9.1.7 V_BUS Slew Control using GDNG C_(SLEW)

Care should be taken to control the slew rate of Q1 using C_(SLEW); particularly in applications where C_OUT >> C_(SLEW). The slew rate observed on VBUS when charging a purely capacitive load is the same as the slew rate of V_(GDNG) and is dominated by the ratio I_(GDNON) / C_(SLEW). R_(SLEW) helps block C_(SLEW) from the GDNG pin enabling a faster transient response to OCP.

Figure 47. Slew-Rate control Using GDNG

There may be fault conditions where the voltage on VBUS triggers an OVP condition and then remains at a high voltage even after the TPS25740B configures the voltage source to output 5 V via the CTL pins. When this OVP occurs, the TPS25740B opens Q1 within t_FOVP + t_FOVPDG. The TPS25740B then issues a hard reset, discharge the power-path via the R_(DSCG), and waits for 795 ms before enabling Q1 again. Due to the fault condition the voltage again triggers an OVP event when the voltage on VBUS exceeds V_(FOVP). This retry process would continue as long as the fault condition persists, periodically pulsing up to V_(FOVP) + V_SrcSlewPos x (t_FOVP + t_FOVPDG) onto the VBUS of the Type-C receptacle. It is recommended to use a slew rate less than the maximum of V_SrcSlewPos (30 mV / µs) allowed by USB (refer to 文档支持), the slew rate should instead be set in order to meet the requirement to have the voltage reach the target voltage within t_SrcSettle (275 ms). This also limits the out-rush current from the C_OUT capacitor into the C_(PDIN) capacitor and help protect Q1 and R_S.