Unexpected power-on sequencing of LTC3554

Hi,

We have an LTC3554 connected to our board as shown in the figure. I expected that when the battery was connected, the LTC3554 would transition to HR state after 1s. When external power (5V in the schematic) was applied it would transition to PUP1 where it would power the board for 5s. Once this timer elapsed, it would transition to PON state, followed by PDN2 (due to PWR_ONx being low). Then it would enter and remain (essentially forever) in POFF state. VOUT would power the board from battery or 5V since the first time 5V was applied.

What I am experiencing however, is the following.

  1. Battery is connected.
    1. Board is powered for exactly 1s, then unpowered.
  2. 5V is enabled.
    1. Board is powered.
  3. After >5s 5V is disabled.
    1. Board is no longer powered.

Why is it that LTC3554 does not drive VBAT when 5V is disconnected?

For testing purposes I'm using a power supply instead of a battery. I noticed then that if I disable the battery voltage before I disable the 5V (i.e. between step 2 and 3 above) and then enable the voltage again, the board is powered from the 'battery' when 5V is removed. In the steps listed above, with a battery instead of power supply, the effect is the same though.

I have also attached the VOUT trace when 5V is disabled (battery connected).

Schematic. U124 and U125 are voltage regulators (buck-boost and boost, respectively). U122 is a battery gauge.

VOUT when 5V is disabled. Battery connected (~3.9V).

Would greatly appreciate any input on the behaviour, or any other tests that may shed light on the issue. Thanks!

  • 0
    •  Analog Employees 
    on Oct 24, 2018 9:24 PM

    Hello,

    The power path should remain on to supply voltage to VOUT either from the battery or from VBUS when present, not in current limit and not in suspend.

    What it the total load from the down stream regulators in the above picture?

    Can the same scope shut be recreated with a probe on the bat pin at the IC with VOUT on pin 18?

    VOUT in the scope shot is droping below UVLO which will shut down the LTC3554 regulators which are not used in this schematic.

    Thanks,

  • Hello Marty,

    Thanks for the reply.

    The power path should remain on to supply voltage to VOUT either from the battery or from VBUS when present, not in current limit and not in suspend.

    I interpret this that the behaviour I'm seeing is not the expected one?

    What it the total load from the down stream regulators in the above picture?

    I can't see the down-stream load, but the total load drawn from the power supply (i.e. 5V) is 20.7mA. The battery is rated for 115mA discharge current.

    Can the same scope shut be recreated with a probe on the bat pin at the IC with VOUT on pin 18?

    If I understood your instructions correctly, here is the scope shot.

    Scope shot of VOUT (yellow) and BAT (blue).

    VOUT in the scope shot is droping below UVLO which will shut down the LTC3554 regulators which are not used in this schematic.

    Yes, we are regulating the voltages using U124 and U125 instead.

  • I think I found the source of error.

    I forgot that between the power supply and 5V there were two diodes (not shown in the schematic). Thus, when I set the supply to 5V, the delivered voltage to the LTC3554 was 5V - 2*Vf, which is 4.36V. This is consistent with VOUT from LTC3554. When I increased the supply to the intended 6V everything seems to work as intended.

  • It seems I was a little hasty in my assumptions. It would appear that it is still having difficulties switching from external power to battery power. We now have a test jig to better sample the signals. For the following results we have thus been able to sample the VBUS signal directly and also apply the voltage directly to VBUS.

    We now get the following traces on the oscilloscope. Note that VOUT remains active once VBUS is set to 5.4V. The sequence was:

    1. Connect battery (@10s)
    2. Enable VBUS=5V (@30s)
    3. Disable VBUS (@35s)
    4. Repeat 2-3 with VBUS incremented 0.1V until VBUS = 5.5V

    Traces captured after using the sequence described above. Yellow = BAT, Cyan = VOUT, Pink = VBUS, Blue = Vcc (3.3V).

    For the sake of completeness, I have added the circuitry that is on the other end of 5V in the previous schematic. J131 is unconnected. D131 is an ESD suppressing diode (Littlefuse XGD10402).

    Once again, appreciate any input!

  • 0
    •  Analog Employees 
    on Nov 2, 2018 2:18 PM in reply to oscar.e

    Thanks for the scope shots. Are the measurements taken near the IC pins? Is the load changing during the process? It looks like the part is in VIN current limit for the first few times VBUS is connected. Is it possible to take a scope shot zoomed in (~20us/Div) when VBUS is removed with the probe on VOUT near the VOUT pin and the part shuts off? I am interested to see if VOUT droops below UVLO when VBUS is lost because of the load.