• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 64 subscribers
  • Views 1038 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

bq25898 - Can Boost converter be used to power System?

Fred Arnold
Intellectual 680 points
Other Parts Discussed in Thread: BQ25898, BQ24295, BQ25895, BQ25898C

Hi,

We need a tiny 1-cell charger with 6V to 12V input range which can charge a Li-Ion battery with about 500mA while powering the System with about 3.4V and 3A.

The bq25898 seems to be a good choice, while the bg25898C has a higher efficiency at lower charging currents but lacks a boost converter.

The charger is together with the battery in the system and cannot be separated by the user. It is also never connected to a USB port and uses a 6-12V high power supply which also powers other circuits which do not need battery backup.

The datasheet of the bq25898 is not very clear, but it says the charger will keep the VSYS Voltage at the programmed level of e.g. 3.4V, even if the battery drops to 3V.
Looking at the block diagram of the IC, I assume this is only the case if an external power supply is connected to the charger. Without external power, VSYS will drop to the actual battery voltage of e.g. 3V, bringing it below the required System voltage.

Question:

Can the boost regulator be used to step-up the battery voltage when no external power is present to provide 3.4 to 5.5V for the System, e.g. via the PMID pins?
It should not output the boost voltage on VBUS as other circuits are connected to this pin which would unnecessarily drain the battery.
In other words, can the boost functionality be enabled while RBFET (Q1) stays off?

Thanks,

Fred

  • 0
    TI__Guru**** 166626 points
    You are correct in that SYS is maintained to MINSYS of ~3.4V only when external power is supplied. When no power is supplied, VSYS ~= VBAT. In OTG boost mode, the 898 provides the boosted voltage back to VBUS, in other words, the RBFET is on. If you want a device that provides the 5V OTG boosted output on PMID with RBFET off, I suggest bq24295 or bq25895. Keep in mind that if the VBUS pin detects an external voltage above the UVLO threshold, the boost converter automatically shuts down.
  • 0 in reply to Jeff F
    Intellectual 680 points

    Hi Jeff,

    Thanks for the confirmation.

    Ignoring the boost mode, there are a few specs in the 898C datasheet which appear to be a copy and paste issue.

    1. The efficiency at low charge currents stated on page 1 is supposed to be better on the 898C than on the other 898x variants. However, the efficiency graph (Figure 1) is identical to the graph of the other 898/D variant.

    2. The battery discharge current in the 898C datasheet states "High-Z Mode, No VBUS, BATFET Disabled (REG09[5] = 1), Battery Monitor Disabled typ 12uA" but this device does not use Reg 09[5] - meaning it would be around 32uA (~3-times higher).

    Questions:

    a) Is there an efficiency graph of the 898C available or which 898 variant is more efficient at a charging current of ~500mA?

    b) What is the discharge current of the 898C with no VBUS and no load attached to VSYS? Do the 898/D variants, which can disable BATFET Q4, have a lower battery discharge current?

    Thanks again,

    Fred

  • 0 in reply to Fred Arnold
    TI__Guru 57651 points

    The efficiency for bq25898C and bq25898/D are very similar all through the charging current range up to 3A.

    The bq25898/D can disable Q4. When Q4 is disabled, Ibat is only 12uA typically. When Q4 is enabled, Ibat is 32uA typically.

    Thanks!