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BQ51050b Minimum Charge Current

Jason Luzinski
Prodigy 100 points
Other Parts Discussed in Thread: BQ25100, BQ51003, BQ51050B, TIDA-00318, BQ51221, BQ500212AEVM-550, CSD25310Q2, BQ500212A, BQ500211, TPS28225

Hello,

I have a very small single cell LI battery with a fast charge current rated at 6mA.  Can the BQ51050 support this low of a charge current?

Thanks,

Jason

  • 0
    TI__Genius 14890 points
    Jason,

    The bq51050B is optimized for higher power solutions. I would not recommend it for such low current charging. My suggestion for this solution is using the bq51003 wireless power supply along with the bq25100 low power charger. It is quite accurate at lower currents and the solution is quite small.

    Please see the TI reference design TIDA-00318. It can be found at www.ti.com/tool/tida-00318 or by searching ti.com for TIDA-00318. This solution is 5mm x 15mm and we've tested it with a wide variety of transmitters and also with many receiver coils.

    Regards,

    Dick
  • 0 in reply to Dick Stacey
    Prodigy 100 points
    Dick,

    I will definitely look into the TIDA-00318. Even though the BQ51050 is not optimized for such low current, would it work? The 5.25mm x 5.5mm footprint of the TIDA-00329 is very appealing.

    Thanks,
    Jason
  • 0 in reply to Jason Luzinski
    TI__Genius 14890 points
    Jason,

    You will definitely have problems with termination. In fact, at a charge current of 6mA, you may never terminate or you may not charge. The A/D conversions taking place on the device are not accurate enough for that current. The issue here is statistical. You could grab a few parts and work quite well. But, if you ran a large number, you will find problems due to the very low currents (and resulting voltages).

    As you look at the TIDA-00329, you'll notice it is not a Qi-certifiable solution. It does not have the Cs / Cd capacitors. If you need a Qi-certifiable solution, you'll need to add that. At your low currents, there are a lot of things you can do with the TIDA-00318 to make that solution smaller. If you put together a schematic, I can review it for you. Take a look at the reference schematic on the TIDA-00318 and see what you could do to reduce components to a minimum for your specific application.

    Additionally, if you're looking for a small transmitter solution, definitely look at TIDA-00334. The whole solution can be the size of its 30mm TX coil.

    Regards,

    Dick
  • 0 in reply to Dick Stacey
    Prodigy 100 points

    Dick,

    Thank you very much for your quick replies and the suggestions.  I think I will go about minimizing the TIDA-00318, but I have a few questions.

    1. I have been following this article, Designing a Qi-compliant receiver, regarding Rx design.  I have been using the test setup for load-line (Figure 10) and equation 4, the voltage ratio of V2/V1 = k * sqrt(L2/L1), to help in choosing a Rx.  In the paper it states to use a 19V input for V1, which is the DC input into an A10 system.  I am wondering if I were to use the TIDA-00334, would V1 and the input voltage into the load-line schematic be 5V?
    2. One idea I have to reduce size is to change any 0603 components to 0402.  The problem with this is that voltage rating will go down.  At these power levels, is a 25V rating necessary for the input pins?
    3. Is there a refrence for what values Crect, Ccomm, Cboot need to be?  I remember reading in the dual mode BQ51221 datasheet that Ccomm should be between Cs/3 to Cs/8.

    Once again thank you for your quick replies and advise.

    Thanks,

    Jason

  • 0 in reply to Jason Luzinski
    TI__Genius 14890 points
    Jason,

    1. Great article. Figure 10 discusses evaluation of an RX coil. In this case, the TX and RX are "open loop" meaning you force the frequency and measure the resulting voltage at different current. The TIDA-00334 is a 5V system and is closed loop meaning it requires a full RX capable of communication. A 5V TX will create higher voltages on the coil than 5V so the input power is not really relevant for the evaluation. Evaluating the coil with the full load line is a great idea. However, for lower powers that you're looking at, you may not need to do as detailed of an evaluation.

    2. As noted in question 1, the voltages can (and will) get quite high. The EVM uses 50V capacitors, but 25V is generally okay. I definitely would not go below 25V. Please keep in mind the DC offset and it's impact on the actual capacitance as you chose the voltage rating and the values.

    3. For your solution, I'd suggest using the same values as in the EVM or the TIDA designs. The COMM capacitors change the level of the communication pulse fed back through the magnetic field to the TX. I suggest leaving the BOOT capacitors as is in the reference designs. RECT capacitors are used really for two things. One is to smooth out the rectification stage and the second is to allow for load steps on the output without crashing the system. For your solution, at 6mA, the main concern is the smoothing of the rectifier. Monitor that voltage during your charging to see the value. Note that the voltage on the RECT pin can get high, particularly during start up. I suggest a 16V capacitor there.

    For whatever solution you end up with, the efficiency of the system will be dominated by the transmitter quiescent power. The optimization you can do will be setting the current limit to a lower value with the RILIM resistance.

    Regards,

    Dick
  • 0 in reply to Dick Stacey
    Prodigy 100 points

    Dick,

    My understanding of your response to my first question is that in the article you have a system with no feedback ("Open loop") and the output will be directly influenced by the frequency as you mentioned. Closed loop, in the TI chipsets case, is the dynamic efficiency scaling which effectively moves a system too and from resonance at 100kHz to achieve the appropriate Vrect based on current out.

    Is there a way I can utilize this open loop system to help design an appropriate Rx?

    For example, a problem I have encountered with past Rx is that the system was operating constantly around 205kHz, which I believe is a problem of L2 having to large of a value. In the article, they used the open loop system to find a Vrect value at a desired frequency, in the paper 5.15V at 150-160kHz.

    Why did they use 19Vpp as the AC input? Could I use 19Vpp input to help design a system that meets my Vrect and Frequency requirement?

    Thanks a lot,
    Jason

  • 0 in reply to Jason Luzinski
    TI__Genius 14890 points
    Jason,

    The TI chipset will change the frequency to get the VRECT target. In general, the frequency is increased for lower power and decreased for higher power. There are some exceptions (at very low power, the frequency may be a bit lower to prepare for load steps).

    The open loop can help evaluate the coils and their response to loads. The goal is to find a coil that has a high enough voltage at the ping frequency (175kHz) to power the RX and will support the load you anticipate.

    19V is used since many of the Transmitters operate at 19V. Some operate at 12V and some at 5V. The result for any system is a change in the voltage seen on the TX coil.

    For your system and the low 6mA requirement, the 205kHz indicates the system is somewhat overpowered for your RX.

    A similar experiment can be done with the closed loop system. Just monitor the frequency. By changing the inductance of the coil, you change the operating frequency. Remove a single turn at a time to watch the movement of the frequency. Additionally, take care to set the Cs and Cd capacitors correctly as the inductance changes.

    Regards,

    Dick
  • 0 in reply to Dick Stacey
    Prodigy 100 points

    Dick,

    Thank you very much for answering all my questions! I have one final question. I just created a BOM for the TIDA-00334 on digikey and I have run into a problem with a missing TI component. On the BQ500212AEVM-550 there are two CSD97376CQ4M NexFET Power stages. On the TIDA-00334, I am guessing there is not enough room, so seperate components were used. I am trying to find CSD25302Q2, which is a MOSFET, P-CH, -20V, -5A, SON 2x2mm. It seems like it has been discontinued, I cannot find it on the TI website, Digikey, Mouser. I even tried to find substitute parts with the same footprint, but no luck. Could you help me with find a replacement part or lead me in the right direction?

    You have been immensely helpful.

    Thanks,
    Jason

  • 0 in reply to Jason Luzinski
    TI__Guru 65736 points

    Hello Jason

    You have a couple of options for the power section on the TIDA-00334.

    1.)   Low cost logic drive used on the TIDA-00334---replacement for CSD25302Q2 is the CSD25310Q2 (same footprint) and Mouser has them in stock.  I will update Sch and BOM in the next few weeks to show this and fix other bom issues.

    2.)   Small integrated solution using CSD97376CQ4M---Design used on the bq500212A EVM, it can be used here also.

    3.)   MOSFET Drive solution using TPS28225—Design used on the bq500211 EVM.  This driver has worked very well for us with no problem.  Any number of external N-type mosfets can be used but the lower the RDSon the better.

    Also I have other small TX designs in work.

  • 0 in reply to Bill Johns
    Prodigy 100 points
    Bill,

    Thanks for the information. I am looking forward to testing this Wearable Tx. I have a few more discrepancies with the TIDA-0334 and I should be ready to go.

    1. R24: Sch - 48.7 Ohms, BOM - 0 Ohms
    2. R33: Sch - 243 Ohms, BOM - 475k Ohms
    3. R70,R71,R72,R73: Sch - 0 Ohms, BOM - 4.7 Ohm
    4. R99: Sch - 86.6k Ohms, BOM - 0 Ohms

    I greatly appreciate the help.

    Best,
    Jason
  • 0 in reply to Jason Luzinski
    TI__Guru 65736 points
    1. R24: Sch - 48.7 Ohms, BOM - 0 Ohms----Should be 48.7k ohms but recommend open
    2. R33: Sch - 243 Ohms, BOM - 475k Ohms----Should be 243ohms
    3. R70,R71,R72,R73: Sch - 0 Ohms, BOM - 4.7 Ohm---Should be 0 ohms
    4. R99: Sch - 86.6k Ohms, BOM - 0 Ohms---Should be 86.6k ohms but recommend open

    R24 & R99 are FOD fault protection, with R24 open function is disabled. Value for R24 and R99 are determined during FOD calibration. This function is probably not needed on a small coil low power design.
    R27 is PMOD fault protection, with R27 open function is disabled. Above applies to this function also.
  • 0 in reply to Bill Johns
    Guru 21675 points
    Hi Dick, Bill,

    My customer also want to use bq51050B with less than 500mA charge current setting. They hope to set 200mA or 300mA. I understand you can't guarantee to set less than 500mA and the accuracy of current regulation should be degrade. And also the termination issue may happened you commented in this thread.

    What do you think other risk in addition them?

    Best Regards,
    Sonoki / Japan Disty
  • 0 in reply to S.Satoshi
    TI__Genius 14890 points

    Sonoki-san,

    The charge current will regulate well at 200mA to 300mA.  The termination current has a wide distribution at low currents.  If the user sets the termination current to 20mA, there may be a wide distribution of actual termination.  For a small sample size, the results may be very good, but as the number of devices increases, there will be an increased likelihood that there will be a wide spread in termination current.  We do not spec the termination at that level, but I would expect actual termination currents to possibly range from 10mA to 30mA.

    Let me know if you need further clarification.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Guru 21675 points

    Hi Dick,

    Thank you for your quick response. I understand we need to care about distribution of termination current. And also I understand that the possibly range of termination current will be from 10mA to 30mA, then can you please let me know the guaranteed minimum termination current? Is it 120mA as attached?

    Best Regards,

    Sonoki

  • 0 in reply to S.Satoshi
    TI__Genius 14890 points

    Sonoki-san,

    The bq51050B will terminate, the concern at lower currents is only the accuracy of that termination current.

    Regards,

    Dick

  • 0 in reply to Dick Stacey
    Guru 21675 points
    Hi Dick,

    Let me change my question. Do you have specified accuracy of termination current at any Iterm setting? In other words, I'd like to know your recommended minimum termination current that bq51050B keep accuracy.

    Best Regards,
    Sonoki