9.1.3 Resistor Divider on GD for Programmable Start Up

Figure 44 shows an alternative usage of the GD pin can help protect against shorts on the V_BUS pin in the receptacle. A resistor divider is used to minimize the time it takes the GD pin to be pulled low. Consider the situation where the VBUS pin is shorted at startup. At some point, the device closes the NFET switch to supply 5 V to V_BUS. At that point, the short pulls down on the voltage seen at the VPWR pin. With the resistor values shown in Figure 44, once the voltage at the VPWR pin reaches 3.95 V the voltage at the GD pin is specified to be below V_{GD_TH}(min). Without the 700-kΩ resistor, the voltage at the VPWR pin would have to reach V_{GD_TH}(min) which takes longer. This comes at the expense of increased leakage current.

Figure 44. Programmable GD Turn On

The GD resistor values can be calculated using the following process. First, calculate the smallest R_GD1 that should be used to prevent the internal clamp current from exceeding I_GD of 80 µA. For a 20 V advertised voltage, the OVP trip point could be as high as 24 V. Using V_GDC(min) = 6.5 V and V_OUT = V_FOVP20(max) = 24 V, provides Equation 2:

Equation 2.

The actual clamping current is less than 80 µA as some current flows into R_GD2. Next, R_GD2 can be calculated as follows:

Equation 3.

where V_(VPWR) = V_{(PWR_TH)} falling (max) and V_{(GD_TH)} = V_{(GD_TH)} falling (min).

For this case, V_VPWR = V_{PWR_TH} falling (max) and V_{GD_TH} = V_{GD_TH} falling (min).