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.
- Battery is connected.
- Board is powered for exactly 1s, then unpowered.
- 5V is enabled.
- Board is powered.
- After >5s 5V is disabled.
- 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!