1.7 Fast Turn-OFF Break-Before-Make Switch

The turn on time of a typical driver is related to the input capacitance of the MOSFET it is driving. t_ON is 0.25 × 10⁶ × (MOSFET C_ISS). Similarly, the turn OFF time of the driver is 10⁶ × (MOSFET C_ISS).

In this situation, t_OFF is greater than t_ON. For a system using multiple switches, however, there may be a problem since some switches will close before others open. Using the HT0440 as shown in Figure 1-8 will provide a solution to this problem and ensure that the correct switches open before others close. To achieve turn off time that is less than turn on time, a small depletion-mode MOSFET placed on the output of the HT0440 (as shown) will slow t_ON such that the circuit exhibits break-before-make switching.