Semtech LR1121 - High Power PA; Figure 9-4: High Power PA VR_PA Voltage Vs. Txpower

LR1121
User Manual Rev 1.1
UM.LR1121.W.APP Mar 2023
87 of 130
Semtech
Proprietary & Confidential
www.semtech.com
9.1.2 High Power PA
For maximum efficiency, the high power PA should be operated with a maximum VR_PA near 3.1V.
To get VR_PA = 3.1V the user should set TxPower= 22. At this setting,
• The PA can deliver up to 22dBm output power. The output power in this case varies when the battery voltage drops
below 3.3V.
• The actual maximum output power can be set according to the PaDutyCycle and PaHpSel.
• If needed, the output power can be decremented in steps of 1dB from maximum by using TxPower < 22.
The VR_PA variation over the programmed power TxPower for different supply voltages and duty cycle (PaDutyCycle)
conditions is depicted in Figure 9-4: High Power PA VR_PA Voltage vs. TxPower below.
The internal regulator for VR_PA has 200mV of drop-out, which means VBAT must be 200mV higher than the VR_PA voltage
in order to attain the corresponding output power.
For example: For Pout = +20dBm,
• VR_PA = 2.5V is required (brown curve)
• The high power PA can maintain Pout = +20dBm on the 2.7V < VBAT < 3.7V voltage range (2.5V +200mV = 2.7V)
• Below 2.7V, the output power degrades as VBAT reduces
At 1.8V of supply voltage, the maximum VR_PA value is 1.6V (1.8V - 200mV), allowing therefore a +17dBm output power.
Figure 9-4: High Power PA VR_PA Voltage vs. TxPower