In an analog circuit, I have a line that drives something wicked fast, and needed to ensure the load device disconnects after a non-critical timeout period.
+5V Q1 -+- MMBT4401 | .-----------. \ ^ |--. ----- Rt | .--- | | 2.2M | |\ | 0|O-----+--/\/\/---+----/\/\/-----+-----| >o-----O|EN | | R1 | |/ | U1 | 330 Ct --- U2 |
74LVC139 | 4.7n --- 74AHCT14 | LOAD tpd=2nS | | | (typ) | === | | | '----------------------------------------O|CS | '--- OPERATIONQuiescent Normally U1.0 = high, Ct is fully charged, and EN=low. Idling 'enabled' avoids any excess delay when CS* comes along later.
Active U1.0 selects the load by driving CS* low. Ct begins discharging through Rt, forming a timer.
Timeout Normally U1.0 will go high within 50nS < t < 2mS. Should it linger low ~>=10mS, Ct discharges & U2's output goes high, disabling the load.
Recovery Recovery < 100nS is desired. Since the timing current is
2.5uA, the reset current needs to be at least 10mS/100nS = 100k * 2.5uA = 250mA.U1 drives 32mA--not shabby, but not enough to drive a diode across Rt. Also, we don't want to slow the critical signal path to CS* during normal operation.
Q1, then, boosts U1's output to ~400mA, effecting a prompt reset.
(I fiddled with timer ideas using the usual suspect ICs, but none of them had both a fast enough response, and a high enough reset-to-timing current ratio.)
Cheers, James Arthur