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Power-Up/Down Protection Circuit for a DAC Driver

IP.com Disclosure Number: IPCOM000044319D
Original Publication Date: 1984-Dec-01
Included in the Prior Art Database: 2005-Feb-05
Document File: 2 page(s) / 37K

Publishing Venue

IBM

Related People

Rittenhouse, LE: AUTHOR

Abstract

Output from a digital-to-analog converter (DAC) driver is inhibited from turning full on during the power-up or the power-down of the +5 volt supply. The clamp or inhibit signal is generated by the same TTL device that has the undesirable characteristic of trying to turn the DAC output on. Output 10 of the DAC circuit shown in the figure should remain off (not conducting) when the voltage from +5 volt power is coming up. Unfortunately, the internal circuitry of the 7407 TTL module is such that with their inputs up or floating the associated output transistors are hard 0's (conducting) when the +5 volt supply voltage is between approximately +1 and +3 volts. This condition forces all DAC summing inputs low, which results in the DAC driver output voltage 10 going to its maximum voltage. This is not desirable.

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Power-Up/Down Protection Circuit for a DAC Driver

Output from a digital-to-analog converter (DAC) driver is inhibited from turning full on during the power-up or the power-down of the +5 volt supply. The clamp or inhibit signal is generated by the same TTL device that has the undesirable characteristic of trying to turn the DAC output on. Output 10 of the DAC circuit shown in the figure should remain off (not conducting) when the voltage from +5 volt power is coming up. Unfortunately, the internal circuitry of the 7407 TTL module is such that with their inputs up or floating the associated output transistors are hard 0's (conducting) when the +5 volt supply voltage is between approximately +1 and +3 volts. This condition forces all DAC summing inputs low, which results in the DAC driver output voltage 10 going to its maximum voltage. This is not desirable. A fix for this power-up/power-down problem is to use one 7407 circuit 11 (six circuits per module) to enable clamp circuit 13 to clamp or short the base-emitter junction on drive transistor 12. The 7407 circuit 11 has its input tied to the +5 volt power. As the +5 volt power is coming up and is between +1 and +3 volts, circuit 11 will have an output that goes low, turning on transistor 14. When transistor 14 is on, drive transistor 12 is held off (nonconducting). Accordingly, when the +5 volt power is between +1 and +3 volts, the DAC output is held off. The 7407 TTL circuit 11 used for generating the clamp sign...