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Battery charger with load sharing for portable Raspberry Pi 3

Juan Jesus Pinuaga Cascales
Intellectual 490 points
Other Parts Discussed in Thread: TPS61090, BQ25892, BQ25895, TPS61030, BQ24070, PMP11536

Hello, i'm doing portable Rasberry Pi 3 and i need help choosing the right battery power system requiered for the project.

The project has:

- Raspberry Pi 3. Normal power consumtion around 700 mA with peaks of 1 A.

- TFT screen. Power consumtion 100-500 mA

- Speakers: 10-100mA

- 4.2 V lipo battery.

So the battery power system must be capable of supply 1.5-1.6 A of current at 5 V. The 5 V must be very stable because the RPI is sensitive to input voltage vriations and can cause reset of the device.

The requirements for the system are:

- Output 5 V and max output current >1.6 A in all the range of the batery charge level (from 3V to 4.2 V).

- Load sharing/power path (i don't know the exact name), to alow a 5 V wall  adapter to power the system at the same time it charges the battery.

- If the adapter can't support the system current the battery must suply the extra current needed by the system.

- If the sysetm if working and the baterry is low, and the adapter is pluged while runing, the system must be capable of mantain the stability of the output in the transient to avoid reseting in the raspberry.

- Smaller ic and pasives  as posible (tring avoid BGA packages). It is a portable system, i have a limited space.

- Cheaper as posbile.

I have two solutions in mind. In one hand a 1 cell battery charger ic with load sharing and input current >= 2 A plus a DC/DC boost converter to output the 5V to the system. The charger ic must be capable of support 1.6A at the input plus the eficiency of the converter ( ~2A input). For example, i used this device for previus versions of the rasoberry pi which consume less power. It uses the TPS61090 as boost converter, but i think, looking at the graphs of the datasheet that i can't use this converter because at low battery levels (3V) it can't support the 1.5 A. A boost converter with this requirement if easy to find, but i din't find a ic charger with this characteristics.

In the other hand i also searched for ICs for power banks, that are capable of charging battery and suply 5V output directly in one single package, but i didn' find one that alows being conected to a wall adapter (charing the baterry) and supply 5V at the same time.

I need guidance in witch system may be better for my requirements and if you know about some IC that have this characteristics.

Thanks

  • 0
    TI__Expert 8690 points

    Hi Juan,

    You can use bq25892 + TPS61235 (or any other boost converter from our catalog). The bq25892 has USB detection capabilities, even for high power USB adapters. You will need to connect the TPS61235 between the SYS pin and your system, this will provide a constant 5V output and able to use the battery even at 3V up to 3A discharge, which is what you need for your 8W system.

    The bq25895 can in fact provide 5V at PMID while charging the battery (assuming input is 5V), the problem here is that when you disconnect the adapter and turn on OTG, the transition will have a small drop, which could reset your system. I suggest looking at the solution in the first paragraph.

    Regards,
    Steven

  • 0 in reply to Steven Bangdiwala
    Intellectual 490 points

    Hello StevenB,

    Thanks for your reply. Before i saw your answer i searched a bit more in the TI devices and i found a similar solution like the first you said. I selected the bq24070 for the battery charger and the TPS61030 for the boost converter. They are a tighter solution, but they are cheaper. I don't know if because their limit current are more tight to my requirements it's gonna cause some trouble with the temperature of the devices.

    Before doing the first prototypes i ask you if they are a good choice for my project, or is better to choose a better ones.

    Thanks.

  • 0 in reply to Juan Jesus Pinuaga Cascales
    TI__Expert 8690 points

    Juan,

    It looks like this may work for your case if linear charger is enough for you. The boost converter can output 1.5A at any battery input voltage which is what you want.

    Regards,

    Steven

  • 0 in reply to Steven Bangdiwala
    Prodigy 20 points
    Hi,

    I'm also trying to power a Raspberry Pi 3 with a capacitive touch screen LCD. My circuit is similar to the one described by Juan, i use a 4V LiPo battery and a TPS61235 in order to boost the voltage to 5V for the Raspberry.

    The problem is, i can't obtain a stable 5V output voltage with this circuit. The voltage fluctuate between 5V and 4V and is very perturbated which leads to a permanent rebooting of the Raspberry Pi. These oscillations are probably due to the behavior of the boost...

    I reproduce the boost part of the schematic given in the PMP11536 application , took the right electronic components but its not working. I also tried to add the Battery protection circuit of this schematics (BQ29700DSER) and the battery manager circuit (BQ25895) in default mode (no I2C), it reduce a little the output ripple voltage but it do not solve the problem.

    I managed to validate both circuits with a simple resistive charge but i think i'm missing something in order to power the Raspberry.

    Do you have any advice for me ?

    Regards,

    Mickael
  • 0 in reply to user4832203
    TI__Mastermind 25800 points

    Hey Mickael,

    Did you reproduce the schematic of PMP11536 exactly, or was it just the boost converter circuitry.

    Have you tried taking a waveform shot of the SW node to ground? Have you tried increasing the output capacitance on the converter to see if reduces the ripple voltage you are seeing? What is the load current of your Raspberry Pi and what is the capacity and max discharge current of your LiPo?

    Regards,

    Joel H