Apollo3 BLE BUCK Q&A

Background:

We are using Apollo3blue for wearable production and a few of the boards are found brown-out issue. The reset register value is 0x484 which indicates there was a BLE buck brown out. While measuring the VDDBH voltage, some of the boards have 100mV~200mV voltage dip.  

Question 1:  

Could you explain Apollo3blue BLE buck brown-out mechanism?

Answer:

BLEBUCK brownout is flagged when the blebuck_comp1 comparator (compares VDDBH to a reference voltage) asserts continually for a programmable period of time. We have this trim set at 0xF which equates to comp1 asserting for 81us. BLEBUCK_COMP1 compares attenuated VDDBH to a 580mV reference.· The brownout circuit does not look at voltage, it senses how long the comparator continually determines VDDBH is lower than its trimmed value.· 100~200mV may or may not cause a brownout. Brownout depends on time duration of dip not magnitude. 100us with a 10mV dip may brownout, but 10us with 100mV should not.

Question 2:

Is the voltage dip on VDDBH normal? Why does the dip happen?

Answer: 

A small dip may be seen on the BLEBUCK when a change in load is seen. This is because of the time constant of ton adjust loop means it may take a short period of time for BLEBUCK to respond. On most chips a dip may not be seen, on some it may due to a combination of zero-crossing trim offset, inductor quality and load dynamics. A 100mV dip will not cause any issues to the circuits running on the VDDBH domain. There is headroom there and it's ok to use.

Queston 3:

What is the recommendation of Apollo3 BLE buck inductor?

Answer:

For operation across full voltage range (1.755V - 3.63V), recommend to use high saturation current (>800mA) inductor. E.g. 1uH with less than 20% loss at 1A current preferred. Inductor's max DC resistance should be less than 0.55 ohm and inductor's operating frequency should be more than 20MHz.