User's Manual

page 21/51
HiAll
NC
User Manual 2012/06/28
Figure 14: Example with Linear LT1913
3.4.4 Avoid side effects of a retro supply (current re-injection )
Interactions or connections between HiAll
NC
module and the external systems can lead to retro power supply
side effects, or current re-injection through pads while the module is not yet fully powered up (means VBAT
lower than its minimum 3.3V).
If some precaution and simple rules are not followed, those effects can in worst case result in a deadlock
module, not able to start up or to communicate.
Deadlock could happen if the retro supply occurs before the module start. The flow back current could in the
worst case prevent the module to start.
The same behaviour can happen in a normal use conditions when the lines connecting to the module to the
external system uses a non compliant voltage higher than the module IO power domain. This results in a current
flow back inside the module and can lead to a deadlock system on the next start if this retro supply has
continued while the system was powered off or under powered (under 3.3V).
An over voltage on any line can also damage HiAll
NC
module.
Those consequences are rare but exist. Therefore, the rules and advises given on every chapter of this
application note must be followed.
To avoid any power up issue, here are the rules:

Avoid any over voltage on the bus lines connected to the module.
• Use the same power domain voltage for HiAll
NC
lines.
• Use voltage level translators when the power domain requires it

When the module is powered-off, do not apply any voltage on lines connected to the module.
• Power-off the bus lines connected to the HiAll
NC
module, to avoid any flow back current (re-injection).
• Power-off the I/Os connected to the HiAll
NC
, to avoid any current loss.

Recommendations for power domains
• To avoid any current re-injection on VANA (2.85V),
o Use a 10µF serial capacitor to block the DC voltage when an external bias voltage over VANA
is used for the microphone.