PF1550 VBUS_LIN_ILIM resetting

[ben_foose]

During brownout testing of a PF1550 design, it was observed that the VBUS_LIN_ILIM setting was resetting to the default value (0x68 400mA) after it was previously set to the desired value of (0xA0 1500mA).

When VBUS falls below 4.2V, the VBUS_LIN_ILIM register value is reset.  No other registers appeared to be effected by this transition.   The documentation in section 8.8.3 of the RM may help to explain this, but it is not clear that the user would need to re-set the ILIM to the desired level afterwards.   Is this the intended operation?

The input-voltage regulation loop automatically reduces the input current limit in order to

keep the input voltage at VVBUS_DPM_REG. Once VBUS drops to the 4.5 V threshold (this

threshold itself is selectable via the VBUS_DPM_REG[2:0] bits), the charging current

is scaled back and this brings back the VBUS voltage higher.

[guoweisun]

Hi

What do you mean about brownout? could you please give us more information about your brown out operation?

[ben_foose]

Brownout : a period of reduced voltage of electricity caused especially by high demand and resulting in reduced illumination  

Basically, this is when the AC power grid voltage drops below the standard output.

This reduced AC power level can cause an AC/DC power transformers output voltage to dip below the specified range.

Out testing is dropping the VBUS input to simulate this condition.  Normally the VBUS input would be 5.0 to 5.2 VDC, but for this test, I am ramping the voltage down to 4V.    As VBUS passes 4.2V moving downward, I see the change in the register value for VBUS_LIN_ILIM.

[guoweisun]

Simply understand brownout operation is drop down VBUS from 5V to 4V,right?

[ben_foose]

For this discussion, that is a correct statement. 

To test on to the pf1550 eval board.

1. Set VBUS input to 5V

2. Read  VBUS_LIN_ILIM, should be 0x68

3. Set VBUS_LIN_ILIM to 0xA0

4. Read back to confirm

5. Drop VBUS to 4.1V

6 . Read VBUS_LIN_ILIM again.

[guoweisun]

HI

I test your procedure in the EVB,the test result same with you no problem!

[ben_foose]

Are you saying the 0xA0 value was retained through the voltage transition?   Or that you saw the register reset to 0x68 like my original issue?

Thanks

[guoweisun]

1. Set VBUS input to 5V

2. Read  VBUS_LIN_ILIM, should be 0x68

3. Set VBUS_LIN_ILIM to 0xA0

4. Read back to confirm

5. Drop VBUS to 4.1V

6 . Read VBUS_LIN_ILIM again,shows 0X68.

[ben_foose]

Thanks.   So to summarize the result of the testing and expected results.

If Dynamic Power Mode is entered, the PF1550 will reset the VBUS_LIN_ILIM to its default level and the VBUS_DPM_I interrupt will be asserted.  The user will need to manually restore the desired VBUS_LIN_ILIM (potentially as part of an ISR),  after the VBUS level is restored.