User's Guide

LM940 HW Design Guide
1VV0301352 Rev. 2 Page 29 of 68 2017-07-19
is crucial for the correct operation of the circuitry. A misplaced component can be useless
or can even decrease the power supply performances.
 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 drop-less 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 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.
This table lists the VREG_L6_IP8 signal of LM940.
LM940 Reference Voltage
PIN Signal I/O Function Type Comment
11 VREG_L6_1P8 O Reference Voltage power 1.8V