Two very interesting things have emerged about the Mac Portable and the Power Manager chip, based on my increased understanding of the Hybrid module through all this research and work.
1: The A/D line voltage, intended to convey battery and charge level information to your system,
can kill your power manager chip
First, I present the voltage tolerance section of the original power manager chip datasheet, a Mitsubishi M50753.
These 'absolute maximum' statements tend to be pretty accurate, and running outside these ranges will cause chip damage.
The A/D input pin of the power manager is in the "IN" bank of the chip, third row down from the top (not including the headers). Its maximum allowable input voltage is the chip's Vcc (main power) plus 0.3 volts.
Have you ever wondered why the main 5 volt power rail of the mac Portable is 5.2 volts instead of, you know, 5? I think this is part of the reason. This "IN" port can accept voltages up to 5.5 before there's a real problem. And because these are absolute max values, you really want to stay under 5.5, if not near 5 volts to be completely safe. The A/D line on some portables I've measured runs up to about 4.7 volts.
So, you ask, where's the problem? We're safely under the 5.5 volt absolute max.
Well, actually that's not guaranteed. The opamp Vcc (power input) determines how high the output can swing. And this can range most of the way up to battery terminal voltage. See where this is going? Drifting resistors on the hybrid from a variety of causes will change the A/D output multiplier and definitely can swing that high. After replacing the capacitors on my friend's Portable, I tested the A/D voltage and found it
spiking to 6.2 volts when plugged in. That's a clearly dangerous voltage knowing what we do about the power manger chip's input voltage limitation.
On to #2, what else is in the A/D circuit which could help mitigate this issue?
Check out the Portable schematic. On page 12 we find a current limiting resistor on the A/D line before it goes to the power manager chip, highlighted in red. Next to it is a little capacitor to ground, forming an R-C network which basically smooths out any big spikes in the A/D line voltage.
This little R-C network probably doesn't provide much protection. Voltage at the "A/D Filter" pin on the power manager will still go too high if your hybrid is putting out more than 5.5 volts. The little 1k resistor is a current limiter at least.
I suggest that everyone with a working Portable should check if the A/D voltage is too high when charging the battery (also check on-battery use). If your A/D voltage ever heads above 5.5 volts your system is probably in danger.
A solution would be to stick a 5v zener diode on the C107-side of R151 to clamp it and prevent damage to the power manager from an A/D line which is running too high. And an A/D line running too high is a good indication that your hybrid is degrading, so you should check the other voltage rails to see if things are still in spec.
