Tim,

I've passed your message on to the guy at WMT who is currently trying to debug 
this; his hardware knowledge is superior to mine, so hopefully we can work 
something out between us.

I've embedded a few responses below.

Thanks for this.

On Thursday, 4 October 2018 08:56:09 BST t...@ls83.eclipse.co.uk wrote:
> Don't know the background to this but that driver is a bootstrap device for
> use with high-side NFETs in an H bridge motor driver. It must be driven by
> PWM and with that type of device it's important that the PWM is kept below
> 100% otherwise the high side gate drive voltage falls and the high side FET
> is destroyed (for starters).

Even after you've pointed this out I haven't been able to spot anything in the 
Datasheet that would give me any clue about this point.  Maybe it's there, but 
it certainly isn't obvious to me.

>  From the data sheet, Iil is 100uA, so through a 100k resistor that's a 10V
> drop. Ends up settling at 1V as the bias current reaches equilibrium.

Thanks for that, I'd missed it.  I've used fairly high value resistors on 
logic inputs before, but looking back through the Datasheets of the devices 
I'd used, they all had significantly lower input currents.

All other things being equal, we'd probably have realised that and reduced the 
value of the resistor significantly or (more likely) buffered the input with a 
suitable gate.

I still don't understand why the inputs were sitting at 2 to 3 V with nothing 
connected.  Unless my Ohms Law is rustier than I thought, there can be no 
input current if there is no connection.  Normal logic gate inputs might float 
a bit if they are left disconnected, but they normally wouldn't deliver enough 
current to destroy a Pi!

> This is not the type of circuit which lends itself to breadboarding. Lots of
> high peak currents so need really good ground and power planes and careful
> decoupling.

I only stuck the chip onto a breadboard (in isolation) to try to understand 
the input voltages that I was seeing.  I assume however from your comment, 
that Veroboard would be no better than breadboard for this circuit,

We had intended to develop a proper PCB once we had got a prototype working. 

-- 



                Terry Coles



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