Tool/software:
Hello
I am doing a test to find out the response at BAT and SYS pins when no battery is installed. The power input is a USB2.0 or USB3.0 with 5V 500mA or 900mA to VBUS pin.
The board is a home-made PCB based on the typical application (see attached) with RICHG set 240R and RILIM set 159R with an aim to set IINDPM 3A and ICHG 2.8A, respectively.
Waveforms on BAT and SYS output is attached. When there is no battery installed, BAT pin oscillates between 3.0V - 4.2V and SYS pin oscillates around 3.8V-4.2V.
Question:
1) Is the oscillation normal? Although I am going to design a switching DC-DC after SYS to step it downto 3.3V anyway, oscillation like that make me feel uncomfortable and I wonder if I have made any mistake to the circuit.
There is another question not related to this post.
With a 5000mAh 3.7V nominal battery installed, on deep discharge when BAT terminal drop to 3.0V, the SYS pin measured something like 2.9V. IMPORTANT: USB cable is now removed.
On the datasheet of BQ25606, it states that "The power path management regulates the system slightly above battery voltage but does not drop below the 3.5V min. system voltage. With this feature, the system maintains operation even when the battery is completely depleted or removed. "
From my understanding, the statement above implies a min. 3.5V at SYS no matter if the battery has depleted below 3.5V or even completely removed. On the contrary, I find SYS pin minimum drops as low as 2.8V on my PCB.
Why?
Look forward to hearing from you.
John
