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The easiest design solution to circumvent unwanted injection current with remotely powered interconnected
PCB’s is depicted in the following figure. A lot of regulators, like MCP1727, MIC5528, MIC29151/301/501/751,
have either an ENB pin, a SHUTDOWN pin, or a POWER GOOD function pin. The solution depicted below
ensures that the MCU is always powered first before the remote PCBs are allowed to be powered on, hence the
remote PCBs cannot power the MCU by injection current through the MCU I/O internal high side input protection
diode. This also acts as a power sequencer. It is important that the MCU is always powered first. Whether the
MCU uses a regulator PWRGD or even an MCU I/O pin, it is always recommended that the user use a control
signal from the MCU PCB to make the other PCBs a power subject to the MCU PCB.
Method 1: A POWER GOOD on most regulators is an open collector output normally low until the MCU power has
reached 90% of nominal value. The output is Tri-stated and the 10k pull-up forces it to a logic high enabling the
remote PCB(s) regulator power.
Note: Using an MCU PCB regulator with POWER GOOD can also be used as an MCU reset supervisor when
connected to the MCU reset pin in the configuration shown in the following figure.
Method 2: Same concept and net result of Method 1, except using the MCU PCB VOUT as remote PCB regulator
ENABLE signal.
MCU Start-up Problems
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Manual
DS70005439B-page 17
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