User's Guide

LM940 HW Design Guide
1VV0301352 Rev. 2 Page 52 of 68 2017-07-19
10. APPLICATION GUIDE
Debug of the LM940 Module in Production
To test and debug the mounting of the LM940 module, we strongly recommend to add
several test pins on the host PCB for the following purposes:
 Checking the connection between the LM940 itself and the application
 Testing the performance of the module by connecting it with an external computer
Depending on the customer application, these test pins include, but are not limited to the
following signals:
 SYSTEM_RESET_N, W_DISABLE_N, WAKE_N
 VBATT, GND
 VREG_L6_1P8
 USB_D-, USB_D+
 USB_SS_TX_M, USB_SS_TX_P, USB_SS_RX_M, USB_SS_RX_P
In addition, the following signals are also recommended (but not mandatory):
 WAN_LED_N
 GPIO_01, GPIO_02, GPIO_03, GPIO_04
Bypass Capacitor on Power Supplies
When a sudden voltage step is asserted to or a cut from the power supplies, the steep
transition causes some reactions such as overshoot and undershoot. This abrupt voltage
transition can affect the device causing it to not operate or to malfunction.
Bypass capacitors are needed to alleviate this behavior. The behavior can appear
differently depending on the various applications. Customers must pay special attention to
this issue when they design their application board.
The length and width of the power lines must be considered carefully, and the capacitance
of the capacitors must be selected accordingly.
The capacitor will also prevent ripple of the power supplies and the switching noise
caused in TDMA systems such as GSM.
Especially, a suitable bypass capacitor must be mounted on the following lines on the
application board:
 VBATT
Recommended values are:
 100uF for VBATT
Customers must still consider that the capacitance mainly depends on the conditions of
their application board.
Generally, more capacitance is required when the power line is longer.
And if customers use the fast power down function, then more bypass capacitors should be
mounted on the application board.