voltage sag when switching on circuit

First a bit of the background:
I am putting together a circuit that runs on a 3.6VDC, 19Ahr battery. The
circuit has a LDO voltage regulator (TC1108) to output 3.3VDC for a local
PIC microcontroller (PIC18LF2620). The PIC has an output pin that controls
an N-Channel MOSFET (IRLML2502), which controls a P-Channel MOSFET (FDS6675)
to switch the 3.6VDC to an external DC-DC converter. The DC-DC converter
will be used to turn on a 12VDC device with a current draw of about 1A, max.
This translates to about 3.7A at the 3.6VDC side of the DC-DC converter.

Now the problem:
I have the circuit put together and have a piece of test code in the PIC to
exercise the on/off function for the MOSFETs. Initially there is no load
connected to the switched output side (the DC-DC converter is disconnected).
When I try to turn on the MOSFET switch, it temporarily tries to swtich on
but then immediately switches back off. The PIC gets reset and
re-initializes. I traced the problem down to the voltage from the 3.6VDC
input side sagging (actually a series of transients going down as far as
1.2VDC and up to 4.1VDC and lasting for about 2us) when I attempt to switch
on. I've tried putting a 100uF or 1000uF cap on the 3.6VDC power supply
side with no effect. Scope traces show the same response. I'm using a
HY3003 bench power supply for the tests. The supply can output up to 3A at
any voltage up to 30VDC. The normal circuit draw at 3.3VDC is about 110mA.
I have breadboarded the switching circuit and verified it works correctly.
Is there any thing that can be done about the initial droop of the voltage
from the power supply causing this problem? Is there any way to desensitize
the 3.3VDC circuit? If I issue a command to switch the circuit on in the
PIC, it immediately resets. If I immediately send another command to switch
the circuit on again, it does switch on. I can't get the circuit to switch
on at all, though, if there is any load at all on the switched output side.

Any insight or constructive suggestions would be appreciated. I am trying
to avoid having to put in a separate 12VDC power supply if I can. I realize
this would probably be a cleaner solution, though.

Thanks.

Dave


.

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