AN4198

Application circuit

Doc ID 023916 Rev 1

3/12

1

Application circuit

The SPIRIT1 integrates a switched mode power supply (SMPS) regulator allowing operation

from a battery voltage ranging from +1.8 V to +3.6 V. With power conversion efficiency of at

least 80%, the SMPS saves current consumption in the active mode.

In the default configuration the transmitter power amplifier (PA) output (pin 12 of the

SPIRIT1) is biased by the 1.4V SMPS voltage output through the L0 external inductor

(Figure 1). This limits the maximum output power at about +11 dBm, measured at the 50

Ohm connector via the reference design.

Biasing the PA output through the inductor L0 directly connected to the battery, instead of

the SMPS output (as shown in Figure 2) allows the maximum output power delivered at the

50 ohm connector (or at the antenna) to be increased. The maximum output power changes

with the voltage level applied at the PA output. Section 2 reports the maximum output power

achievable changing the voltage level in each ISM sub-band.