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USB Charging

Martin Risso
Intellectual 470 points
Other Parts Discussed in Thread: BQ24295, BQ25895

USB Battery Charging

 

The more I read about USB battery charging, the more questions I have :.) First question, does T.I. have a self enumerating USB battery charger chip? So far, I have only found one company that makes a self enumerating USB battery charging chip and unless I’m missing something, it isn’t as “smart” as I would expect. First it uses resistor and current sink techniques to determine if the host is a dedicated USB charger, a charging downstream port or a standard downstream port. If it determines it is a standard downstream port, it will communicate with the host and enumerate.  It will request 500 mA, if request is denied, it will default to 100 mA and set the charger chip input current limit accordingly. For the “smarts” to be able to enumerate, this seems not so smart to me. Let’s say the host couldn’t provide 500 mA but it could provide 400 mA. I would think it would make a lot more sense to then “agree” with the host on 400 mA and set the input current limit to 400 mA.

 

The device that I am developing does not have a microprocessor but it does have a 1 Amp Hour Li-ion battery. For simplicity, I will use the term “full charge” as the time from fully discharged until the 4.2 Volt switch over to constant Voltage mode. In other words, a 1 hour “fast charge” at a 1C charge rate. So at 100 mA, it would take 10 hours but at 400 mA, it would only take 2.5 hours (assuming the device is off during charging and 100% of the current is used for battery charging). That is a fairly big difference.

 

To further complicate matters, a USB 3.0 standard downstream port can supply 150 mA minimum and 900 mA maximum. So a “smart” self enumerating Li-ion (or LiPo) charger I would think should be able to set the input current limit at 100 mA, 150 mA, 200 mA, 300 mA, 400 mA, 450 mA, 500 mA, 600 mA, 750 mA and 900 mA to be able to fully optimize charging from USB 2.0 or USB 3.0 standard downstream ports as well as set higher currents for charger downstream ports and dedicated charger ports.

 

Am I missing something here in my thinking? Does T.I. make a stand alone charger chip that can do this?

 

Martin Risso

 

 

  • 0
    TI__Genius 10240 points

    TI has switching chargers to support BC1.2 spec and set the input current limit automatically based on D+/D- source detection. Some examples are bq24295, bq25895 http://www.ti.com/lit/ds/symlink/bq25895.pdf.

     

    If the input source does not have the capability it is supposed to provide, the chargers have the Input Voltage Dynamic Power Management (VIN DPM) function to maximize the adaptor utilization and avoid the adapter from crashing. When the adaptor output current exceeds the maximum current under regulation, the output voltage falls to VINDPM threshold and the charger reduces the charging current to keep the charger input voltage (adaptor output voltage – cable voltage drop) from falling below the input voltage limit (VINDPM threshold). It can maximize the output current from the adapter or the USB port without changing the input current limit. For an example, the charger sets the input current to 500mA automatically when it detects SDP but the adapter can only provide 400mA. If the charging current is set at 1A, the USB output voltage will start to drop when the charging current increases to certain level Ichg1 (<1A). The VINDPM will not allow the charger input voltage drop below VINDPM threshold. When the charging current reaches Ichg2 (1A > Ichg2 > Ichg1), the charger input voltage reaches VIN DPM setting and the charger will not further increase the charging current to maintain the input voltage. The charging current could not reach the setting value. The real input current could be slightly higher than 400mA depending on the VINDPM setting.