• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 98 subscribers
  • Views 531 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Power Management Solutions C55 / RTC Only / DCDC / LDOs

Nicolas Boulot
Intellectual 330 points

Hello,

I am a little bit confused about power-hardware solutions (LDOs vs DCDC) to provide power to the "low power" C5000 DSP, more precisely the C5535 AND enabling the RTC Only mode.

My typical application set-up is :

  • battery powered (Li-Ion / min=3v3, max=4v3, typcial 3v6-3v7), 
  • DSP is running at 100 MHz,
  • CVDD, at 1v3, sinks approx 30 mA.

I would like to use a DCDC to provide the CVDD 1v3 rather than the integrated DSP_LDO because of efficiency. In fact, using :

  • a DCDC, input DCDC current will be around 13mA taking into account the 85% efficiency,
  • integrated LDO_DSP, input LDOI current will be around 30 mA, so more than twice compared with the DCDC.

The problem is that the in IDLE MODE 3 (RTC only), the C5535 requires

  • 1v8 for LDO_In (ANA_LDOI, DSP_LDOI and USB_LODI), and
  • 1v3 for CVDDRTC,
  • 3v3 for DvDDRTC, and
  • DSP_LDO_EN_b tied to low.

This information can be found at http://processors.wiki.ti.com/images/b/ba/RTCOnlyMode.pdf

So is it possible to use the DCDC 1v3 output as CVDD 1V3 rather than the DSP_LDO_O, even if DSP_LDO_EN is tied low ?

I am afraid now that in RTC Mode 3, my DCDC efficiency will low to provide 3v3 just for some µA (perhaps more than the DSP itself ...)

Any idea to avoid this issue ?

Thanks.

Nicolas 

  • 0
    Intellectual 330 points

    Hi, 

    I answer my own question and confirm that CVDD can be powered via external DCDC 1v3 PMU, even if DSP_LDO_EN is tied (low) to ground, rather from the DSP_LDO output at 1v3. Furthermore, Idle "RTC Mode only" is still operationnal (you can get in and get out).

    According to section 4.2.1.1.2 LDO Outputs, page 57 from SPRS737B –AUGUST 2011–REVISED MARCH 2012 :

    Optionally, the CVDD pins may be powered by an external power supply; in this configuration the DSP_LDO_EN pin must be tied (high) to LDOI, disabling DSP_LDO.

    So something is wrong somewhere, or perhaps an errata in the datasheet.

    May some on from TI confirm please  ?

    Thanks.

    Nicolas

  • 0 in reply to Nicolas Boulot
    TI__Mastermind 30120 points

    Hi Nicolas,

    When using the DSP_LDO, RTC-only mode disables the DSP_LDO which removes power from CVDD. The RTC alarm or wakeup pin can then trigger the DSP_LDO to turn back on causing the core to boot.

    When using an external supply for CVDD, the external DCDC must instead remove power from CVDD to enter RTC-only mode. In this scenario, the wakeup pin can be configured as an output to trigger the external DCDC to turn on (active low, open drain). This leaves you with the RTC alarm to trigger wakeup pin output to exit RTC-only mode by re-enabling the DCDC.

    If you do not shut-down the external DCDC, then RTC-only mode is not enabled - the core will not turn off, and the power will remain higher.

    When using an external supply for CVDD instead of using DSP_LDO, DSP_LDO_EN must be high to disable the DSP_LDO. In this configuration an external voltage monitor must monitor CVDD and release RESET pin when stable.

    Refer to http://processors.wiki.ti.com/index.php/C55xx_RTC_ONLY_MODE_SUPPORT

    and http://processors.wiki.ti.com/index.php/C5505/15/35_Schematic_Checklist

    Hope this helps,
    Mark