BQ25895: With a resistor across D+/D- USB DCP will not negotiate to 3.25A as designed.

Hi Jeff,

D+ needs to be shorted to D- with a trace or <10 ohm resistor in order for the IINDPM register to be set to 3.25A.  With ILIM resistor set to clamp the input current limit to 2A, you should not see more than 2A DC input current.  Keep in the mind the ICO feature is default enabled.  If the 2A input current causes the VBUS input voltage to droop below the VINDPM threshold, the ICO algorithm could reduce the effective input current limit (stored in REG0x13b0:5) from 3.25A to below 2A.

Regards,

Jeff

Jeff,

I don't appear to be falling below the VINDPM threshold. I'm powering vbus with a bench top supply set for 5V/5A. Due to your feedback, I've since shorted D+/D- together with no change. I have ILIM going to ground using a 196 ohm resistor (2A per Iinmax = Kilim/Rilim eq. 3). I can only get 190mA of input current.

-Jeff

Jeff,

Can you send a copy of your schematic and inductor part number?  What is the REGN voltage?   Can you send your I2C register settings if not default? 

Regards,

Jeff

REGN = 4.3V and all of the register settings are at their default values. I need the hardware to negotiate to the higher current because the system draws about 700mA at startup. It appears that with the higher current draw,  VSYS is too low and the processor will not come up allowing me to change the registers over I2C.

-Jeff

It appears the schematic was stripped off.

Hi Jeff,

If charger TS pin is not connected to thermistor, then its resistor divider needs to be equal sized so I recommend changing R271 to 100kohm.  Otherwise, the IC thinks the battery is too cold to charge.

Regards,
Jeff

Hey Jeff,

I'm not sure about how the TS divider affects the power up default input current, but that had no affect.

-Jeff

Jeff,

I don't see any other issues with the schematic.  Are you certain that the ILIM resistor is correct value?  The only control loops that affect input current limit are IINDPM register (which is 3.25A if you short D+/D-), ILIM resistor, which should only clamp input current limit to 2A and VINDPM, which by default should be 600mV less than VBUS at startup before loading.  Can you send an I2C register dump?

Regards,

Jeff

Jeff,

The ILIM resistor is 196 ohms as it should be and D+/D- are still shorted.

I can't provide a register dump as the I2C is not alive. With VSYS at 0.31V, the 3.3V is not up to power the processor. Only with a battery does the everything come up. 

-Jeff 

Hi Jeff,

Like most dc/dc converters, the BQ25895 buck converter has a soft start to prevent overstressing the FETs at start up.  The soft start works by limiting the input current limit to ~200mA until V(SYS) >2.2V.  Is your SYS load mostly resistive (instead of capacitive) at startup when V(SYS) < 2.2V?  If so, then you could be trapped in soft start.

Regards,

Jeff