LM940 Hardware Design Guide
1VV0301352 Rev. 18
Page 29 of 75
2022-01-04
Not Subject to NDA
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The bypass low ESR capacitor must be placed close to the LM940 power input pins,
or if the power supply is of a switching type, it can be placed close to the inductor
to cut the ripple, as long as the PCB trace from the capacitor to LM940 is wide
enough to ensure a low voltage drop connection even during the 2A current peaks.
•
The protection diode must be placed close to the input connector where the power
source is drained.
•
The PCB traces from the input connector to the power regulator IC must be wide
enough to ensure that no significant voltage drops occur during the 2A current
peaks.
•
The PCB traces to LM940 and the bypass capacitor must be wide enough to ensure
that no significant voltage drops occur when the 2A current peaks are absorbed.
This is needed for the same above-mentioned reasons. Try to keep these traces
as short as possible.
•
The PCB traces connecting the switching output to the inductor and the switching
diode must be kept as short as possible by placing the inductor and the diode very
close to the power switching IC (only for the switching power supply). This is done
to reduce the radiated field (noise) at the switching frequency (usually 100-500
kHz).
•
Use a good common ground plane.
•
Place the power supply on the board in a way to guarantee that the high current
return paths in the ground plane do not overlap any noise sensitive circuitry, such
as the microphone amplifier/buffer or earphone amplifier.
•
The power supply input cables must be kept separate from noise sensitive lines,
such as microphone/earphone cables.
RTC
The RTC within the LM940 module does not have a dedicated RTC supply pin. The RTC
block is supplied by the VBATT supply.
If VBATT power is removed, RTC is not maintained so if maintaining an internal RTC is
needed, VBATT must be supplied continuously.
Reference Voltage
1.8V regulated power supply output is provided as the reference voltage to a host board.
This output is active when the module is ON and goes OFF when the module is shut down.