Hey team -
I am working on this circuit for a supercapacitor charger based around the TI BQ24640 supercapacitor charger IC. [For reference, supercapacitor bank will be the power supply for an audio amplifier in an open-source, supercapacitor-powered boombox: www.blueshiftPDX.com ]
http://focus.ti.com/docs/prod/folders/print/bq24640.html
The BQ24640 is a PWM controller for a pair of MOSFETs - it sends constant current to the supercap stack. Duty cycle & voltage increase linearly as the caps charge. The PWM square-wave (600kHz) is LC filtered, TI recommends a resonant frequency of 12-17kHz.
My question is about this filter: they recommend different components based on the charge rate. Each set of components has a similar resonant frequency, but more inductance for higher charge currents.
I am trying to implement two different charge rates: fast charge (10A) from wall adapter and slow charge (500mA) from USB, solar, cigarette lighter, etc. Charge current is set elsewhere with voltage divider, so that's easy to switch.
Question(s):
Is the only reason they recommend a small inductor at small charge current space/money savings?
Or is there some (stability?) issue I'm not seeing from running small current through the LC filter recommended for 10A?
i.e. can I set this up for 10A and also run it at .5A?
I think this is a non-issue but I'm guessing, about to build this and I'd love to have it kind of work on the first try...thanks for reading!
Sam Beck
BlueshiftPDX
I am working on this circuit for a supercapacitor charger based around the TI BQ24640 supercapacitor charger IC. [For reference, supercapacitor bank will be the power supply for an audio amplifier in an open-source, supercapacitor-powered boombox: www.blueshiftPDX.com ]
http://focus.ti.com/docs/prod/folders/print/bq24640.html
The BQ24640 is a PWM controller for a pair of MOSFETs - it sends constant current to the supercap stack. Duty cycle & voltage increase linearly as the caps charge. The PWM square-wave (600kHz) is LC filtered, TI recommends a resonant frequency of 12-17kHz.
My question is about this filter: they recommend different components based on the charge rate. Each set of components has a similar resonant frequency, but more inductance for higher charge currents.
I am trying to implement two different charge rates: fast charge (10A) from wall adapter and slow charge (500mA) from USB, solar, cigarette lighter, etc. Charge current is set elsewhere with voltage divider, so that's easy to switch.
Question(s):
Is the only reason they recommend a small inductor at small charge current space/money savings?
Or is there some (stability?) issue I'm not seeing from running small current through the LC filter recommended for 10A?
i.e. can I set this up for 10A and also run it at .5A?
I think this is a non-issue but I'm guessing, about to build this and I'd love to have it kind of work on the first try...thanks for reading!
Sam Beck
BlueshiftPDX