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BQ25895 SYS Settings

Other Parts Discussed in Thread: BQ25890

I purchased the BQ25895EVM and the EV2300 to setup my charger. I have been changing the register settings to test my loads and cant seem to get the SYS output to hold at the SYS_MIN as set in REG03. The voltage on SYS is tracking exactly with VBAT. So the battery voltage is 3.3 under load and the SYS output is matching that voltage. Below is  the register dump from the TI software which gives you the setting. Any thoughts on why the SYS voltage isn't holding at 3.7V?

* Format: Register Name tab Character,\t Hexadecimal register value.
* Device: bq25890
* BQZ Container: Charger_1_00-bq25890_895.bqz
*
Input Current Limit, HIZ and ILIM pin 7F
OTG Temperature thresholds, and Input Voltage DPM Offset 05
ADC controls, OTG Frequency, ICO, DCP, and Input Detection 3D
Battery Load, WDT Reset periods, OTG and Charge Enable, and MINSYS setting 3E
Fast Charge Current Limit settings and MTK Pump enable 20
Pre-charge and Termination Current Limit settings 13
Charge Voltage Regulation Limit, Battery Low Voltage and Recharge threshold 5E
Termination Current Enable, STAT pin, WDT, Safety Timer, and JEITA Charge Current 9D
IR Compensation and Thermal Regulation thresholds 03
ICO, Extended Safety Timer, BATFET control, MTK Pump Voltage, and JEITA Charge Voltage 44
OTG Regulation Voltage and Current Limit settings 93
Charger VBUS, Charging Mode, PG, USB Input, and System Voltage Regulation status 76
WDT, OTG/VBUS OVP, Thermal Shutdown, Safety Timer, and TS pin faults 80
Input Voltage DPM settings 8D
Thermal Regulation Charge Status, and ADC Battery Voltage conversion 00
ADC System Voltage conversion 00
ADC TS pin voltage as percentage of REGN conversion 00
VBUS status, and ADC VBUS Voltage conversion 98
ADC Charge Current conversion 00
DPM status, and Input Current Limit setting 7F
Registers reset, Input Current Optimizer status, Device ID, and Temperature Profile 79

  • You have Enable OTG and Enable ILIM bits high. Is input power connected to VBUS? If not then you are in OTG mode and the IC is in supplement mode. If so, then your input current is limited by the ILIM resistor and if you SYS load is too much for the input current limit setting, then you are also in supplement mode.
  • I have no VBUS attached in this test. My goal is to see what the output of SYS is when the battery voltage drops below SYS_MIN. I'm trying to simulate my actual use condition of extended use of the battery. As you can see from the screen shot, the battery voltage and SYS are the same. Nothing I do seems to make the battery charger maintain a voltage about SYS_MIN. PMID does a great job of maintaining 5v and I can charge my phone easily. I just can't seem to figure out why SYS doesn't boost BAT voltage at SYS to maintain the minimum. I have to be missing something.

    I included a second screenshot where I plug in VBUS, even then its not producing the 3.7V at SYS

  • I should also say that the result is the same with NO load on SYS
  • With no input at VBUS, the buck converter is turned off and so the output cannot be regulated. SYS is tied to BAT through a FET so V(SYS) = V(BAT) - I(SYS)*RDSONfet.
  • Okay, I put 5v on VBUS. Here is the screen capture. It shows that I have 5V in, the BAT voltage is 3.064V and SYS is 2.864 with nothing connected to SYS output (I_SYS=0). I have not changed any jumpers or settings on the EVM. The datasheet says it should be above SYS_MIN setting. But its not. Below is a capture from the datasheet that shows the output I'm expecting. Any thoughts on why my EVM won't perform like this?