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BQ24190: VSYS cuts off when below VSYS_MIN when powered with a USB DCP without battery

Edwin Mui
Prodigy 20 points
Part Number: BQ24190

Hello all,

I am doing some load tests on the BQ24190EVM evaluation kit where I connect a DC load at the SYS pin. The BQ24190EVM is powered by a USB DCP which provides 5V@3A at VBUS. No battery is connected.

I have set up the following register values.

- Write 0b111 to IINLIM[2:0] in REG00  // Set Input current limit to 3A to match DCP above. 

- Write 0b00 to CHG_CONFIG[1:0] in REG01 // Disable charger to use BQ24190 as buck converter.

- Write 0b111 to SYS_MIN[2:0] in REG01 // Set SYS_MIN voltage to 3.7V. VSYS will be 3.85V since the BQ24190 regulates VSYS 150mV above SYS_MIN.

With this setup I can now switch on the DC load to see how much current can the BQ24190EVM provides. As I increase the DC load's current, VSYS would very slowly drop and then it will reach a point where the BQ24190EVM just suddenly cutsoff VSYS and no longer supplying the demanded current by the DC load. After the cutoff, VSYS falls instantly to 0.1V and the DC load current is 670mA, as if it was a short circuit. My cutoff point for the above settings was about 3.75V@3.04A.

I found in in page 7 of the datasheet which says at VSYS = 3.5V, the typical value of VSYS_MIN is 3.65V while minimum is 3.55V. From this point, does that mean for VSYS = 3.7V, the minimum VSYS_MIN allowed is 3.75V before the BQ24190 cuts off VSYS? Or could there be other explanations?

Best regards,

Edwin

  • 0
    TI__Expert 8230 points

    Hello Edwin,

    Have you monitored the VBUS voltage and current during the transient cut off event? If VBUS voltage dips below the UVLO threshold (because the input power reached its limit) and bounces back up, the converter will perform it's power up sequence. If SYS voltage has crashed to ~0.1V (because there is no battery to supplement the maxed out input), the converter would enter it's soft start sequence when SYS is below 2.2V and clamp the input current limit to 200mA until SYS raises above the 2.2V threshold. 

    I would also verify whether the converter is shutting off/on due to the input being maxed out and the lack of supplement power. The ~670mA current you are observing might be the average current flowing into the load every time the converter tries to turn on.

  • 0 in reply to Fernando Lopez
    Prodigy 20 points

    Hello Fernando,

    Thanks for the quick reply. I reran the tests on the BQ24190EVM with the DC load and observed the following. The yellow trace on the oscilloscope is the VBUS while the blue trace is the VSYS.

    [1] On the picture below,  the BQ24190 is supplying about 10W to the DC load. The DC load is taking 3.66V@2,75A.  VBUS maintains at about ~4.8V and VSYS drops to ~2.4V before rising back to ~3.6V. From here, it seems the BQ24190 has no problem supplying the 10W the DC load demands.

    [2] On the next picture below, the DC load is set to 11W. We observe that VBUS still maintains at about ~4.8V. This time, VSYS seems to experience a sharp drop and continuously reduce to below 2.2V and then cutting off completely. The BQ24190 from there just supplies 0.1V and ~670mA as previously reported.

    From the waveforms, it seems both your points are valid. The cutoff occur because of VSYS falling below 2.2V. This could be caused by BQ24190EVM not being able to supply enough power using the USB DCP  without the battery to the DC load.

    Do you know if it is possible modify to the BQ24190EVM circuit to increase its maximum output power to VSYS? E.g. changing inductor value, etc? We hope to be able to implement a power supply via BQ24190 where we can use its Power Path management to supply direct DC power via USB DCP, a 1S Lithium Ion battery and possibly both combined to supply up to 12W of power.

    Best regards,

    Edwin