Re: [Usbpicprog-technical] Troubleshooting a low Vpp

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Dear David,

Thank you for your suggestions. There is one thing that I wouldn't like
to do in this schematic, and that is inverting the signal of the VPP_CTL
pin. The reason for this is that it wouldn't be compatible anymore with
older hardware. So either I would have to drop support for older
hardware (a thing that I would never do) or I have to make two firmware
versions. The second option is also not very convenient, especially for
people who want to update their firmware and then need to search for
which version they will need.
Also in your suggestion, the G-S voltage will be 13.8V which is too high
for the TSM2301. Of course another one can be used but I won't consider
a P-channel mosfet in my design for the above reason.
I would rather just use a bipolar transistor for Q4 in exactly the same
circuit as it is in 0.3.1. The zener diode is in the circuit because
some programming specifications say that 12.0V is the max VPP, but if
you look in the datasheet of those devices, the absolute maximum ratings
say 13.5V so the zener can also be removed from the schematic.
I will soon come with a new hardware release, taking a higher VPP into
account, but I will also try to be able to use the old PCB so that
people will be able to alter their device to the new schematic without
too many problems.

Kind regards,

Frans Schreuder

On 01/10/2011 02:07 AM, David wrote:
> Indeed, Q5 turned out to just be a diode -- thanks for the tip.  The extra
> pad to Vcc was throwing me off.  This means what I really have is rev 702,
> minus R13.
>
> I was able to get Vpp to 12.5V with no load by removing the Zener and
> bypassing Q4 (so, Q1 shorts Vpp to turn it off).  That solved my problem.
>
> Alternatively though, consider the attached schematic tweaks.  I switched
> out Q4 for an P-MOSFET.  The resistance across Q4 should be in the milli
> ohms in this setup, so the voltage drop should be negligible.  It should
> also easily give you 13V for Vpp (no load) as it also removes D4 which isn't
> needed anymore.
>
> This could also allow for a slightly higher voltage Zener shunt and allow R2
> to be reduced, letting Vpp to remain above 12.5V at the 0.45mA load
> prescribed in the PIC10F200 programming spec.
>
>
> Either way, thanks for the quick reply.  I hope this helps.
>
> David
>
>
>
>
> On Sun, Jan 09, 2011 at 08:56:53PM +0100, Frans Schreuder wrote:
>> Dear David,
>>
>> On the version you have, Q5 is actually a diode in an SOT23 casing, so
>> it seems that it doesn't match the 0.3.1 design but it actually does.
>> The problem you are facing is probably that the resistors R15/R16 were
>> still 47k/100k. In the new schematic they are now 470k/1M because they
>> were loading the circuit too much. They can be removed if you are not
>> interested in measuring the voltage in the IO Test dialog. (you can also
>> remove the 12V zener)
>> When that is done, the circuit should go to at least 11.5V which is
>> enough for 99% of the PIC devices.
>> It is also possible to replace Q4 with a bipolar transistor (BC547 or
>> something similar) which will bring the voltage drop to only 0.7V in
>> stead of almost 2. I am still considering to do this action in a future
>> hardware release.
>>
>> Kind regards,
>>
>> Frans Schreuder
>>
>> On 01/09/2011 08:02 PM, David wrote:
>>> I think there's a design issue in how Vpp is being switched.
>>>
>>> I figured out my board is closer to rev 644 (minus R13).  To that end, I
>>> removed Q5 altogether as a test, and I still get the same 10.78V for Vpp.  I
>>> would have suspected body-diode conduction from Vpp to the 5V supply as the
>>> culprit (as this appears to be the design oops Frans refers to), but alas no
>>> change.
>>>
>>> On the other hand, I think the real issue is Q4: It's an N-MOSFET being used
>>> to switch the 12V supply and the only thing to generate Vgs is the drop
>>> across the MOSFET itself.  The BS170 datasheet calls for a Vth anywhere from
>>> 0.8V minimum to 3V max, with 2.1V typical.  As such, depending on the device
>>> tolerances, the ~1.25V drop between the pump and the Zener shunt isn't
>>> enough to drive the MOSFET into good conduction.  Moreover, the problem is
>>> exacerbated with R1 being ahead of the pull-up for Q4, so as the Vpp current
>>> increases (via the shunt or the load), Vgs drops even more.  The result
>>> significantly limits the current on Vpp.
>>>
>>> I may have *completely* misread the circuit and I'm a bit rusty on my analog
>>> electronics (had to pull out the old text books), but this appears to be
>>> what's happening.
>>>
>>>
>>> The fix would be to use a P-MOSFET in place of Q4 (source towards the pump,
>>> drain towards the Zener), with the rest of the circuit remaining unchanged
>>> (though D4 could also be removed).  VPP_CTL would change from being active
>>> low to active high.  The P-MOSFET won't conduct so long as the gate is held
>>> towards the source, but will conduct quite well when Q1 pulls the gate down
>>> to ground (Vgs=-13-14V, which will be too much for the TM2301.  Fix with a
>>> different MOSFET or an extra resistor to make a voltage divider on the
>>> gate.)
>>>
>>> Thoughts?
>>>
>>> Of course, another, less elegant fix is to bypass Q4 altogether and use Q1
>>> to short out the 12V when not in use.  This will cost about 6mA in draw, but
>>> won't require any other circuit changes.
>>>
>>> If I have time later, I might try it.  Nothing like doing some hotwired SMT
>>> soldering!
>>>
>>> David
>>>
>>> On Sun, Jan 09, 2011 at 12:03:45PM -0500, David wrote:
>>>> Hello everyone!
>>>>
>>>> I'm trying to troubleshoot a low Vpp on my usbpicprog v0.3 hardware.  I'm
>>>> only getting about ~10.78V with no load attached.  USB supply voltage is
>>>> 4.98V.
>>>>
>>>> I'm getting a solid 13.25V on the pump side of R1, with only a 0.17V drop
>>>> across it, confirming little load.  However, by the time I get to the Vpp
>>>> header, I'm showing only 10.79V.  The anode of D4 is also low at 11.27V.
>>>>
>>>> Unfortunately, the board I have (SMT version, pre-assembled) doesn't exactly
>>>> match any of the schematics.  I have what appears to be a Q5, and R13 is not
>>>> assembled (which is what the v0.3.1 hardware calls for), yet I still have
>>>> the 47k and 100k R16/R15 voltage divider on the Vpp sense (v0.3 design).  I
>>>> also have no D10.  So, thus far, until I trace Q5, I'm a bit of a loss to
>>>> troubleshoot further.
>>>>
>>>> FWIW, my Vpp may have always been low -- I recall having some issues reading
>>>> a device a while back, but I moved the interface to a powered hub and all
>>>> was well.  I didn't investigate further.  Now I'm on a powered hub and I'm
>>>> unable to get a PIC10F206 to respond, which I'm assuming is because Vpp is
>>>> well below spec for program mode entry.
>>>>
>>>> Any thoughts?
>>>>
>>>> Thanks!
>>>>
>>>> David
>>>>
>>>>
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