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External passive balancing controlling

Category: Hardware
Product Number: LTC6811-2

Hi,

I have following questions about the PWM controls of MOSFET for external passive balancing.

1. What is PWM frequency of the S pin for cell discharging?

2. In datasheet its say " Each PWM signal operates on a 30 second period" it's means PWM frequency is 1/30 ?

3.What is voltage at S pin on?

4.When 2 or more cell unbalance than all S pin controlled by DCC simultaneously or one by one for external passive balancing ?

5. What is current limit of S pin? I want to calculate the s pin resistance. or I just put 3.3K ohm?

Thank you in advance of the help.

  • Thank you for your inquiry. I am looking into this and will get back to you as soon as possible.

  • Question 1 & 2: It seems you've answered your own question here. According to the datasheet (page 38), the PWM duty cycle for S pin passive balancing control is configurable (anywhere from continuously on to continously off with increments of 2 seconds, see Figure 17). The frequency is as you've stated 1/30 (0.033Hz). This is assuming you are only talking about controlling the S pins using the DCC register group.

    Question 4: Since all of the DCC bits are part of the config register group, it seems to me that all the S pins could theoretically be controlled (opened or closed in any combination) at the same time.

    Question 5: Page 71 states that balancing currents should not exceed 60mA due to excessive heating above that level. The balancing current goes through the S pin if you're using the internal MOSFETs so I'd probably consider that the maximum limit.

    I do not understand what is meant by your question 3 and have thus skipped it.
    Note I am not an employee of Analog Devices, so do not take these answers as 100% truth, however I am reasonably confident in the answers I've provided.

  • 1. What is PWM frequency of the S pin for cell discharging?

    2. In datasheet its say " Each PWM signal operates on a 30 second period" it's means PWM frequency is 1/30 ?

    Yes, you are right. The frequency of the PWM on S Pins for External Cell Balancing by configuring the DCC bits in the device is 0.033 (=1/30) Hz. You can find more info about it in the Table.17 of the Datasheet(Page No.38).

    What is voltage at S pin on?

    When S-Pin is configured for External Cell Balancing, an internal PMOS pulls the S-Pins to VREG so the output voltage on S-Pin will be within 0 to VREG(i.e, 5V). You can find more info in Cell Balancing with External Transistors section of the Datasheet (Page No. 72)

    4.When 2 or more cell unbalance than all S pin controlled by DCC simultaneously or one by one for external passive balancing ?

    Yes, all S-Pins can be controlled simultaneously. I guess you have a doubt regarding the switching internal mentioned in the datasheet page no.39, it refers to only the switching interval i.e, the turn-on or turn-off transition on the S-Pins or the High to Low edge or Low to High edge switching transition on the S-Pins being timed one after another and not all switch on or off simultaneously. 

    5. What is current limit of S pin? I want to calculate the s pin resistance. or I just put 3.3K ohm?

    For external balancing, we would suggest you refer to the application information on Page No. 72 for choosing the right discharge resistor.

  • Thank you for your reply.

    I want to calculate the gate resistor value for connecting the S pin and the external MOSFET gate terminal.

    In the datasheet, it shows 3.3K is used.

    I am using RRH050P03 P channel Mosfet as external balancing.

  • In the datasheet, it shows 3.3K is used.

    You can use the same in your case as well. That gate resistor is to reduce the noise and dampen/prevent ringing on the FET input.