Russell McMahon wrote:
>
> >From their block diagram it appears that the body diode is absent - on
> reflection this is probably the case with all synchronous rectifier
> ICs with this topology and I have never looked into it. The FET is a
> truly isolated one, unlike discrete units which have the substrate
> implicitly connected to Drain by virtue of the means of construction.
> Something new every day ... :-). Someone may care to correct me on
> this.
>   
They mention that the output capacitor will not discharge back into 
input supply. Since it's a P FET, isn't the substrate at Vin ? So as you 
say how do they isolate when Vin > Vout?  Millions of years ago I 
remember signal MOSFETs that had no source/drain diode. The 4066 quad 
switch obviously has "isolating" FETs. But it's unusual to see one that 
can do 800mA (mentioned someplace as max SW terminal current).

My samples of this are shipping from Microchip on 15th. I'll let you 
know how I get on with a 3.3V from 1 AA alkaline, 3.3V from 2x NiMH 
maybe, and 5V from 2x AA (Alkaline or NiMH). Also what a single cell -> 
3.3V followed by second 1640 to get 5V so as to have 3.3V and 5V.

They show a single Li cell. I don't fancy using this with a 2.5V to 4.2V 
LiPoly for 5V. Seems like wrong starting point in terms of charger 
circuit and discharge cut off. Maybe they mean a disposable primary 
Lithium battery?



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