BQ24640: Sense Resistor to the input

Dear Simon,

I have several questions/comments:

1. Could you please provide some more information about your input current limit, what your input voltage is, and what your charging voltage is?  Bear in mind the I = C dV/dt calculation. For example, if you charge your 50 F capacitor combination to 2.5 V in 3 seconds, assuming you don't taper charge, you would need 41.67 A.

2. This charger is capable of charging at 10 A. What is the output current when you increase the limit?

3. The sense resistor is to be used in conjunction with the SRN and SRP pins. I would not recommend configuring the sense resistor with the input. As this is a buck converter, the input current to the buck is discontinuous even though the charge current is continuous.

Thanks,

Mike Emanuel

Please click "This resolved my issue" button if this post answers your question.

Dear Mike,

thanks the fast Response.

To 1) The Input Comes from an ACDC converter with 12V and 15W. So if i don´t limit the input current to 1,25A, the overload function of the power supply could shutdown itself. My charging voltage is 10,5V for the 5 series 10F SuperCaps.

In this Picture you can see. Violet (Vin=12V), Blue (Vout), Yellow (I_in), Green (I_out)

To 2) So the output current is limited by ISET (100k/6,8k) to 1,05A and the sense resistor is at output (original) to limit output current. As you can see the Input current at the end of charging comes close to the limit to the ACDC converter. So when I changed the ISET to 100k/10k, I exceed my limit, but don´t used the power at the beginning of the charging. My idea was that i could reach 10,5V Vout faster to have a Input current limited charging. In this case I wanted to use the sense resistor at the Input. My concern is that at the beginning of SuperCap charging the inrush current might exceed 6A at the output.

Do you have a idea what i could do to reach my requirements to Change the Charger circuit or how to limit the input current in a propar way? Then I could Limit the output current to 6A as intended with the solution of the sense resistor at the output.

Best regards

Simon

Simon,

If you wish to change the charge current as the supercap voltage changes, I would recommend using a micro-controller in tandem with several FETs and sense resistors to adjust the charge current on the fly. Changing the equivalent sense resistor using equation 2 in section 7.3.2 will change the charge current. Note however, that the charge current will need to be scaled back as the supercaps reaches full voltage in order to prevent reaching the input current limit of the AC/DC converter.

Thanks,

Mike Emanuel

Please click "This resolved my issue" button if this post answers your question.

Hi Mike,

so I will have the sense resistor of BQ24640 at the output current. What are the FETs supposed to do?

My idea was to sense the current and a micro-controller sets the current limit at ISET with a voltage calculated from equation 2.

Is there a additional circuit recommended from your side?

Best regards

Simon

Dear Simon,

There are actually two ways to perform this task. One is mentioned above as changing the equivalent sense resistor. However, as this involves changing the high current path, I would recommend a second option. The second option is to use a microcontroller to change the VISET voltage.

To make this work, you would need to establish a baseline charging current you want. For example, you could program the VISET divider to be 2 A. At this current, you could charge to approximately 7.5 V before drawing too much from your adapter. Around this point, you would have a second resistor to ground from ISET turn on through a FET. To do this, you would need a way to sense the battery voltage is 7.5 V and have a way to turn on the FET to the second resistor. You would then turn on this resistor to add it to the network, and lower the charge current to 1 A. You would be able to charge all the way up to 10 V with this current without overloading the adapter. This works because adding a second lower resistor in the divider will lower the equivalent total lower resistance in the divider, lowering the VISET voltage. 

Thanks,

Mike Emanuel

Please click "This resolved my issue" button if this post answers your question.

Hi Mike,

thank you. I understand. This makes sense. I will build this up and perform more Tests.

I have another task:

To increase the lifetime of the SuperCaps i want to implement with the BQ24640 a variable charged voltage for the SuperCaps. So if the full energy of the SuperCaps is not needed, the voltage should be reduced.

For a variable set voltage the idea was also to use a microcontroller for setting different voltages at FB Pin. The first option i see, is to built a circuit like that:

But in this case I could only set discrete values.

The second option I see, is to put 2,1V at the pin in case the value is reached which should be set. Is it necessary in this case to put the voltage according the formula which I set up for a proper regulation?

Best regards,

Simon

Dear Simon,

Could you please repost with the schematics? For some reason they did not upload correctly.

However, I would not suggest trying to change the FB pin voltage. This voltage is regulated. Changing the resistor divider would change the output voltage according to equation Section 3.7.1.

Thanks,

Mike Emanuel

Please click "This resolved my issue" button if this post answers your question.

Hi Mike,

so here is the schematic. where the voltage divider could be changed by turning on the MOSFETs via microcontroller. With two MOSFETs I see the possibilty to have 4 different voltage levels, if necessary.

To come back to the topic with the ISET: We want to set two different values for current limiting. Maybe i need to use also analog output from the microcontroller, if there are not enough digital Outputs left. In this case the current is sensed in another path so it could be possible to set an analog voltage value at ISET. Or do you also suggest to put a MOSFET for changing the voltage divider at ISET for some reason?

Best regards,

Simon