• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 63 subscribers
  • Views 537 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

BQ25504 - SLUA629A Application note

Somayeh Imani
Prodigy 40 points
Other Parts Discussed in Thread: BQ25504, BQ25570

Hi!


I am trying to use BQ25504 IC for an energy harvesting application. My schematic is similar to Figure 2 of SLUA629A application note, in which VBAT and VSTOR are tied together. I have few questions regarding this schematic:


1) For this case, can I use CSTOR larger than 4.7 uF?! like 100uF?!

2) If I disable the MPPT by grounding VREF_SAMP and connecting VOC_SAMP to VSTOR, what will be the input voltage?! Could it be still around 300 mV?

Thanks!

  • 0
    TI__Guru**** 166416 points

    Regarding 1, yes; however, you still need a 0.1uF bypass capacitor on VSTOR and I recommend a smaller 1.0-4.7uF capacitor on VSTOR to help with transient response.  The larger the capacitor, the longer the startup time.

    Regarding 2, by disabling MPPT, the input voltage is no longer regulated so it will be at the voltage provided by the source.  If the source has a high impedance output (like solar panel, TEG, etc.), I do not recommend this because the boost converter may collapse the source.  The main boost converter is powered from VSTOR when VSTOR is above 1.8V and will continue to operate down to VIN_DC of 100mV or so. When VSTOR < 1.8V, the cold start circuit is actively trying to raise VSTOR to 1.8V and needs at worst 450mV and 50uW on VIN_DC in order to operate.

  • 0 in reply to Jeff F
    Prodigy 40 points

    Thank you Jeff!

    Right now, I have my PCB ready and I am using 220uF capacitor connected to VSTOR and VBAT (which are tied together). I have started doing test by a sourcemeter (1V and 1mA). The problem is that it takes too much time (almost 1min) for the output to reach 3.5 V. The input voltage is set by the IC to 330mV, almost for the whole charging time and the current is 1mA. Is it possible that the IC still works in cold start mode?! Another thing is that I have my inductor almost 1cm away from the IC. Can I assume it close enough, or that might be problematic?!

    Moreover, I have programmed my VBATOK_L2H for 3.2 Volts. But the VBATOK never gets high...

    Many Thanks,

    Somayeh

  • 0 in reply to Somayeh Imani
    TI__Guru**** 166416 points

    If the input voltage is clamped to 330mV, then the IC is in cold start as it charges the 220uF capacitor to 1.8V.  Inductor placement is not the issue.  The cold start circuit is not very efficient.  If your source voltage were greater than 1.8V, then I would suggest that you bypass cold start with two external FETs and the source directly charge the capacitor to above 1.8V.  The only option I see is to increase the source's output current.

    You might consider changing to bq25570 and tie the capacitor to the OUT pin as explained here

    http://www.edn.com/design/power-management/4422103/Fast-charging-a-supercapacitor-from-energy-harvesters

    VBAT_OK goes high when VSTOR rises above VBAT_OK_HYST threshold and goes low when VSTOR falls before VBAT_OK threshold.

  • 0 in reply to Jeff F
    Prodigy 40 points

    Thanks!

    I still do not see the reason for staying in the cold start... If this is the case, VBATOK never gets high, when the IC is in the cold start mode?!

    My real input is going to be even smaller (0.5V, 50uA). So it seemed that I could exactly use the SLUA629A Application note...

    Thank you so much for your time!

    Somayeh