DC Swich mode on/off

[kasparbp]

Hello 
I am trying to design a PCB which have both a LDO and a switch mode DC-DC converter (MAX1688), I wan't to preserve as much power as possible so I want to shutdown the switch mode converter most of the time.

According to the datasheet I can shut the switch mode supply off by driving the ON pin low on the IC, so thats easy, however my atmega that are controlling the supply is running on 3.3v from the LDO and the input to the switch mode supply is 3.6v.
The datasheet says that the ON pin only need to get above 1.5v(or thereabout) to turn on, so the 3.3v should be enough, however I am concerned about current running back or something weird like that, I never mixed 2 different supplies like that.

In all the schematics I have been able to find they always seem to tie the ON pin to Vin, so always on, no help there. The datasheet does not say much about the pin either.

I have attached 2 solutions I am considering, are they any good? (i probably need to adjust the resisters but besides that is an transistor even needed?)

[T3sl4co1l]

Input pins will not source current, so it is acceptable to drive an input (to a device on a higher supply) from an output (on a device with a lower supply).  The second case is preferred.  Probably 10k ohms would be fine for a pull-down.

Tim

[RR]

That N mosfet will not work like that. Dont forget that for switching its voltage between gate and source of mosfet not just gate-gnd. And 1m is what 1mili or Mega? Anyway just driving it from mcu with pulldown should be okay.

[kasparbp]

Thanks guys, going with the pulldown
Looks like the mosfet option was bad design either way, i will have to read some more on the subject before trying to use any of those 

/Kaspar