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Tool/software:
Hi,
We use the BQ24780S for different configuration of use.
The first configuration is using the BQ24780S with a battery.
The second configuration is using the BQ24780S without a battery.
The issue we've is the drop voltage for the cold start without a battery, the ACDET goes below 2.4V and the BQ24780S opens the ACFET.
The system is unable to power on because the drop voltage reappears each 1.3s
I'm wondering if there is a way for the BQ24780S to accept a level below 2.4V without opening the ACFET ?
Thanks
Nicolas
Of course we've already reduce the voltage threshold by changing the resistor divider for the ACDET, but there is the maximum battery voltage limit.
The datasheet says : An external resistor divider attenuates the adapter voltage before it goes to ACDET. The adapter detect
threshold should typically be programmed to a value greater than the maximum battery voltage
Why does the datasheet use the term "should" ?
What does it mean exactly "greater" ?
What is the minimum voltage margin between the adapter detect threshold and the maximum battery voltage ?
Thanks a lot
Hello Nicolas,
Why does the datasheet use the term "should" ?
What does it mean exactly "greater" ?
What is the minimum voltage margin between the adapter detect threshold and the maximum battery voltage ?
This is a buck charger, the adapter needs to be higher than the battery for the device operate correctly. We recommend 1.5V-2V higher than the maximum battery voltage limit.
The issue we've is the drop voltage for the cold start without a battery, the ACDET goes below 2.4V and the BQ24780S opens the ACFET.
The system is unable to power on because the drop voltage reappears each 1.3s
I'm wondering if there is a way for the BQ24780S to accept a level below 2.4V without opening the ACFET ?
No, the ACDET has to be higher thn 2.4V, why is the voltage dropping below 2.4V?
Best Regards,
Christian.
Hi Christian,
Thanks for the information.
The voltage drops because the AC/DC has difficulty giving the current peak during the cold start, this is only when the battery is removed (= cold start without battery).
In reality, by shorting the BQ24780 it is possible to start the system, it's just that the BQ24780 opens the ACFET because of the voltage drop.
We tried also to reduce the commutation speed of the ACFET to reduce the voltage drop (increase of 4K resistors or Cgs/Cgd), but the mosfet didn't survive.
Knowing that our AC/DC outputs a fixed voltage of 24V, why not allow a voltage drop going lower than the maximum battery voltage ?
Finally we know that nominally the voltage is 24V and that the drop only lasts a very short time. We know also that this voltage drop only appears when the battery is not present.
Thanks again
Hello Nicolas,
Knowing that our AC/DC outputs a fixed voltage of 24V, why not allow a voltage drop going lower than the maximum battery voltage ?
If the battery is not present when the voltage drop occurs, I see no issue for allowing this voltage drop. However the ACDET pin has to be 2.4V or higher, this is not something we can change.
If the ACDET is set below the maximum battery voltage, but the adapter voltage does not go below this vlaue when battery is present, I do not see any obvious issue.
Best Regards,
Christian.