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Design IdeasMarch 3, 1997 |
Stuart Smith, Elantec Inc, Milpitas, CA
If you need a wideband, high-current analog driver that can also switch its output to a high-impedance state, you can use a triple or quad amplifier with a built-in disable feature (Figure 1). Relays are often too slow to be a viable option, CMOS switches cannot handle currents of 100 mA, and building a power MOSFET switch takes a moderate number of parts, especially if the switch needs to be compatible with TTL/CMOS logic.
Several amplifiers are available, including the EL4393, that have fairly high output-current capability and provide separate output-disable controls for each amp. However, to get 100 mA of output current, you need to connect several amplifiers in parallel. In the case of the EL4393, each amp can sink or source approximately 40 mA, generating 100 mA or more, as well as providing a TTL/CMOS-compatible output-disable feature. You don't need to set the gain of each amplifier separately, and some small resistors (RBAL1, RBAL2, and RBAL3) ensure that the amplifiers share the output current equally.
Connecting all the output enables in parallel provides the required on/off capability; a TTL/CMOS low turns on the outputs, and a high turns them off. If you need a very high-impedance off, you must omit the three RG resistors, and the gain is unity.
Figure 2a shows a 2V pulse's switching off approximately 50 nsec after the active-low enable pulse goes high, which is the usual performance for this amplifier. The upper trace is the paralleled amplifier output, and the lower trace is the paralleled enable signal of the three amplifiers. Figure 2b shows the amplifier's performance as a high-current driver, with a 10V pulse driving a 100 ohms load. The amplifier's turn-off at these high currents is much slower than the 2V-type signals that this amplifier normally handles. However, the paralleled amplifier still manages to switch 50- and 100-mA load currents in around 300 nsec, which is faster than many MOSFET switches and much faster than any relay.
If the amplifier is in current limit, the disable function exhibits a modest additional time delay before the output stage starts to turn off (Figure 2c). Here, the top trace is the output of all three channels in parallel, driving 75 ohms to ground. The figure shows a delay of 300 nsec before the output starts to turn off. This delay depends on the degree of the amplifiers' saturation and current limit.
When switching this high-current amplifier on and off, watch for stray inductance in the signal path. With currents changing at rates of 5 mA/nsec, just 0.2 µH will drop 1V.
As well as causing high power dissipation, continuous output currents of more than about 35 mA/channel can cause early device failure, especially if the device operates at a junction temperature approaching 150°C. The SOL package's thermal resistance when soldered to a pc board is 90°C/W. With ±15V supplies and a 10V output into 100 ohms (all three channels connected in parallel) the total dissipation PD is
So, for a maximum junction temperature of 150°C, the maximum ambient temperature is:
Even for a 25% duty cycle, the dissipation would still be 0.935W, translating to a maximum ambient temperature of 66°C. A heat sink attached to the package and a forced-air cooling system can significantly improve the dissipation capability of the package. However, just as you can connect the individual amplifiers for greater drive capability, so too can you parallel additional packages of amplifiers. This approach often is more cost-effective than is using heat sinks and forced-air cooling systems, especially when you consider the full assembly cost. (DI #2000)
| Figure 1 |
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| By connecting triple amplifiers in parallel, you can design a high-current analog driver with a disable feature. |
| Figure 2 | |
A B B
B (c) |
 HORIZONTAL SCALE=100 NSEC/DIV
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| With a 2V output, the paralleled amplifier switches off in approximately 50 nsec (a). With a high-current, 10V output, switch-off time is around 300 nsec (b), which is much faster than either MOSFET- or relay-based designs. If the paralleled amplifier is in current limit (c), an additional turn-off delay occurs. | |
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