VCORE Modifications

The current state of the power supplies on the A1 is very noisy, with the major component synchronous with VCORE switching. This noise was triggering the +1.8VSUS shutdown on VCORE startup, a time of maximum current draw from Vin. The noise is high frequency (1 MHz) ringing coupled through Vin onto all power supplies (3VPCU, etc.). +1.8VSUS has improved immunity due to Vin decoupling through a ferrite.

The suggested circuit modifications to minimize this noise are:

In all figures, the display channels are assigned to the same signals. The amplitude and position of the display channels may vary from figure to figure.

VCR2
VCR2: MB I w. no modifications

VCR2 is a trace from the same unmodified motherboard, taken during the startup of VCORE (high load). Here we see the induced voltage at the lower MOSFET gate exceed the Vgs conduction threshold, inducing direct Vin to ground feedthrough and triggering a noise burst that conducts 0.5+V of noise onto the (filtered) +1.8V rail, along with every other rail. VCORE shown at 500mV/div.

VCR1
VCR1: MB I w. no modifications

VCR1 is a trace from the Isabel MB, unmodified except for those modifications made to all A1 motherboards during bring-up. The trace was taken while OFW was running (light VCORE load). The noise is 125 MHz ringing, with an amplitude of 0.2V on a 0.8V power rail.


Modified performance

VCM12
VCM12: MB I w. suggested modifications wo. ferrite

VCM12 is a trace from the same Isabel MB, after modification, taken during startup of VCORE. The modifications were the suggested ones, except for no ferrite. The value of the low MOSFET gate decoupling capacitor was 6800 pF.

VCM7
VCM7: MB I w. suggested modifications

VCM7 is a trace from the same Isabel MB, after modification, taken during VCORE startup. The modifications were the suggested ones. The value of the low MOSFET gate decoupling capacitor was 3300 pF.

VCM8
VCM8: MB I w. suggested modifications

VCM8 is a trace from the same Isabel MB, after modification, taken when OFW was running. The modifications were the suggested ones. The value of the low MOSFET gate decoupling capacitor was 3300 pF.


Tests missing one modification

VCM9
VCM9: MB I w. suggested modifications wo. ferrite

VCM9 is a trace from the same Isabel MB, after modification, taken during startup of VCORE. The modifications were the suggested ones, except the Vin ferrite wasn't present. The value of the low MOSFET gate decoupling capacitor was 3300 pF.

VCM10
VCM10: MB I w. suggested modifications wo. ferrite

VCM10 is a trace from the same Isabel MB, after modification, taken during when OFW was running. The modifications were the suggested ones, except the Vin ferrite wasn't present. The value of the low MOSFET gate decoupling capacitor was 3300 pF.

VCM5
VCM5: MB I w. suggested modifications wo. BST resistor

VCM5 is a trace from the same Isabel MB, after modification, taken during startup of VCORE. The modifications were the suggested ones except for no resistor in the BST circuit. The value of the low MOSFET gate decoupling capacitor was 3300 pF.

VCM6
VCM6: MB I w. suggested modifications wo. BST resistor

VCM6 is the same as VCM5, with the trace taken while OFW was running.


Wrong Direction

VCM1
VCM1: MB I w. ferrite and extra input cap

VCM1 is the Isabel MB after adding a ferrite (PL1 part) to path from Vin to PQ11/PC33/PC35 and an extra 10 uF input cap. No other modifications were used, and the results of this brute force approach were worse than no modifications! This trace was taken during startup of VCORE. VCORE shown at 500mV/div.

VCM2
VCM2: MB I w. ferrite and extra input cap

VCM2 was the same as VCM1, taken while OFW was running (light load).


Right Direction

VCORE4
VCORE4: MB G w. basic improvements except series R in DH

VCORE4 is a trace from the Graham MB, modified with the suggested modifications, except no ferrite on Vin and no series resistor in DH. The lower MOSFET gate bypass cap was 3900 pF. The trace was taken while VCORE was starting up (heavy VCORE load). VCORE shown at 500mV/div.

VCORE5
VCORE5: MB G w. basic improvements except series R in DH

VCORE5 is a trace from the Graham MB, modified with the suggested modifications, except no ferrite on Vin and no series resistor in DH. The lower MOSFET gate bypass cap was 3900 pF. The trace was taken while OFW was running (light VCORE load).


Is the extra input bypassing crucial ?

VCORE2
VCORE2: MB G w. basic improvements except extra input cap
VCORE3
VCORE3: MB G w. basic improvements except extra input cap

VCORE2 and VCORE3 are traces from the Graham MB, modified with the suggested modifications, with the exception of not having a Vin ferrite, third input bypass cap, or series resistor in DH. The lower MOSFET gate bypass cap was 3900 pF. There were recorded during the startup of VCORE, which is a worst case for current draw from Vin. Watch how intermittent the startup is (Vin droop detected ?) Note the 0.5V influence on the (filtered) +1.8V supply.VCORE in VCORE3 shown at 500mV/div.

VCORE1
VCORE1: MB G w. basic improvements except extra input cap

VCORE1 is the same MB and modifications as VCORE2. The trace was taken while OFW was running (light VCORE load).


Is the lower MOSFET gate bypassing crucial ?

VCORE7
VCORE7: MB G w. basic improvements except gate bypass

VCORE7 is a trace from the Graham MB, modified with the suggested modifications, with the exception of not having a lower MOSFET gate bypass capacitor, a ferrite, or a series resistor in the DH line. The trace was taken during startup of VCORE. VCORE shown at 500mV/div.

VCORE6
VCORE6: MB G w. basic improvements except gate bypass

VCORE6 is a trace from the Graham MB, modified with the suggested modifications, with the exception of not having a lower MOSFET gate bypass capacitor, a ferrite, or a series resistor in the DH line. The trace was taken while OFW was running (light VCORE load).


Does slowing down DL help at all ?

The 1907DL signal shown significant over and undershoot. Does adding a series terminating resistor to it help prevent accidental turn-on ? No:
VCM11
VCM11: MB I w. suggested modifications plus series resistor in DL

VCM11 is a trace from the same Isabel MB, after modification, taken during startup of VCORE. The modifications were the suggested ones. plus a series resistor in the DL line. The value of the low MOSFET gate decoupling capacitor was 3300 pF. The series resistor in 1907DL was 3.3 ohm.