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BQ24650 with variable MPPSET/Alternate USB input

F Iannce
Prodigy 100 points
Other Parts Discussed in Thread: BQ24650

Hello,

We're trying to use the BQ24650 to create a solar powered single cell LiPo battery charger with an MCU programmable MPP voltage, as well as the ability to use an optional USB charger instead of the solar panel.

To create a programmable MPP voltage, we replaced the ground of the MPPSET pin's voltage divider (453k on top and 38.3k on bottom) with a DAC capable of outputting a voltage between 0 and 2.5V.  Our intention was to drive the DAC voltage above 1.2 to enable the use of the USB charger.

We were under two impressions from the datasheet:

1) A voltage on the MPPSET pin above 1.2V will NOT do any current regulation

2) The MPPSET pin is just an input to a couple internal comparators.

We are unable to get proper USB charging to work (we haven't tested with a PV panel yet--we'll deal with that later).  We set the DAC voltage to 2.5V and we get no output current.

IN order to try and see what was happening, we connected a 5V PSU to the input and swept the DAC voltage from around 0.75V to 0.84V we had a linearly increasing charging current (0 to about 1.5A), and the MPPSET voltage remained constant at ~1.2V.  A few mV above 0.84V and the charging current abruptly dropped to 0A and the MPPSET voltage went above 1.2V.

The result is puzzling in two aspects:

1) The MPPSET pin appears to be doing some sort of regulation to maintain the voltage at 1.2V.  With a PV panel I could see this, but we were driving it from a low impedance voltage regulated bench top DC PSU (and the voltage never changed from its 5V value).

2) Applying a voltage to the MPPSET pin above 1.2V appears to cause the charger to not produce any output, when we were expecting maximum charging current (~2A).

Any help would be greatly appreciated.

F Iannce

  • 0
    TI__Mastermind 31120 points

    The MPPSET pin is a voltage feedback pin to a control loop.  The buck converter delivers power until the input (impedance source) is pulled down to a voltage that divides to 1.2V and holds the power transfer there.

    Your application may be tricky to implement since the response time of the micro may interfere with the input voltage control loop.  The only way I can see to do it is by iteration.

    The micro checks to see if the source voltage is where desired and if not, say too high of input voltage, the micro bias to the MPPSET pin is raised proportional to the divider so the converter will load the input a little more.  The micro checks the input voltage again and adjusts if need.  If the input voltage is too low then the opposite is done.

  • 0 in reply to Charles Mauney
    Prodigy 100 points

    Thanks for the response.

    Is there an easy way of bypassing the input voltage regulation for the case in which we are using a constant voltage 5V USB charger input?

  • 0 in reply to F Iannce
    TI__Mastermind 31120 points

    just pull the MPPSET pin high.  This will load the input until the charge current and system load are satisfied.

    thus you must make sure that these loads are less than the source capability of the input so it does not crash the input.

  • 0 in reply to Charles Mauney
    Prodigy 100 points

    This is how we believed that the IC would operate, so we did try pulling the MPPSET pin to ~2.7V (the DAC was set to maximum output of 2.5V), but unfortunately we got no output from the BQ24650.  We can only get it to produce an output over a narrow range of DAC setings (from around 0.8 to 0.9V).  Above or below this range we get no output.

  • 0 in reply to F Iannce
    Prodigy 100 points

    Any idea why we'd get zero output with a 5V input and the MPPSET pin pulled to 2.7V?

    I can provide this schematic page to TI if necessary, but can'[t put it on a public forum.

    Thanks,

    -F

  • 0 in reply to F Iannce
    TI__Mastermind 31120 points

    I would use a scope look at each pin on the IC and see if it within the specification.  Also set the scope on single sequence trigger and see if there are any glitches.

    I would remove any external controls and see if the designs works as a stand alone with a low impedance power supply with a higher input voltage to start with.

    If you have an EVM, this is the best bet since it should be working when delivered.