9.2.2.1 VBIAS Capacitor Selection

As noted in the Recommended Operating Conditions table, a minimum of 35-V_BUS rated capacitor is required for the VBIAS pin, and a 50-V_BUS capacitor is recommended. The VBIAS capacitor is in parallel with the central IEC diode clamp integrated inside the TPD8S300A. A forward biased hiding diode connects the VBIAS pin to the C_CCx and C_SBUx pins. Therefore, when a Short-to-V_BUS event occurs at 20 V, 20-V_BUS minus a forward biased diode drop is exposed to the VBIAS pin. Additionally, during the short-to-V_BUS event, ringing can occur almost double the settling voltage of 20 V, allowing a potential 40 V to be exposed to the C_CCx and C_SBUx pins. However, the internal IEC clamps limit the voltage exposed to the C_CCx and C_SBUx pins to around 30 V. Therefore, at least 35-V_BUS capacitor is required to insure the VBIAS capacitor does not get destroyed during Short-to-V_BUS events.

A 50-V, X7R capacitor is recommended, however. This is to further improve the derating performance of the capacitors. When the voltage across a real capacitor is increased, its capacitance value derates. The more the capacitor derates, the greater than 2x ringing can occur in the short-to-V_BUS RLC circuit. 50-V X7R capacitors have great derating performance, allowing for the best short-to-V_BUS performance of the TPD8S300A.

Additionally, the VBIAS capacitor helps pass IEC 61000-4-2 ESD strikes. The more capacitance present, the better the IEC performance. So the less the VBIAS capacitor derates, the better the IEC performance. Table 3 shows real capacitors recommended to achieve the best performance with the TPD8S300A.