I will design PCB for 20A charge current. I have some questions.
1) There is no enough information about Inductor, Capacitor calculate in LTC4015 datasheet. Is there a other reference document?
2) M3A and M3B MOSFETs have 12A current capacity. I would like to connect parallel MOSFET for high current. Is it possible this condition?
In addition, Vsense voltage is very low (32 mV). For 20A charge current, resistance value is 16 mOhm. Low resistance, it is hard to find. Why 32 mV?
I am waiting for your answer. Thanks. Have a nice day.
1. You can just use general buck regulator equations. There are many resources for this online. That said, there is detailed information about the inductor and capacitor selection in the LTC4015 datasheet. See page 43.
2. Yes, but you may have trouble if the combined gate capacitance of the switching FETs is very high. My suggestion is that you try it first and let me know if you run into any trouble.
The lower the sense resistor, the less resistive loss you will get when charging. 16mOhm resistors are not hard to find. Here is a link to a Digi-Key parametric search showing 44 entries of 16mOhm sense resistors currently in stock.
For this high-current design, I advise that you use at least 4 PCB layers, keeping layer 2 as a GND plane. Copy the demo board layout as much as possible, reference the Layout Considerations section in the LTC4015 datasheet, prioritize the proximity and routing of the bypass cap on 2P5VCC, and reference this document for the switcher component layout.
Thanks for your answer.
I will use SIR422 MOSFET (2 parallel for M3B, 2 parallel for M3A). This FET input capacitance is 1785 pF, gate charge is 48nC. What do you think?
Also, for 20A charging current, battery sense resistance must be 0,0016 mOhm. 16 mOhm resistance only apply to 2A charging current. This value is very low. I looked for a 1,6 mOhm resistance but I did not find. Therefore, I will use 1,5 mOhm sense resistor. In this condition, charging current will be 21,3A.
For this high-current design, I will consider AN136.
Thanks. Have a nice day.
That's pretty high gate charge. You may want to consider operating at a lower switching frequency for this application. Also, consider driving the DRVCC pin from your own supply rather than INTVCC.
Ah, yes 1.6 mOhms has many less options. Another option is to use two parallel 3.2 mOhm sense resistors and there seem to be a couple of those in stock. Might be more options on other sites.
I changed M3A and M3B FETs. I will use SQJ848EP-T1_GE3 FET (2 parallel for M3B, 2 parallel for M3A). And I will give external 5VDC to DRVCC. Are these FETs usable?
In addition, I have some questions.
1) If I connect 1 mOhm resistor to RSNSI (In this condition, input current is 32A), Can I change as 20A input current with I2C communication?
2) For 25,2V battery voltage, What should be minimum input voltage?
Also, I changed switching frequency as 250 KHz