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  Searing   - Vhyst specification wrong in datasheet?
2008-11-19
In the MAX16025-MAX16030 19-0525; Rev 3; 1/07 datasheet
page 2 gives the Vhyst specification as 0.5 % (typical).
When we try some MAX16029
parts we see the OUT pins flip to the shutdown state
when the IN pins fall below about 0.470 V (this matches
well with the Vth typical spec of 0.472 V [with
TOL = Vcc]).  But when we raise the IN pins, the voltage
has to reach about 0.602 V before the OUT pins flip
to the enable state.  This is a hysteresis of about 28 %.
(Or over fifty times more than the spec.)  I realize
there is only a typical value given in the datasheet,
but how high can it go?  If some parts are fifty times
typical, might others be hundreds or thousands of times
the typical specification (maybe requiring hundreds
of volts to turn on)?  Seems unlikely.  But we will
stop using this part until a maximum hysteresis is
specified.

  Eric   - Re: Vhyst specification wrong in datasheet? (no rating)
2008-12-05
Hi Searing,

A quick check in the lab verified that the MAX16029 hysteresis acts as specified in the datasheet (the typical spec is 0.5%, and the measured value was 0.53%).

There are several possible causes for the behavior that you are seeing. The most likely explanation is that your circuit has some ground shift.

This means that when the OUT outputs are low (as when the input voltages are less than their thresholds), they are sinking significant current. The sum of the sink currents of all the outputs flows through GND, creating a voltage drop due to the internal resistance on the IC die, the bond wire, the solder connection, and the ground trace on your PC board. The voltage drop creates an "apparent" shift in the internal reference voltage, increasing the rising threshold.

In your circuit, try measuring the ground current by wiring an ammeter in series with the ground pin of the MAX16029. Make sure you use an ammeter with a low burden voltage.
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