> The 1640 has variants which are not available in the 1623/24
> which instead of turning off the PFEt in disable mode, turn it hard
> on. This allows the battery to be connected to the output in
> shutdown. If Vin is high enough this is most useful

Let me see if I understand this. The datasheet says

4.1.4 INPUT BYPASS OPTION

The MCP1640C/D devices incorporate the input bypass shutdown
option. With the EN input pulled low, the output is connected to the
input using the internal P-Channel MOSFET. In this mode, the
current draw from the input (battery) is less than 1 uA with no load
The Input Bypass mode is used when the input voltage range is high
enough for the load to operate in Sleep or low Iq mode. When a
higher regulated output voltage is necessary to operate the application,
the EN input is pulled high enabling the boost converter

Now, I've got a sensor which works from 3.2V to 5.5V. If I
use 3 x alkalines (4.95V when fresh) and set the converter to
eg 3.3V output, the converter can be shut down until battery V
is below 3.3V. Apart from the 1uA quiescent during bypass, the
converter efficiency will not figure in the battery drain until then.
The PIC (12F675) associated with the sensor can periodically
monitor the battery voltage and enable the converter when needed

The figure I haven't seen in the datasheet is the "low Iq" value,
but I suspect it might be 1uA, which isn't enough for the sensor
to run, although is the 1uA just to turn the FET on, with the load
able to take its operating current through the FET ? The block
diagram isn't full enough, and the description flips unclearly
between current and voltage

"A "true" load disconnect mode provides input to output isolation
while disabled by removing the normal boost regulator diode
path from input to output. A bypass mode option connects the
input to the output using the integrated low resistance P-Channel
MOSFET, which provides a low bias keep alive voltage for circuits
operating in Deep Sleep mode. Both options consume less than
1 uA of input current"

"low bias keep alive voltage" ?

The converter consumes 1uA or that's all the converter will
pass for the load ?
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