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TPS61236EVM-676: Voltage drops immediately, even under very light loads. Can't get stable 3A @ 5V, or anything even close.

Part Number: TPS61236EVM-676
Other Parts Discussed in Thread: TPS61236P


I am trying to get 3A at 5V from the TPS61236EVM-676 Evaluation Module.  However, the output voltage drops substantially even under very light loads.  Here are the test conditions:

Test #1

Unprotected Li-Ion battery supply at 3.56V without load (20A discharge rate capability)

Battery connected to Vin and J3 GND using both pins on each jumper.

Load is a 1.8 Ohm 100W resistor (5%) connected to both pins of Vout and J8 GND

Output voltage is sagging to 2.6V (instead of 5V as expected)

Output current is 1.3A, but TPS61236EVM makes a weird, soft clicking sound approximately 20 times per second when current is being measured

Input voltage drops to 3.39V when load is attached

TPS61236EVM is enabled/started without load attached, and then load is connected

Expected to get 5V at 2.78A with that load, but it's not even close on either

Test #2

Similar setup to test #1, but load is replaced with an e-load.  The e-load shows 5.02V with no current, but when current is increased the voltage drops immediately.  By the time the current reaches 1A or 1.5A, the e-load shows the voltage dropped to about 4.6V and eventually cuts off.  E-load is added to circuit after enabling the TPS61236EVM with the jumper at J4.

I have measured the resistor at R1 to ensure it really is 0 ohm and that the constant current feature is therefore disabled.  That doesn't seem to be the problem.

I suspect the cause has something to do with the soft start feature, but I'm confused as to why this doesn't work "out of the box".  Do I need additional resistors or capacitors at the output to ensure the soft start is successful?  The odd clicking sound is also referred to in another thread on this forum, so I know I'm not going crazy.  Maybe it's cutting off and restarting over and over?  If so, what would cause this to happen?

Basically, I'm looking to get a stable 5V 3A from a Li Ion battery that has more than enough discharge capacity, in theory.  Any help or guidance you can offer would be greatly appreciated!!



  • Hi Graham,

    You findings are really strange. 

    1. Want to check with you first, is the EVM that your are testing bought on No components on the EVM were changed. right?

    2. I would suggest you use E-load to test the board first. Since TPS61236P precharge mode current limit is lower than normal switch mode current limit, it's suggested to enable the device first. When output voltage reaches 5V, then apply load.

    3. When you found current reaches 1A or 1.5A, the e-load shows the voltage dropped to about 4.6V. Please make sure the input and output power cables are short and thick enough to have the capability of flowing >5A current. Power cables also have voltage drop so it's better to measure the input and output voltage on EVM sense pins with DMM.

    4. Please provide the Vin, SW, Vout pin waveform if the issue still exists. Thanks.

  • Hi Zack,

    Thanks very much for the reply.  I have done some further testing, and to my chagrin I can confirm that the wires connected to the TPS61236EVM (both coming in from the battery and going out to the e-load) are responsible for a huge amount of voltage drop.  They were nowhere thick enough, and also longer than necessary.  When I tested at the sense pins, the TPS61236EVM is producing exactly 5V.  I think that when I was drawing too much current, the voltage drop on the input side was enough to go below the device's Vin minimum, causing the reset oscillation I observed.

    Thanks again for your advice - much appreciated!