Can't draw enough current from Analog Discovery 3

[Aykut]

Hi, I want to test a device while powering it with wavegen. Device specifications indicates 800mA with AUX but I do not think it gives that much. I tested the device with other supplies and it draws 700mA max but I cannot power it enough with analog discovery 3, I do not think there is a problem with connections etc. I can get 5V but It cannot power the device. Am I doing something wrong? Kinda new using the Analog Discovery 3, so thanks for any advice.

[JColvin]

Hi @Aykut,

What voltage are you attempting to get your 800 mA at? As per the Specifications, https://digilent.com/reference/test-and-measurement/analog-discovery-3/specifications#programmable_power_supply, the real limit is 2.4 W per channel, as you'll start running into thermal dissipation problems after that point. At 3 V, this is where you'll be able to get up to 800 mA output (achievable with an auxiliary power supply) before hitting that 2.4 W limit.

What sort of power supply are you using to power the Analog Discovery 3, and within the Supplies instrument, what do you have the Hardware Power Limit set to? As mentioned in the Reference Manual, https://digilent.com/reference/test-and-measurement/analog-discovery-3/reference-manual#power_supplies, the value listed is the overall limit for the supplies hardware circuitry.

Thanks,
JColvin

[Denspa]

Related to this topic.
I have two types of Analog Discovery 2/3.
I'll call them (AD-2/AD-3).
I have a 2A power supply connected to AUX-5V.
 

Let's set it to the state we're checking with the power system monitor.
(When no measurements are being made).
When you connect the AUX-5V to the AD-2, 600mA is immediately consumed from AUX, and this drops to 2.6mA from USB.
However, with the AD-3, the USB current is 600mA and the AUX current is 30mA.
Why is this?
It seems that the AUX is recognized properly.
 

I would like to reduce the current from the USB to less than 100m when connecting the AUX-5V, just like with the AD-2, but is there a setting for this somewhere?
 

[attila]

Hi @Denspa

Make sure the AUX power supply is in the 4.5-5.5V range and check that the USB Power setting is set to Stop with AUX.

 

[Denspa]

Thank you for your reply.
However, the situation has not changed.
I am attaching the current situation.
Also, the power display for Auto is different.

 

[attila]

Hi @Denspa

I see it now. The AUX voltage and current is lower than the USB. Due to this the USB supply is not stopped.

Initially both USB and AUX are supplies are enabled. To validate the AUX supply the voltage on this needs to be between 4.5-5.5V and I also set a 154mA 'firmware' threshold to validate this supply. This, to make sure the AUX supply is really present, the voltage reading on AUX it not just a back powering. Which can only occur only if the 'USB Supply' option is set to 'Always on' and the AUX supply is removed.
In you case the AUX voltage/current is lower probably because this has higher resistance than the USB supply. If you have, you could try to use a different switching supply.
I'll try to lower this 154mA limit to see if it causes any problems.

[Denspa]

Thank you for your reply.
I see.
When I changed the power supply to 5.2V, AUX became enabled as shown below.
Is the function here structured slightly differently from the AD-2?
In the AD-2, AUX was 4.99V, and even when the USB was 5.05V it still drew current from AUX,
but with the AD-3, I understand that it won't switch if the voltage is reversed.
Is that correct?

[attila]

Hi @Denspa

In AD2 hot swap controller fet+diode was used which had no backpowering. Here the USB was stopped based on AUX voltage reading.
In AD3 e-fuse with integrated fet is used which has a slight backpower before it turns off. So the decision can't rely on voltage before this ic. I'll try to reduce the current threshold to eliminate the problem you noticed.