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C6748: USB0_VDDA18/USB1_VDDA18 too high

Hi,

 

a customer is seeing a strange behavior with the C6748 on USB0/1_VDDA18, at powerup and during the initialization phase the voltage is reaching 2.8V until the USB block is initialized. 2.8V would be above the absolute max per datasheet for this supply. Something is feeding into this supply, potentially from another internal rail in the C6748.

Here is how power is supplied with the TPS650250 to the C6748

DCDC1: VCC_3V3D (DVDD3318, USB0/1_VDDA33)

DCDC2: VCC_1V8D (DVDD18)

DCDC3: VCC_1V3D (CVDD, RTC)

LDO1: 1V2_LDO (SATA, USB_CVDD12, PLL0, PLL1)

LDO2: 1V8_LDO (USB0_VDDA18, USB1_VDDA18)

External TPS2530: USB_OTG_VBUS  (USB0_VBUS)

 

With this when the C6748 is powered up (power sequencing is per datasheet), the USB0_VDDA18 ramps up to 2.8V and only after the USB block is initialized by the firmware, does the voltage drop to normal 1.8V.

 

So the customer cut the trace from the LDO2 to the USB0/1_VDDA18 pins. After this the TPS650250 showed correct ramp to 1.8V. The USB0/1_VDDA showed an immediate ramp to 3.2V at power up and after the USB block was initialized by the firmware the USB0/1_VDDA18 dropped to 400mV.

Somehow it looks as if another voltage rail internally is feeding into the USB0/1_VDDA18.

 

Could someone comment on what could be happening?

 

Thanks,

--Gunter


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