Page 94 - Troubleshooting Analog Circuits

Power Transistors May Hog Current 81

substrates of the MM74HC00 family of chips, the NSC LMC660 and LPC660
family, and most of the dielectrically isolated op amps from Harris. So, be aware of
your IC’s substrate connection. If an LMlOlAH op amp’s metal can should happen
to bump against ground or +V,, you have a problem. Similarly, you shouldn’t let an
HA2525’s case bump against ground or -Vs.
MOSFETs are widely used in digital ICs but are also very popular and useful in
analog circuits, such as analog switches. The quad switches such as CD4016 and
CD4066 are popular because of their low (typical) leakages and low price. Op amps
with MOSFET inputs are starting to do well in the general-purpose op-amp market.
MOSFETs used to have a bad reputation for excessive noise, but new IC devices,
such as the LMC662 dual op amp, demonstrate that clean processing can cure the
problem, thus making MOSFETs competitive with BiFETs. They offer an advantage
of a 1OOO: 1 improvement in input current, decreased from 10 pA down to 10 fA. Just
be careful not to let ESD near the inputs. Most MOSFET-input linear ICs do have
protection diodes and may be able to withstand 600 V, but they usually can’t survive
2000 V. If you work with unprotected MOSFETs, such as the 3N160, you must keep
the pins securely shorted until the device is soldered into its PC board in which the
protection diodes are already installed. I do all of that and wash the transistor package
with both an organic solvent and soap and water. And, I keep the sensitive gate cir-
cuits entirely off the PC board by pulling the gate pin up in the air and using point-to-
point wiring. Air, which is a superior dielectric, is also a good insulator (Ref. 2). So
far, I haven’t had any blown inputs or bad leakages-at least nothing as bad as 10 fA.
On the other hand, when using CMOS digital ICs, I always plug them into live
sockets; I never use conductive foam; and I never wear a ground strap on my wrist.
And I’ve almost never had any failures-with one exception. One time I shuffled
across a carpeted floor and pointed an accusatory finger at a CMOS IC. There was a
small crack of ESD-probably 5000 V-followed by a big snap as the IC blew out
and crowbarred the entire power supply. Since ESD testing is usually done with the
power OFF, then if you did some tests with the power ON, you might get some
messy failure modes like the one I just mentioned. Always be wary of any devices
that manufacturers claim are safe from ESD.
One reader reminded me that in some cases, if you abuse CMOS ICs with ESD.
they may not fail instantly, but they may become unreliable and fail at a later time.
So, I must caution you that fooling around with CMOS ICs while you are not prop-
erly grounded might cause latent unreliability problems. If you do have to do trou-
bleshooting of CMOS ICs while you are not grounded, if you decide to plug in
CMOS ICs while the power busses are hor, just be aware that you might in some
cases do some long-lasting harm to an occasional IC. But you have to use your judg-
ment and trade off that possibility against the advantages of more free-swinging
troubleshooting approaches.

Power Transistors May Hog Current

As you build a bipolar transistor bigger and bigger, you may be tempted to go to
extremes and make a huge power transistor. But there are practical limitations. Soon,
the circuit capacitances cause oppressive drive requirements, and removing the heat
is difficult. Still, no matter how big you build power transistors, people will find a
use for them. Their most serious limitation on just building transistors bigger and
bigger is secondary breakdown, which is what happens when you drive a transistor
outside its “safe operating area.” When you operate a power transistor at very high
currents and low voltages, the distributed emitter resistance of the device-which
includes the resistance of the emitter metal and the inherent emitter resistivity-can