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UCD9246: UCD9246: VOUT_MAX has no effect above 1.6 V

Love
Prodigy 50 points
Part Number: UCD9246
Other Parts Discussed in Thread: UCD7242

Hi,

I have a board with one UCD9246 and two UCD7242s. I use the Fusion Digital Power Designer to connect to the UCD9246 via the USB-to-GPIO. The intended use case is 4 separate rails, each with one phase, three of the domains are supposed to run at a nominal 1.0 V (independently configured 0.8-1.2 V), the fourth rail should supply a 2.5 V IO-domain (input for the power stages is a common 12 V):

12 V in  +---------------------+
                               |
                               |
UCD9246|---+-+-+-+----|1/2 UCD7242| --> 1 V (coreX)
           | | |               |
           | | +------|1/2 UCD7242| --> 1 V (coreY)
           | |                 |
           | +--------|1/2 UCD7242| --> 1 V (coreZ)
           |                   |
           +----------|1/2 UCD7242| --> 2.5 V (IO)

The issue:

When I set VOUT_MAX to anything above 1.6 V it does not have the expected effect on rail#4, it is always capped at 1.6 V or lower. For example:

  1. VOUT_MAX = 1.4, VOUT = 1.5 -> actual output voltage = 1.4
  2. VOUT_MAX = 2.9, VOUT = 2.5 -> actual output voltage = 1.6 (no FLT flags)

So for whatever reason I cannot get my rail above 1.6 V. I assume that I am configuring something wrongly, since the rail otherwise behaves as expected. What I have found in the "UCD92xx Digital PWM System Controller PMBus Command Reference"  is that the VOUT_MAX has a hardcoded default of 1.6 (page 35, SLUU337.pdf, August 2008).

I am grateful for any help I can get... I really need to get the IO-voltage running to be able to do anything useful with my ASIC.

Best Regards,
Love C

  • 0
    TI__Genius 13700 points

    Does the fourth rail have a resistor divider on the feedback input, the other three rails would operate without one as the maximum voltage that can be resolved by the error ADC is 1.6V.

    Could you forward your configuration file?

    Regards

  • 0 in reply to Brad
    Prodigy 50 points

    Dear Brad,

    Well, I did not, I have now macGyvered a parallel 678 Ohm (sns_p and sns_n) in addition to a series 1 kOhm on my rail #4. Of course I should have done this from the beginning since the ADC input range is 0 to 2.5 V. Actually the details of the relation between VOUT_MAX and VOUT_SCALE_LOOP were only clear to me after checking the command reference (see below), reading the datasheet again, and using the Fusion Digital Power software.

    I am not sure if I am the only one who ever made this mistake, but maybe it would be good to clearly specify in the data sheet that the VOUT_MAX is bounded by 1.6 and can only be set above this if one also adapt VOUT_SCALE_LOOP?

    Very good that you spotted that as the potential problem, thank you!