LTC4065L connection to active circuit

We're curious on how the LTC4065L performs in the following situation, and what the recommended design is.

There's a battery connected to LTC4065L, a 5V input to the LTC4065 and a circuit connected to the battery as well. See the attached diagram.

If I have understood this correctly, during charging, some of the charge current will go to BAT1 and some to U2. However, LTC4065L will have no way to determine this, and if U2 constantly draws a current >1.9mA (1/10 of the charge current), LTC4065L will never detect a fully charged battery. Further, the expected time to charge BAT1 will depend on how much of the programmed current U2 draws. Is this correct?

Further question; how would one design a circuit with the LTC4065L where the battery is in integrated into the product and therefore cannot be disconnected during charging? It is important that U2 receives no power dips as this can cause a restart (I figure some ripple that can be suppressed with capacitors is OK). Any pointers or reference designs would be much appreciated!

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  • Hi WATaylor,

    Once again thank you for your answer.

    I realise the battery will eventually get charged, but I'm more curious on how it affects the CHRG output. We read the CHRG output and use this to drive a LED to indicate battery status to the user. Since the charging "comes and goes" when fully charged, we 'latch' CHRG the first time it goes high impedance (i.e. fully charged). We then keep displaying a fully charged battery, even if CHRG goes low (i.e. re-charge). We also monitor Vcc to detect when a charger voltage is applied. When Vcc is removed we indicate discharging. When Vcc is once again connected and CHRG is low, we indicate charging, and so forth.

    My thinking was that if the battery is not fully charged within the first timer cycle (lets assume it takes 6 hours to fully charge the battery), the LTC4065 will indicate that it is fully charged, while in reality its only 50%. Correct?

    Our design spec is currently the following, but it there is room for some tweaking if necessary.

    • Input voltage (to charging circuit) range: 4.5-6V

    • Maximum permissible charging-circuit input current: 150mA

    • Acceptable load-output voltage range: 3.3±0.03V (processor and sensors), 5±0.5V (LED - not included in the original question)

    • Range of load current: 20-45mA (active), <500µa (sleep)

Reply
  • Hi WATaylor,

    Once again thank you for your answer.

    I realise the battery will eventually get charged, but I'm more curious on how it affects the CHRG output. We read the CHRG output and use this to drive a LED to indicate battery status to the user. Since the charging "comes and goes" when fully charged, we 'latch' CHRG the first time it goes high impedance (i.e. fully charged). We then keep displaying a fully charged battery, even if CHRG goes low (i.e. re-charge). We also monitor Vcc to detect when a charger voltage is applied. When Vcc is removed we indicate discharging. When Vcc is once again connected and CHRG is low, we indicate charging, and so forth.

    My thinking was that if the battery is not fully charged within the first timer cycle (lets assume it takes 6 hours to fully charge the battery), the LTC4065 will indicate that it is fully charged, while in reality its only 50%. Correct?

    Our design spec is currently the following, but it there is room for some tweaking if necessary.

    • Input voltage (to charging circuit) range: 4.5-6V

    • Maximum permissible charging-circuit input current: 150mA

    • Acceptable load-output voltage range: 3.3±0.03V (processor and sensors), 5±0.5V (LED - not included in the original question)

    • Range of load current: 20-45mA (active), <500µa (sleep)

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