If the current limit threshold is less than the current defined by the power limit threshold at maximum V_DS the circuit operates only at the current limit threshold during turn-on. Referring to Figure 9-5a, as the drain current reaches I_LIM, the gate-to-source voltage is controlled at V_GSL to maintain the current at I_LIM. As the output voltage reaches its final value (V_DS ≊ 0 V) the drain current reduces to the value defined by the load, and the gate is charged to approximately 13 V (V_GATE). The time for the OUT pin voltage to transition from zero volts to V_SYS is equal to:

Equation 4.

where

• C_L is the load capacitance

For example, if V_SYS = –48 V, C_L = 1000 µF, and I_LIM = 1 A, t_ON calculates to 48 ms. The maximum instantaneous power dissipated in the MOSFET is 48W. This calculation assumes the time from t1 to t2 in Figure 9-5a is small compared to t_ON, and the load does not draw any current until after the output voltage has reached its final value, and PGD switches high (Figure 9-3).

Figure 9-3 No Load Current During Turn-on

If the load draws current during the turn-on sequence (Figure 9-4), the turn-on time is longer than the above calculation, and is approximately equal to:

Equation 5.

where

• R_L is the load resistance and V_SYS is the absolute value of the system input voltage

Figure 9-4 Load Draws Current During Turn-On