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BQ25606: BQ25606 and TPS61236 connected together, max current problem.

Part Number: BQ25606
Other Parts Discussed in Thread: TPS61236EVM-676,


We are using BQ25606EVM-772 and TPS61236EVM-676 to build a UPS.
The required output is 5V with a max current Iout=2.5A .

Sys pin of BQ25606EVM is connected to Vin of TPS61236EVM, no battery connected, /CE-GND  jumper on the BQ25606EVM not installed (charger disabled).

The input power supply can provide 5V-2.5A  (we tested it and it can go up to 3A without issues).

Connecting a electronic load in CC mode on the Vout pin of the TPS61236EVM, we can’t reach 2.5A. The BQ25606 shut the output down at about 1.3A (current out from the TPS61236)

R11, which is connected to ILIM, has been set to 170Ω, considering that KLIM/ILIM=500/3=166.7Ω

Placing a Fluke 87V in different points to check the current, we noticed the following:

With no battery connected, when a current of 1.3A from Vout of the TPS61236 is reached:

1. current out from Sys pin is 2.185A
2. current out from the power supply is 1.934A

Connecting a 1400mAh battery, the system can provide 1.6A before it starts discharging the battery (tested with charging disabled). At 1.7A there are 0.136A coming out from the battery.

Connecting the electronic load directly to Sys pin of the BQ25606EVM, with no TPS61236, we can go over 2.5A without any problem. When a current of 2.5A is coming out Sys pin, the power supply is at 2A.

Is this behavior related to the inductor on the BQ25606EVM (from the BOM, 1 μH, 3.2 A, 0.028 ohm)?
How could we fix it and take 2.5A from Vout of the TPS61236?

Thank you,

  • Hey Francesco,

    What did you do with the D+/D- pins on the charger? Are they floating?

    What is the voltage seen at the VBUS pin of the charger?

    My current guess would be that you are overloading the input or the charger is going into a DPM state. The bq25606 is a 3A charger. Doing some rough calculations using efficiency from both our charger and the TPS61236, if you are trying to get 5V/2.5A out of the boost converter, the charger needs to provide ~3.5A at the SYS pin. Considering that VSYS it typically ~3.7V, your input adapter needs to provide ~5V/2.97A.

    Now while you stated your input source could seemingly provide 5V/3A, you may still run into an issue at the VBUS pin itself. If VBUS drops too low (possibly due to a lot of IR drop), we will trigger our VINDPM regulation loop to prevent the input source from crashing.

    Less likely is that the charger is hitting it's HS cycle by cycle current limit @ around 5.2A(min). But I suggest you look at the SW pin to verify this is not the case (basically look and make sure the f_sw is ~1.5MHz).

    Joel H

    Joel H
  • Hi Joel,

    Thank you for your support.

    D+/D- pins on the charger are floating.

    The voltage at the VBUS pin of the charger varies as follows:

    Vac       |    Vbus    |    Vsys      |    Iout  (from TPS61236EVM)
    5.274    |    5.274    |    3.7        |    0.000A
    4.767    |    4.699    |    3.669    |    1.000A
    4.650    |    4.560    |    3.667    |    1.100A
    4.570    |    4.480    |    3.663    |    1.200A
    4.631    |    4.555    |    3.663    |    1.250A
    4.530    |    4.495    |    3.663    |    1.290A
    4.580    |    4.530    |    3.661    |    1.300A
    4.560    |    4.440    |    3.660    |    1.320A
                     shutdown                |    1.350A            

    I’m having some issues trying to take a precise measurement at the SW pin. What is the best way to measure it? What bandwidth do you suggest for the Oscilloscope?

    Best regards,

  • Hey Francesco,

    Just judging on your input voltage, know that the charger has a VINDPM feature that attempts to hold the input votlage to the higher of 4.3V or VBAT+200mV.

    In order to measure the SW node, there should be a hole labeled "SW" near the inductor. Place your probe in the center hold and hook your ground connector to a ground on the EVM. I would recommend 200MHz BW if your scope has the option. If not, full BW. 20MHz BW may cut off what we are looking for. 


    Joel H