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LT3562HV CHRG Pin Diode and Resistor to Vfb

Hi,

On the development board DC1688A for the LT3652 charger there is a diode D8 and a 1M resistor R12 between the Vfb potential divider and the CHRG pin. This is the same as on the very last example on the LT3652 datasheet.


What is the purpose of these? I cannot see any notes in the datasheet for the purpose of this part of the circuit?

Many thanks

Alex



added image from datasheet
[edited by: alexc63 at 10:44 AM (GMT -4) on 24 Oct 2018]
  • Hi Alex,

    Those exist to create a wider range between the charge voltage and the recharge threshold.

    When charging, the /CHRG pin is pulled low, so the added current through this path sets the charge voltage. When not charging, /CHRG is high-Z, so this path is off meaning that the voltage at the FB node will be higher than it is for the same battery voltage as a result of an effectively higher bottom resistance in the FB voltage divider. So, the charge voltage is set lower. Since the recharge threshold is a ratio of the charge voltage (97.5% of the recharge voltage), that threshold is moved as well.

    So, this allows you to effectively lower the recharge threshold by changing the charge voltage depending on whether the part is charging or not.

    The diode exists to prevent a pull-up resistor or LED on /CHRG from interfering with this function. Note that if you do not have a pull-up resistor or LED on /CHRG, you do not need the diode.

    Regards,

    Zack

  • Thanks Zack, that makes perfect sense! 

    Is the advantage of this to allow the battery to discharge more before the recharge restarts?

    Thanks

    Alex

  • Hi Alex,

    Ok, great! Yes, that's exactly it. Depending on your battery and application, you may or may not want to do this. One benefit is that there will be less charge cycles if you do, which may extend the life. However, it will decrease the minimum run-time of your system if your battery is allowed to fall to a lower voltage without being recharged. So, there is a tradeoff there.

    Regards,

    Zack

  • Hi Zack, I wanna ask something too since it's related to this schematic. What's the purpose of D7? Is this for transient protection? Please expound. THanks!