User's Guide

LM940 HW Design Guide
1VV0301352 Rev. 2 Page 27 of 68 2017-07-19
Information – The electrical design for the power supply must ensure
a peak current output of at least 2A.
General Design Rules
The principal guidelines for the Power Supply Design embrace three different design
steps:
 Electrical design
 Thermal design
 PCB layout
4.3.1. Electrical Design Guidelines
The electrical design of the power supply depends strongly on the power source where
this power is drained.
4.3.1.1. + 5V Input Source Power Supply – Design Guidelines
 The desired output for the power supply is 3.8V. So, the difference between the
input source and the desired output is not big, and therefore a linear regulator can
be used. A switching power supply is preferred to reduce power consumption.
 When using a linear regulator, a proper heat sink must be provided to dissipate the
power generated.
 A bypass low ESR capacitor of adequate capacity must be provided to cut the
current absorption peaks close to the LE925A4 module. A 100 μF tantalum
capacitor is usually suitable (on both VPH_PWR and VPH_PWR_RF together).
 Make sure that the low ESR capacitor on the power supply output (usually a
tantalum one) is rated at least 10V.
 A protection diode must be inserted close to the power input to protect the
LE925A4 module from power polarity inversion.
Figure shows an example of linear regulator with 5V input.
4.3.2. Thermal Design Guidelines
The thermal design for the power supply heat sink must be done with the following
specifications: