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Self Limiting Off Chip Driver

IP.com Disclosure Number: IPCOM000080652D
Original Publication Date: 1974-Jan-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 50K

Publishing Venue

IBM

Related People

Elliott, JC: AUTHOR [+2]

Abstract

This driver features auxiliary drain contacts at the output devices to prevent damage to the semiconductor chip, due to overload or a short circuit of the output line.

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Self Limiting Off Chip Driver

This driver features auxiliary drain contacts at the output devices to prevent damage to the semiconductor chip, due to overload or a short circuit of the output line.

Referring to Fig. 1, assume that the output current in P channel transistor Q4 is excessive. Then the voltage VDS across Q4 and, as such, at the auxiliary drain of Q4, is high enough to turn on device Q2 through Q3. Node B is thus made more positive, thereby limiting the drive and the current in Q4. The sensitivity of the limiting action can be increased by increasing the width-to- length ratio of Q2, or by decreasing the width-to-length ratio of Q5. The output current in device Q8 is limited in a similar manner. When the output line is fully charged up, or down, both Q2 and Q6 are cut off because the IR drops across Q4 and Q8 are zero.

The limiting action can be delayed by using long lengths in Q3 and Q7 and/or by adding extra capacitances Cf and Cg. The limiting threshold can be shifted by applying bias E1 to device Q2 or E2 to device Q6.

In Fig. 1 the slanting terminals at devices Q4 and Q8 are the auxiliary drain contacts. In Fig. 2 a surface view of the device shows the auxiliary drain contact as being separated from the standard drain contact. The operation of the device shown in Fig. 2 can be improved as shown in Fig. 3. In Fig. 3 the auxiliary drain is used as a very smell field-effect transistor (FET), which uses the drain of the main device as its source and...