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BQ25504: How to have standalone fixed MPP

AGM63
Prodigy 30 points
Other Parts Discussed in Thread: BQ25504, TL431

I'm using a power source that has very slow recovery from its maximum power point to open circuit voltage. The recovery rate is also variable, so the tricks used in SLUA632 don't work. I'd like to define a fixed MPP with a voltage applied to VREF_SAMP, but what's the best way to do this? The simplest would be with a resistor divider from VSTOR. But what happens on cold start when VSTOR=0V?

  • 0
    TI__Guru**** 166136 points
    As shown in datasheet figure 28, connecting VOC_SAMP to VSTOR disables the MPPT sampling routine. If you then apply a voltage to VREF_SAMP (instead of grounding it per the figure), the IC will only pull current from the harvester source until the voltage at VIN_DC falls to that voltage.
  • 0 in reply to Jeff F
    Prodigy 30 points
    I know I can apply a voltage to VREF_SAMP, the question is where can I get that voltage from? I do not have any other voltage sources that I can use as an external voltage reference. I would like to use the IC's own VSTOR output as the voltage source. But I'm unsure about behaviour on cold start.

    Say, for example, I connect VREF_SAMP to 0.5 * VSTOR. On startup, VSAMP = 0.5 * VSTOR = 0V. So the chip will attempt to operate the input source at short circuit, where the power is zero. Under these conditions will it start?

    How do MCU controlled MPPT systems deal with this? Presumably the MCU is powered by the bq25504. Is it better, or necessary, to have a system which actively switches from the standard MPPT sampling scheme to a fixed (self-supplied) operating voltage after the cold-start phase?

    Thanks for the help, I'm still trying to determine whether this is worth pursuing further for our application.
  • 0 in reply to AGM63
    TI__Guru**** 166136 points
    The input voltage regulation is disabled during cold start. Since VSTOR will be moving around alot, I would not recommend it as the reference. Standard voltage references like TL431 take alot of current. A high beta bipolar transistor might be a good option as it will provide ~0.6V across its base to emitter junction. Another option for a reference might be a low Iq linear regulator such as TPS78001.
  • 0 in reply to Jeff F
    Prodigy 30 points
    Great. It sounds like this might work. The key is that the power point tracking is off for startup.

    Thanks for your help.
  • 0 in reply to AGM63
    TI__Guru**** 166136 points
    Keep in mind that during cold start, VIN_DC is clamped to around 0.35V.