wide as possible, which reduces any voltage drops on the input
or output paths of the converter and maximizes efficiency.
- Minimize trace length to the FB pin.
Place both feedback resistors, RFB1 and RFB2, close to the FB pin.
Place CFF (if needed) directly in parallel with RFB1. If output
setpoint accuracy at the load is important, connect the VOUT sense at the load.
Route the VOUT sense path away from noisy nodes and preferably through a layer
on the other side of a grounded shielding layer.
- The RON pin is sensitive to noise. Thus,
locate the RRON resistor as close as possible to the device and route with
minimal lengths of trace. The parasitic capacitance from RON to GND must not exceed 20
pF.
- Provide adequate heat sinking for the LM5013 to keep the junction temperature below 150°C. For
operation at full rated load, the top-side ground plane is an important heat-dissipating
area. Use an array of heat-sinking vias to connect the exposed pad to the PCB ground
plane. If the PCB has multiple copper layers, these thermal vias must also be connected to
inner layer heat-spreading ground planes.
- Reference Layout Example.