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BQ25504: Connecting BQ25504 to an existing MCU Board

Eric Bieri
Prodigy 160 points
Part Number: BQ25504
Other Parts Discussed in Thread: BQ25570, BQ25505

Hi,

I'like to design an energy harvesting shield for the adafruit feather boards (e.g. Feather M0 WIFI Board, www.adafruit.com/.../3010).
The schematics of the M0 WIFI Feather Board can be found here learn.adafruit.com/.../downloads.


I have the following questions regarding the BQ25504 energy harvesting IC:

1) What is the current consumption of the charger if the VBatt_UV < VBatt < VBatt_OV and VIN at the solar cell is 0V (e.g. at night time)?

2) Is there a way to completely shut down the IC at night time (Similar to the BQ25570 shipping mode)?

3) What is the best way to connect the BQ25504 with the Feather M0 WiFi Board?

Option 1:

Feather.VBatt -> BQ25504.VSTOR

Lipo Battery connected to BQ25504.VBATT

Advantages:
- Battery charge protection feature protects battery from discharging below UV limit / charging above OV limit

Disadvantages
- The Feather LDO draws up to 500mA, which results in 1V (2 Ohm * 0.5A) voltage drop on the PFET -> too high!

Option 2:
Feather.VBatt -> BQ25504.VBatt (Feather directly connected to LiPo-Battery)
Lipo Battery connected to BQ25504.VBATT

Advantages:
- There's no significant voltage drop if the Feather LDO draws 0.5A.

Disadvantages:
- The battery charge protection feature cannot protect the battery.

-> The VBAT_OK signal could be used to pull the Feathers EN signal low, if the battery voltages drops below a certain level.

Are my assumptions above correct?
Do you see better options to connect the two?

4) The Feather board also has a LiPo charger IC on board. Could charging the LiPo battery via Feather USB damage the BQ25504?

5) The datasheet states a peak current flowing from VIN into VIN_DC input of 200mA (0.5V < VIN < 3 V; VSTOR = 4.2 V). If I connect a power supply (3 Volt)
via 10 ohm resistor to the VIN, I only get a charge current of about 100mA. Is this correct?

6) Is there a way to increase the charge current to 200mA?

  • 0
    TI__Guru**** 165886 points

    Hi Eric,

    Regarding 1,

    Regarding 2, unfortunately, no.  you might consider the bq25505 which has enable.

    Regarding 3, You have the two options.  Your battery likely has internal protection FET that provides UV protection.  You can add external Schottky diode (or PFET controlled by NFET driven by VBATOK) from battery to VSTOR in parallel with internal PFET to handle the high current.  Therefore option 1 might be best.  BQ25505 might be a better fit if you add the external PFET as it already provides inverted VBAT_OK.

    Regarding 4, having another battery charger connected to same battery in parallel with BQ25504 is ok.

    Regarding 5, yes.  The boost converter runs in DCM with 200mA peak current.  Therefore, the highest charge current at D=0.5 is 1/2 of 200mA or 100mA. 

    Regarding 6, unfortunately no, unless you want to put two BQ25504 in parallel.

  • 0 in reply to Jeff F
    Prodigy 160 points
    Hi Jeff,
    Thanks for your quick response...

    1) Unfortunately the picture you attached does not display. Could you please fix this or send me the link to the picture if available online
    3) Thanks for the idea with the PFET and NFETs. This will certainly work too, but if I have to use the VBAT_OK signal, then I go for Option 2. disabling EN with VBAT_OK will cut the power to the 3.3V supply (AP2112K-3V3) and therefore switch all consumers off.
  • 0 in reply to Eric Bieri
    TI__Guru**** 165886 points
    Eric,

    Regarding 1, see I(VSTOR) spec in the middle of datasheet page 6.
  • 0 in reply to Jeff F
    Prodigy 160 points
    Hi Jeff,

    I saw that, but the datasheet only defines the following two cases:
    - VSTOR Quiescent current Charger Shutdown in UV (VIN_DC = 0V;VBAT < VBAT_UV = 2.4V; 330 750 nA Condition;VSTOR = 2.2V, No load on VBAT) -> 330nA
    - VSTOR Quiescent current Charger Shutdown in OV (VIN_DC = 0V; VBAT > VBAT_OV, VSTOR = 4.25; No load on VBAT) -> 570nA

    But what if the VBAT_UV < VBAT < VBAT_OV and VIN_DC = 0V? e.g. VBAT is valid, but no sun is shining? What is the quiescent current then?
  • 0 in reply to Eric Bieri
    TI__Guru**** 165886 points
    I(VSTOR) will be between those values. You can linearly interpolate based on the expected VBAT=VSTOR value.