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LT8490 24V design not working

Category: Hardware
Product Number: LT8490

Hello,

I have designed the solar charging circuit with LT8490 for a 24V 7A lead acid battery. However, it is not charging the battery. The fault LED is off, and the status LED blinks once at 3.5 Seconds.

I have provided the input from a voltage source with 36 V. I have also tried with a solar panels.

Can you help me with this issue? I have attached my schematic for review.


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updated to new Ask a Question form
[edited by: GenevaCooper at 12:52 PM (GMT -4) on 10 Aug 2022]
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  • The schematic looks OK from a functional viewpoint. There are a few things like no cap across CSP/CSN but nothing that would cause it not to function. What do you have for a load? The Output battery voltage needs to be above 35% of the VS2 setting before the LT8490 will charge. See table 3 on page 17 of the datasheet.

    If you want to use a bench power supply on the input, VINR needs to be tied to GND before applying voltage to disable the MPPT function.

  • Thanks for your reply, I will connect the motor control board at the load (24V 5A).

    Currently, my battery voltage is 23V, and I have set the VS2 at 29.6V. 

    Is it necessary to disable MPPT?

    I have observed that at SWEN, the voltage is 0.7V.

    Do you need any additional details?

    Thanks a lot for your valuable time.

  • To troubleshoot the problem, it is best to simplify the function as much as possible. Using a bench power supply to power the board and disabling MPPT is a simplification that helps in troubleshooting. Once you have the LT8490 functioning properly using a power supply then connect the solar panels and enable MPPT.

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  • To troubleshoot the problem, it is best to simplify the function as much as possible. Using a bench power supply to power the board and disabling MPPT is a simplification that helps in troubleshooting. Once you have the LT8490 functioning properly using a power supply then connect the solar panels and enable MPPT.

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  • I have disable the MPPT and tested with external power supply. However still it is not charging the battery. 

    I have observed that at gate of M2 there is 6.3V, however at gate of M1, M3, M4 there is 0V. 

  • Check INTVCC, LDO33 to make sure they are at the correct voltage. Check SHDN to ensure it is logic high. Check CLKOUT to ensure it is oscillating at ~200KHz if you are using 215K ohm on RT. Check voltage at Vin pin. If these are OK, proceed with probing each pin of the IC verifying the voltages are what they should be. Focus on the inputs. Use a scope when probing. The load voltage needs to be above 35% of the output voltage (VS2) setting. If you are using a simulated battery ensure the power resistor is large to minimize current to start and place a power supply in parallel with the resistor and set voltage to somewhere between 35% of VS2 setting and VS2 setting.

    You may also find it beneficial to get a DC2069A demoboard and compare pin to pin between the demoboard and your board to pin point the problem. 

    From your previous responses, the load is correct but verify FBOUT IC pin voltage is correct.

    It is important to verify all inputs at the LT8490 pins which will identify incorrect resistor values, solder issues, etc.

    Be sure to ground the scope probe as local as possible to the LT8490.

  • I have tested the IC pin voltages. Is there any issue with the Clock or voltage divider network? Please find the attachment for the test result.

    PDF

  • IMONOUT should never go to 3V. A simulated load as I described earlier would be best until you find the issue but if that isn't feasible then putting in a series resistor with the load battery to limit the current will help. The LT8490 is likely turning on, output current goes too high hitting the shutdown threshold on IMONOUT and then shutting down. The issue could be instability due to incorrect Vc components or due to a bad layout. It could also be something incorrect with the MOSFETs. It could also be a filtering issue with CSPOUT/CSNOUT or IMONOUT. Is the CSPOUT/CSNOUT sense resistor the correct value and soldered properly? Is the inductor the correct value (15uH) and not shorted?

  • Thanks again for your valuable time. Found One issue with a resistor value in my schematic R35 was 10K, which is supposed to be 10 ohms. I have replaced the resistor and tested the board again. Please find the attachment for the updated result.

    Now IMONOUT is around 500mV. 

    I have also attached the load with the battery.

    The resistance between the inductor terminal is 0.9 ohm. 

    I think there are still some issues at MOSFET. 

    PDF

  • SWEN should not be toggling and it appears from your spreadsheet that it is toggling. Look at your timer settings and Stage 0 threshold as well as Stage 0 and Stage 2 voltage thresholds. Ensure the input voltage stays well above the minimum operating voltage. If the SS pin never goes above a few millivolts then the LT8490 is continuously resetting. If you are seeing noise on the CLKOUT but you do see ~200KHz clock then there may be a layout issue assuming you are using a very local GND on your scope probe. The layer below the IC should be a GND plane.

  • Input Settings: I have set the maximum input current as 5 A with a 10mohm sense resistor. Vx1 is 6V, and Vx2 is 53V.

    Output Settings: Iout Max is 2.5 A, Iout(MaxS0) is 0.625 A; I have used a 20mohm sense resistor. VS2 is 29 V. 

    Do you think there is any issue with this? Do I need to increase the current limit?

    I am using a 24V 7Ah lead acid battery. 

    I have tested the MOSFET, they are working. 

    I am not sure about the battery's internal thermister, Is IC restarting because of the temp sense pin? Is R28 10K resistor creating an issue?

    I don't see any timer expiration on fault led. IC blinks the status LED once at 3.5S, and the fault LED is OFF.

    May I share my PCB layouts with you?

    Thanks a lot for your valuable time and efforts.

  • Rsense2 is too large for 7A. Target a voltage drop across the sense resistor of 0.0497V at max desired current. At 7A, a 7 milliohm resistor would be appropriate. With this value of sense resistor R_IMONOUT = 274K and RIOW = 26.7K.

    Until you get it working properly, disable the temperature sensor.

  • I have changed the Sense resistor and R_IMONOUT; still, SWEN is varying between 0.6 to 3V, the status LED flashes every 3.5 sec, and the fault LED is off.

    Is there anything else that I can check for the reason behind restarting IC?

  • I recommend using a power supply on the input and a resistive load on the output until you find the issue. Tie the VINR pin to GND and apply a 36V power supply that is not current limited. On the output, use a power resistor with a resistance that provides approximately 2A of current. For a 24V output this would be a resistance of approximately 12 ohms. Connect a power supply across the power resistor and set the voltage to approximately 20V. You can now disable the current limits by tying IMON_IN and IMON_OUT to GND. To disable the temp sense have a 11.5k pull up on the TEMPSENSE pin to LDO33 and a pulldown on the TEMPSENSE pin of 10K. Ensure a 100nF cap is in parallel with the pulldown resistor. Ensure no other components are on the TEMPSENSE pin. The LT8490 should now regulate to the set output voltage. If it does not, look at each pin of the LT8490 with a scope focusing on the inputs to find the problem. Once you get it operating properly in this mode, enable the features one at a time until you get back to the application circuit desired.