BQ21040: solar/USB charging options

Hi Dharmesh
The BQ21040 operate from 4.5V to 6.5V higher than that it will be in OVP mode and turn off, it will survive up to 28V input.
It is a linear charger IC, the device will work similar to and LDO. Current in and current out are about the same and voltage is dropped across a pass element, power is dissipated as heat.

The aim is to design the circuit for the worst case, what would happen if the solar input was to gradually decrease during charging phase?---Input voltage would decrease and DPM would become active, see section 8.4.2 of data sheet.

Would there be any need to have any additional protection circuit?---I cannot assist with this question it will depend on your system requirements and regulations you are trying to meet.

Are 0603 caps adequate for applications or should 1206 be better to use?---Input capacitors yes they will support higher voltage better, for out and other signal lines 0603 or 0402 would work.

How large a battery is planned for this system?
What is your required charge time?

Hi Bill

Thank You for your reply, sorry for the delay in my reply.

The battery we use are 2200mA to 4000mA.

The batteries will include its own protection circuit.

We do not require a very fast charging time, even if it was to take a day to charge this is no problem as the aim is for the system to trickle charge eventually or work as a top up charger.

The solar panels we aim to use will be 12V 1A, the type that are used for car battery chargers , unless we get 5V 1A versions.

In regards to the what happens if the solar input was to gradually decrease, what I would expect is a to maximise the charge with input power. So instead trying to output a maximum charge current and having the VIN drop below the minimum requirement, it would be better to reduce the charge current so that battery keep getting charged. If I am correct this device has such function?

What else must I keep in mind, when designing for the worst case?

As you mentioned 4.5V to 6.5V OVP, what impact would this have?

As the device will work as a LDO, if for example Vin was 24V, this will produce some amount of heat. My PCB will also have a Temp and Humidity sensor, will this have a negative effect of that?

In regards to capacitors would I be correct in saying 1206 would be recommended for the input, should I use any value other than 1 UF?

Any update to my questions please?

"As you mentioned 4.5V to 6.5V OVP, what impact would this have? "-- this may be a problem, charger will only operate between 4.5V and 6.5V higher or lower and it is OFF.  You may need a converter in front of the battery charger to keep voltage in range.

Also the bq21040 has a safety timer that will turn off charger after 10 hours--Update, to disable timer pull TS pin high.  See data sheet section 8.4.6.

"As the device will work as a LDO, if for example Vin was 24V, this will produce some amount of heat. My PCB will also have a Temp and Humidity sensor, will this have a negative effect of that?" -- This will be a problem, output from the solar panel needs to be in a narrow voltage range, unit is designed for USB.

Dear Bill

Thank You.

In regards to the OVP, if i make sure that i do not intend to go above the max VIN i guess this will not be an problem. I will also aim to use solar cells no more then 12V and always try to have one with regulator fitted.

I intend to keep the Solar cell always connected, so after 10 hours what would happen? Would i need to disconnect the battery, as this might be a problem,

The 10 hour timer can be disabled, see data sheet section 8.4.6.
But also check with battery manufacture regarding recommended charging profile.

Hi Bill

Thanks

Great,

If i am correct, the 10 hours timer comes into effect if the battery is being charged but there is no load attached to the battery?

My system(PCB) will be connected to the battery terminal in parallel, therefore i am charging and providing power to my system.

Out of curiosity what is the purpose of this device have a large VIN range?

Thanks