From: John Lawton <piclist@jle.co.uk>
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Date: Mon, 20 Apr 2020 02:06:18 -0700
Subject: Re: [EE] LDO reg keeps failing. Looking for suggestions.
Thread-Topic: [EE] LDO reg keeps failing. Looking for suggestions.
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James,

looking at the datasheet for the LDO, the recommended output capacitor=20
is 1uF - 22uF ceramic (low ESR) so 100nF doesn't cut it.
I would always recommend using the vendor's typical application circuit=20
as a basis for my design.

To prevent the regulator seeing a droop in the input supply, add a=20
series diode and reservoir capacitor on the regulator input, so if the=20
motor turn on surge droops the supply, the series diode will stop=20
conducting and the reservoir capacitor will continue to feed the=20
regulator. Use a reservoir capacitor somewhat larger than the regulator=20
output capacitor.

John


On 20/04/2020 09:31, James Burkart wrote:
> Thanks James. I'll try a schottky diode between the LDO and filter cap.
>
> On Mon, Apr 20, 2020, 1:37 AM James Cameron <quozl@laptop.org> wrote:
>
>> Looking at your schematic, when Q2 or Q4 conduct, there may be a time
>> during which VCC will fall below VDD.
>>
>> I've had this happen in exciting ways once before, with C1 catching
>> fire, and the fix that worked for me was a diode between VCC and
>> C1/U2.
>>
>> On Mon, Apr 20, 2020 at 01:06:24AM -0600, James Burkart wrote:
>>> VDD is 5V, and it's provided by the LDO regulator. VCC is the battery
>>> voltage and is anywhere from 6.5 to 8.2V. I don't know what circumstanc=
es
>>> would lead to VCC dropping below VDD. Could you shed some light on this=
?
>>>
>>> On Mon, Apr 20, 2020, 12:25 AM David VanHorn <microbrix@gmail.com>
>> wrote:
>>>> If vcc can rise above the input then most pnp regulators will fail
>>>>
>>>>
>>>> On Sun, Apr 19, 2020, 11:44 PM James Burkart <james@burkartstudios.com
>>>> wrote:
>>>>
>>>>> Hey everyone! Here's a little motor driver board I made that works
>> great,
>>>>> until it doesn't. The voltage regulator keeps failing on me. I've
>> used a
>>>>> couple different LDO regulators and it always happens. It's always at
>>>>> random times. So I am pretty sure it's EMI from the brushed motors.
>> If
>>>>> someone here has a few minutes, could you look over my schematic and
>>>>> recommend changes?
>>>>>
>>>>> The board is powered from a 2S lipo, so 6.5 to 8.2V. For the flywheel
>>>>> output there are 2 motors in parallel that draw about 3A total when
>>>>> running, and a stall current of about 20A. The pusher output runs 1
>> motor
>>>>> that draws about 100mA with a stall current of about 3A. I do use
>> PWM to
>>>>> control the speed of the pusher motor.
>>>>>
>>>>> If you're wondering, this driver board is for flywheel Nerf blasters.
>>>>>
>>>>> --
>>>>> Sincerely,
>>>>>
>>>>> James Burkart
>>>>> *925.667.7175*
>>>>>
>>>>> *Schematic:*
>>>>> https://drive.google.com/file/d/1dgLf3sFNzFxvEuzaqTuIHd4_VSUfNbg8
>>>>> --
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>> --
>> James Cameron
>> http://quozl.netrek.org/
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