Extended Power Management

Dear All,

Could anybody explain to me the details of Extended Power Management Scheme? Or could anybody suggest any good link for all the details on tolerances of 1.2V and 1.8 V supply rails on PCBAs?

Extended Power Management Scheme is normally found in a System On Chip solutions using low drop series pass transistors for 1.8 V and 1.2 V supply rails.

Here is the reason why I would like to know in every details of it:

I am now working on power up testing of a typical PCBA by using Bed of Nails style test fixture.

In one typical testing scheme, I have to measure 1.2V and 1.8V power rails.

The Manufacture Specified that the voltage measurement on 1.2V and

1.8V in given Golden PCBA should not exceed 1.26 V and 1.89 V respectively.

I could verify that by supplying power directly to the PCBA and measuring the rails by Agilent DMM 34401A. I measured more than 10 times.

However, after putting the PCBAs onto the Bed of Nail style test fixture, I found that the measurements are exceeding the limits quite often.

For 9 out of 10 times. 1.8 V rail measures 1.891 ~ 1.894. For 6 out of

10 times 1.2V rail measures 1.260025 to 1.272.

Checking the PCBA schematics, I found that 1.2V and 1.8V rails are controlled by extended power management part of the main processor IC.

It looks like a Linear Voltage Control scheme. 10V adaptor supply is converted to 3.3V by a DC to DC converter. This 3.3V is passing through a low drop series pass transistor (controlled by core processing IC) to give the output of 1.8V. This 1.8V is passing through another low drop series pass transistor to give the output

1.2V.

I am checking on the PCBA and Test System now to find the root cause of it.

Thanks and Best Regards Aung