Hello,
I am planning to use the BQ24295 in a prototype device and after reviewing the datasheet I have a few questions about the implementation. Note that I am using this IC as a simple charger + powerpath (no external device charging). I will have 2x 18650 batteries (limited to 5A current), and will use a 5V/3A USB C connector for charging.
1. I see the sample circuit is provided for 1.5A charge limit. I'd like to use the full 3A capability of this IC (I will be using a USB C connector sourcing from a supply that allows 3A/5V). From section 9.2.2.1, I understand the suggestion is to set inductor to give ripple current of 20-40% of charge current. I don't however see any graphs regarding efficiency vs ripple current/inductor size. I am a bit hesitant to double the ripple current of suggested schematic (e.g. using 1.1uH inductor with and expect ripple current of ~0.75A), and wanted to confirm this is what is suggested for full 3A charge current.
2. A secondary part on using 3A charge current, I want to confirm that shorting ILIM to ground is the correct thing to do to allow for full charge current.
3. I'm planning to use the SYS output rather than driving my system directly from battery so I can take advantage of the powerpath. Does this mean that although my batteries are capable of delivering up to 5A, that my system can only draw up to 3A? If I connect my system directly to the battery, I assume that means that I couldn't use the powerpath (e.g. when battery very low or disconnected, no power would be delivered to my system).
4. For USB connection, I understand charge negotiation is handled by the BQ24295 using D+ and D- lines. For the full 3A I understand I'll need to use a USB-C cable, but the datasheet does not describe the connections required here outside the basic power, ground, and D+/D- lines. Does there need to be any other connections to the USB C Connector (e.g. CC1/CC2)? Note I will not be using USB for data, only for charging.
5. In my simple application I don't plan to use my device for charging any external devices which appears to be the purpose of PMID/OTG pins, however I'm not fully clear here. How should I set up these pins when not trying to charge external devices? From my understanding I should still have the majority of the input capacitance on PMID (e.g. Vbus 1uF, PMID 20uF) and then short OTG to ground, however I would appreciate confirmation.
6. In my prototype, I'm using 18650 batteries (2x in parallel) which have protection built-in. For this simple prototype, I don't plan to include a thermistor as the cells are protected themselves. In other chargers I've seen option of omitting the thermistor by tieing pin to ground, but I don't see that option here. I'm considering setting up a voltage divider to fake the thermistor, but wanted to check to ensure this is reasonable (at least for prototypes).
7. In the layout example on pg 44 of datasheet, there's a resistor between BTST pin and the BTST capacitor, but this isn't shown anywhere else in the datasheet. Can you clarify if/why a resistor might be placed on the BTST output before the capacitor?
8. Is there a simple way to know when battery is charging, fully charged, and charger unplugged without setting up I2C? I understand that STAT will be driven low when charging, but seems like there wouldn't be a difference on STAT beteween charge finished and charge cable disconnected. From my understanding the REGN line will only be driven (to approx 5V) when charger cable is connected, is this correct? I'm also considering directly monitoring the VBUS line via my MCU.
9. Does CE line matter when VBUS is disconnected? Is it okay to simply always drive CE low, or will this cause excessive current when charger is disconnected?
I will look forward to your clarifications and greatly appreciate your support to help me avoid issues.
Kind Regards,
-Jon
