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BQ24092: TTDM MODE (ONLY POWER THE SYSTEM)

Bhargav Jyoti Saikia
Intellectual 420 points
Part Number: BQ24092
Other Parts Discussed in Thread: BQ24074

Hello,

In the datasheet of BQ24092, in page 19 it is mentioned that 

"This IC can be used to charge the battery plus power the system, charge just the battery or just power the system (TTDM) assuming the loads do not exceed the available current. If the system load ever exceeds the output programmed current threshold, the output will be discharged unless there is sufficient capacitance or a charged battery present to supplement the excessive load."

Can you please explain how the BQ24092 can be used to power only the system? And suppose I have set ISET to 800mA and made ISET2 = ISET. So, that makes input current to be a maximum of 800mA. Now, if the battery was charging at 800mA and suddenly the system requires 1A, what will be the scenario? Will the battery stop charging and supplement the extra 200mA automatically? Please help in this matter.

Also, can you suggest another part number, in case BQ24092 doesn't handle such situations automatically?

  • 0
    TI__Guru 69731 points

    The bq24092 is a non power path device and the system load is typically connected directly to the battery.  If they system is operating during charging the battery may not complete charge correctly.  Additional system load prevents the charger from seeing the low output current to set charge complete and safety timer will set a fault.  The safety timer can be turned off on the bq24092 using the TTDM function.  This would be a decision based on the system configuration and how charging / use is planned.  

    A power path device has separate battery and system output with management of current between the two paths, see bq2407X family of devices.

    A couple of application notes are available see below:

    Power-Path Li-Ion Charger Selection http://www.ti.com/lit/ug/sluu332/sluu332.pdf

    Implementations of Battery Charger and Power-Path www.ti.com/.../slua376.pdf

  • 0 in reply to Bill Johns
    TI__Guru 69731 points

    Regarding your questions:

    Can you please explain how the BQ24092 can be used to power only the system?---If the battery and system are in parallel, typical configuration current from charger will go to both cannot only power the system.  If the battery is discharged the output voltage will be low and system may not operate.

    And suppose I have set ISET to 800mA and made ISET2 = ISET. So, that makes input current to be a maximum of 800mA. Now, if the battery was charging at 800mA and suddenly the system requires 1A, what will be the scenario?---The input current is limited at 800mA and the system needs more than this it would come from the battery in parallel with the system

    Will the battery stop charging and supplement the extra 200mA automatically? --- Yes

    Also, can you suggest another part number, in case BQ24092 doesn't handle such situations automatically?---The bq2407X

  • 0 in reply to Bill Johns
    Intellectual 420 points
    Thanks Bill. That helped a lot. I think I will switch to BQ24074.