BQ25173: BQ25173

Hi Meiling

The TPS61299 has a short circuit protection mode that limits input current when output is be 0.5V.  This feature could be what is preventing the supercapacitor from charging at 0-1V.

The BQ25173 has a minimum Vin (UVLO) of 3.0V and will not load the TPS61299 below this point.  This should resolve the start-up problem.

BQ25173 does not have VINDPM to limit the input current as input voltage decreases.  This could result in shutdowns is the TPS61299 cannot support load.  May not be a problem but something to test in you application.

Hi Bill,

Many thanks for your reply and the information you supplied is very valuable. 

Our situation is:

The VR of rectifier output is 0.7-5.5 Volt as the boost input, our capacitor size is 1.5F with a minimum voltage of 3.57Volt at V5V.  we find the VR is clamped at 0-1V for our design. in order to find the reason, we used TPS61299 evaluation board to verify the reason and we know the boost can not connect with the big supercapacitor. Please note that we do not charging the supercapacitor from 0-1 Volt.

You mentioned that the BQ25173 has a minimum Vin (UVLO) of 3.0V and will not load the TPS61299 below this point. I thought this would be very helpful to solve our start-up issue.

You mentioned  BQ25173 does not have VINDPM to limit the input current as input voltage decreases. Our input to BQ25173  is 5v  with 6mA-25mA form TPS61299.  just wonder if the boost can work properly for such  weak inputs when charging the big super capacitor via BQ25173 and if there is direct loading effect of the supercapacitor  to rectifier and boost. Do we need the VINDMP for our case?

In addition, our input is from 0.7V-65volt. we use LMR36520ADDAR buck convertor as well as our input profile of rectifier is from 1.5 to 50 volt  in one charging process. it go through the boost first and after passing the boost and going to buck and then to boost. do you think is it ok for us to use  BQ25173  to connect with Buck as well without loading effect to the rectifier from the big supercapacitor? I want you to know  that our input at VR is  30mA-320mA for LMR36520ADDAR. 

In addition, what is the charging efficiency of the BQ25173?

Many thanks for your help and advice.

Best regards,

Meiling  

Hi Meiling

Thanks the additional information.

Meiling Zhu said:

Do we need the VINDMP for our case?

You may not need this feature if the current is 5v at 6mA-25mA from TPS61299.

Meiling Zhu said:

do you think is it ok for us to use  BQ25173  to connect with Buck as well without loading effect to the rectifier from the big supercapacitor?

The BQ25173 max input voltage is 40V (no damage), max operating voltage is 18V, above this point unit is OFF.  Connecting BQ25173 to Buck will depend on the expected Vout range of buck and your system.

Meiling Zhu said:

In addition, what is the charging efficiency of the BQ25173?

The BQ25173 is a linear regulator so efficiency will depend on input vs output voltage, if they are close efficiency in high but large difference will result in low efficiency.  

Hi Bill,

Many thanks for your reply.

Our buck and boost all have outputs of 5V and the output will connect with BQ25173 input. Based on your email, this should be fine.

The minimum voltage of  our supercapacitor is 3.5 volt and the maximum is 5 Volt. can you estimate the charging efficiency for our case?

Best regards,

Meiling

Hi Meiling

Meiling Zhu said:

Our buck and boost all have outputs of 5V and the output will connect with BQ25173 input. Based on your email, this should be fine.

I agree with this.

Meiling Zhu said:

The minimum voltage of  our supercapacitor is 3.5 volt and the maximum is 5 Volt. can you estimate the charging efficiency for our case?

Efficiency would be Input voltage / Output voltage,  3.5V / 5V = 70%.

Hi Bill,

Many thanks for the response and I will test the method and let you know if it is ok in the next week.

Best regards,

meiling