Pa Supply Scheme; Figure 9-2: Pa Block Diagram - Semtech LR1121 User Manual

9.1 PA Supply Scheme

The PA supply scheme is depicted in
Main supply
1.8 to 3.7V
The PA regulator (Reg_PA) supplies the high power PA (HP PA), the low power PA (LP PA), and the high-frequency PA (HF
PA) through the VR_PA pin. Both the HP PA and the LP PA require a high-Q choke inductor connected externally between
their respective outputs, and VR_PA to provide the bias and control the output power. No external choke inductor is
It is best practice to place a 200 Ohm resistor in parallel to the HP PA choke inductor.
necessary for the HF PA.
The PA regulator is internally connected to the DC-DC /LDO output for the low power PA and high frequency PA, allowing
a +15dBm and +13dBm operation in both DCDC or LDO configurations. For the high power PA, it is connected to the
VBAT_RF pin, therefore to the main supply voltage. This means that the maximum output power generated by the high
power PA depends on the VBAT voltage.
The TX main supply can switch between the battery VBAT and the internal regulator VREG, according to the PA use case.
When operating with VR_PA above 1.35V (e.g. in the case of high power), the battery supply VBAT must be chosen. When
operating with VR_PA below 1.35V (e.g. in the case of low power PA), either supply can be chosen. However, it is better to
choose the internal regulator VREG whenever the required VR_PA is 1.35V or below, in order to benefit from the Buck
converter.
The LR1121 incorporates a precise duty cycle trimmer shared between the three power amplifiers. This duty cycle trimmer
can be used to trade-off the output power, efficiency, and harmonic emission to address the different regional standard
requirements. The duty cycle trimmer is dependent on the impedance presented to the PA, and thus might vary from one
PCB design to another.
LR1121
User Manual Rev 1.1
UM.LR1121.W.APP Mar 2023
Figure 9-2: PA Block
External connection to LP_PA
External connection to HP_PA
VR_PA
VBAT_RF

Figure 9-2: PA Block Diagram

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Diagram.
HP_PA LP_PA
HF_PA
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