Page 154 - Troubleshooting Analog Circuits

Toys Illustrate Some Basics 141

processor RESET function with such crude RC timers that the processor gets con-
fused or faked-out and doesn’t start correctly. I’ve heard about those horror stones.
Sometimes people forget to add the diode to discharge the capacitor, and then the
reset fails to work after a brief power outage. (The same diode function as D2 in
Figure 1 1.2.)
Designers also include start-up circuits in linear circuits. For example, current-bias
circuits ensure that a small but stable bias current starts the whole circuit going. Then
when the start-up circuit is operating properly, another circuit (a “hand“) reaches
around and shuts off the start-up circuit. When the start-up circuit works properly, it
saves power and doesn’t waste much die area. Unfortunately, if the start-up circuit is
broken or inoperative, the main circuit may still start if the supply voltage jumps up
rapidly but may fail to start if the supply ramps up slowly. Once a customer returned
a regulator complaining that the device would start if the supply ramped to 20 V in 30
seconds or less, but would not start if it ramped in 36 seconds or more. We checked it
out, and the customer was absolutely right. We had to change one mask and add a
start-up test to prevent future troubles.
Over 20 years ago, an IC maker designed a micropower IC that didn’t have a true
DC start-up circuit. The IC was supposed to be started up by the transient rise-rate of
the supply voltage. At room temperature, the circuit would always start. no matter
how slowly the supply ramped up. But at cold temperatures, the device wouldn‘t start
even with supplies of +15 V, if the supplies came up slowly. Worse yet, if the device
was running and you hit one of the power supply busses with certain positive or neg-
ative transients in sequence, you could turn the part OFF, and it might never turn ON
again. Needless to say, that part never became popular, nor did anything else from
that company.
So, let me caution you: Whether your circuit is a loop of sequential logic or an
analog loop with positive and negative feedback, be sure to design the start-up circuit
carefully. Add a test to make sure a bad part will be rejected, AND build up a few bad

Figure I 1.2. The addition of RI and Dz to this standard data-book design of a IS-V, 0.5-A switching regu-
lator provides the circuit with better soft-start and restart capabilities.