12.1 Layout Guidelines
When doing the layout of the TPS2477x the following are considered best practice.
- Ensure proper Kelvin Sense of RSNS.
- Keep the filtering capacitor CFSTP as close to the IC as possible.
- Place a Shottky diode and a ceramic bypass capacitor close to the source of the Hot Swap MOSFET
- Do not connect VDD to the Kelvin Sense trace for SET and FSTP
- Note that special care must be taken when placing the bypass capacitor for the VDD pin. During Hot Shorts, there is a very large dv/dt on input voltage during the MOSFET turn off. If the bypass capacitor is placed right next to the pin and the trace from RSNS to the pin is long, an LC filter is formed. As a result a large differential voltage can develop between VDD and SENM if there is a large transient on Vin. This could result in a violation of the abs max rating from VDD to SENM. To avoid this, place the bypass capacitor close to RSNS instead of the VDD pin.
- When using parallel resistors the layout becomes even more critical. Due to PCB parasitics, the current through each RSNS may be different, which results in different sense voltages across the two resistors. It’s important to average these in order to get a proper current measurement. This can be accomplished by forming a resistor divider with the traces. As long as Dis1 = Dis2, the final VSNS will be an average of the drop across the two resistors.