Electronic shutdown circuit uses a practical method of shutting down the LM340 under the control of a TTL or DTL logic gate. The pass transistor Q1 operates either as a saturated transistor or as an open switch. With the logic input high (2.4V specified minimum for TTL logic) transistor Q2 turns on and pulls 50 mA down through R2. This provides sufficient base drive to maintain Q1 in saturation during the ON condition of the switch. When the logic input is low (0.4V specified maximum for TTL logic) Q2 is held off, as is Q1; and the switch is in the OFF condition. The observed turn-on time was 7.0 µs for resistive loads from 15O to infinity and the turn-off time varied from approximately 3.0 µs for a 15O load to 3.0 ms for a no-load condition. Turn-off time is controlled primarily by the time constant of RLOAD and C1.
*Required if the regulator is located far from the power supply filter
**Head sink Q1 and the LM340
Source : Natioanal Semiconductor