BQ25505: Using BQ25505 as a power converter with variable output voltage

Kaveh Mohammad

Part Number: BQ25505
Other Parts Discussed in Thread: BQ25180, BQ25570, BQ25504, BQ25170, TPS62125, BQ25185

Tool/software:

Hi,

I am thinking to use BQ25505 chip in my design to power up a small load. BQ Input power is from a small solar cell. I want the load to operate at its voltage 2V-3.5V.

I want to design BQ25505 feedback resistors so that it gives out 2-3.5V voltage range. Instead of a battery, I want to use a 100uF-1mF capacitor so that the load is only powered up as power is available from the solar cell. So the BQ25505 chip will just function as a variable output power supply that tunes a load depending on its input power.

Can you advise if this application of BQ25505 is reasonable? Or you suggest another power harvesting for such application,

Thanks

2 months ago

+1 Michael Emanuel35 2 months ago

TI__Mastermind 33855 points

Hello,

As show in Section 7.3.2, "The system load should be removed or reduced based on the VBAT_OK threshold which should be set above the VBAT_UV threshold." So the fan may need to operate at a slightly higher voltage than 2V and needs to be disabled if VBAT_OK is not met.

Keep in mind the VBAT_OV setting must be higher than VIN_DC.

Also, "For the VBAT_UV feature to function properly, the system load should be connected to the VSTOR pin while the storage element should be connected to the VBAT_SEC pin."

As shown in the Pin Functions table, a secondary storage element of at least 100 uF equivalent capacitance should be connected so the application should be fine. Just keep in mind the leakage of the chosen element.

I believe this should work.

Regards,

Mike Emanuel

0 Kaveh Mohammad 2 months ago in reply to Michael Emanuel35

Intellectual 585 points

Ok thanks. Lets say load is running at 3.5V=VBAT_OV and it is rated for 100mW max at this voltage. Now if solar cell inputs 200mW to the chip, what will happen to the excessive power?

+1 Michael Emanuel35 2 months ago in reply to Kaveh Mohammad

TI__Mastermind 33855 points

The solar panel will not regulate to the MPPT voltage if it doesn't need to. It will operate at a higher voltage. It simply will not draw the excess power into the chip.

Regards,

Mike Emanuel

0 Kaveh Mohammad 2 months ago in reply to Michael Emanuel35

Intellectual 585 points

Regarding the initial points that you mentioned in your post about voltage range considerations:

I noticed that my solar cell input voltage of ~4.5V at MPP is actually higher than my max load voltage of ~3.5V.

Is there a buck version of BQ25505 or other power management PMIC you recommend to harvest power from solar cell at 4.5V and run my load at 3.5V?

+1 Michael Emanuel35 2 months ago in reply to Kaveh Mohammad

TI__Mastermind 33855 points

Hello,

Unfortunately there is not an ultra low power solar buck charger.

Regards,

Mike Emanuel

0 Juan Ospina 2 months ago in reply to Kaveh Mohammad

TI__Genius 13165 points

Hi Kaveh,

We do have linear chargers that can step down the input from your solar cell. The BQ25180 could work for this application but it would require some configurations. The Battery regulation voltage would have to be set to 3.5V, the input VINDPM would have to be set to 4.5V (your input MPP), and your SYS_REG should be set to "Battery Tracking Mode" for an output voltage close to your battery voltage. These are non-default configurations so they would have to be written on start up and after any register reset, before your solar cell is connected to the device.

The linear topology may not be as efficient as a switching topology, so it would be important to take efficiencies into consideration, such as losses across FETs.

Best Regards,

Juan Ospina

0 Kaveh Mohammad 2 months ago in reply to Juan Ospina

Intellectual 585 points

Hi Juan,

Thanks for your reply. What is the efficiency I can get out of such system?

Also please consider I do not have a battery in my system. I want to use a 100-200uF cap instead of a battery and use the energy to run my load when it is available from solar cell.

0 Juan Ospina 2 months ago in reply to Kaveh Mohammad

TI__Genius 13165 points

Hi Kaveh,

For a charge current of 44 mA dropping from 4.5V down to a battery voltage max of 3.5V you are seeing a theoretical upper limit of 77% efficiency, in reality it might be a bit lower than this. If that is within the realm of functionality for you I can run some tests to get more typical numbers.

Best Regards,

Juan Ospina

0 Kaveh Mohammad 1 month ago in reply to Juan Ospina

Intellectual 585 points

Thanks Juan.

How about bq25570? It seems to be able to buck the voltage at the output at a 3.3V level when connected to a solar cell operating at MPPT of 4.5V.

+1 Juan Ospina 1 month ago in reply to Kaveh Mohammad

TI__Genius 13165 points

Hi Kaveh,

The BQ25570 uses the same boost topology as the BQ25505 and BQ25504 which requires that VBAT_OV be greater than VIN_OC. It just integrates a buck for load rail regulation.

Best Regards,

Juan Ospina

0 Kaveh Mohammad 1 month ago in reply to Juan Ospina

Intellectual 585 points

How about BQ25170? Does it function similar to BQ25180 you are suggesting? I am not interested in I2C interface, but prefer to hardware program charger voltage and currents.

0 Kaveh Mohammad 1 month ago in reply to Juan Ospina

Intellectual 585 points

How about TPS62125? It seems to be a buck converter with a very good efficiency.

For chips like this how do I ensure that the solar cell voltage/current does not collapse and I can extract power at a reasonable operating point?

0 Juan Ospina 1 month ago in reply to Kaveh Mohammad

TI__Genius 13165 points

That charger does not have the VINDPM feature so I don't think this will work for a high impedance source.

0 Juan Ospina 1 month ago in reply to Kaveh Mohammad

TI__Genius 13165 points

I'm not familiar with this device. I recommend you create a new E2E thread with this device as the thread device.

0 Kaveh Mohammad 1 month ago in reply to Juan Ospina

Intellectual 585 points

Ok thanks Juan. Does BQ25180 has a TINA simulation model so that I can check the operating points?

0 Kaveh Mohammad 1 month ago in reply to Juan Ospina

Intellectual 585 points

Is there a version with VINDPM feature I can use that also can be hardware programmed for voltages and currents instead of an I2C interface?

0 Juan Ospina 1 month ago in reply to Kaveh Mohammad

TI__Genius 13165 points

Hi Kaveh,

The BQ25185 is I2C programmable and includes the VINDPM feature. However TINA models don't exist for either of these devices.

Best Regards,

Juan Ospina

Power management

Power management forum

BQ25505: Using BQ25505 as a power converter with variable output voltage