DC-DC Bypass Current Limiter

by <a href="http://community.silabs.com/t5/Welcome-and-Announcements/Community-Ranking-System-and-Recognition-Program/m-p/140490#U140490"><font color="#000000"><font size="2">Hero Employee</font></font> </a> PhillipB on ‎03-31-2017 05:06 PM

Question

The DC-DC regulator available in EFM32 and EFR32 Wireless MCU Series 1 includes a Bypass Current Limiter.  When should I use this feature?

Answer

As described in the application note AN0948: EFM32 and EFR32 Series 1 Power Configurations and DC-DC:

 

"The DC-DC Bypass Current Limit limits the maximum current drawn from the main supply in Bypass mode. This current limit is enabled by setting the BYPLIMEN bit in the EMU_DCDCCLIMCTRL register. The limit value may be adjusted between 20 mA and 320 mA using the BYPLIMSEL bitfield in the EMU_DCDCMISCCTRL register."

 

This can be particularly helpful in cases where you expect to enable Bypass mode when there is a large differential between DVDD and VREGVDD.  In an extreme example, a drop of 2 V (between 3.8 V VDDmax and 1.8 V DVDD) across the ~2 ohm internal resistance of the Bypass switch == 1 A peak current.  Severe, sudden load current steps can strain a weak battery and risk inducing brownout.  Limiting the maximum current that flows through the Bypass path will therefore minimize the voltage droop observed on the VREGVDD power source (battery or otherwise).

 

The MCU will incur an overhead of approximately 10 µA when the Bypass Current Limiter is enabled. This consumption is only present when the DC-DC is in Bypass mode - BYPLIMEN has no effect in other DC-DC modes. For this reason, to avoid an unnecessary ever-present 10 µA draw, you should enable the Bypass Current Limit only when a transition into BYPASS mode is anticipated and only when the expected peak current consumption warrants the limiter enforcement. Once in Bypass and DVDD rises to a level nearer to VREGVDD, the limiter is no longer necessary and can be disabled.