SI4438 Datasheet, PDF (26/44 Pages) Silicon Laboratories – Low active power consumption

Si4438
5.4. Transmitter (TX)
The Si4438 contains an integrated +20 dBm transmitter or power amplifier that is capable of transmitting from –20
to +20 dBm. The output power steps are less than 0.25 dB within 6 dB of max power but become larger and more
non-linear close to minimum output power. The Si4438 PA is designed to provide the highest efficiency and lowest
current consumption possible. PA options are single-ended to allow for easy antenna matching and low BOM cost.
Automatic ramp-up and ramp-down is automatically performed to reduce unwanted spectral spreading.
Chip’s TXRAMP pin is disabled by default to save current in cases where on-chip PA will be able to drive the
antenna. In cases where on-chip PA will drive the external PA, and the external PA needs a ramping signal,
TXRAMP is the signal to use. To enable TXRAMP, set the API Property PA_MODE[7] = 1. TXRAMP will start to
ramp up, and ramp down at the SAME time as the internal on-chip PA ramps up/down.
The ramping speed is programmed by TC[3:0] in the PA_RAMP_EX API property, which has the following
characteristics:
TC
Ramp Time (µs)
0.0
2.0
1.0
2.1
2.0
2.2
3.0
2.4
4.0
2.6
5.0
2.8
6.0
3.1
7.0
3.4
8.0
3.7
9.0
4.1
10.0
4.5
11.0
5.0
12.0
6.0
13.0
8.0
14.0
10.0
15.0
20.0
The ramping profile is close to a linear ramping profile with smoothed out corner when approaching Vhi and Vlo.
The TXRAMP pin can source up to 1 mA without voltage drooping. The TXRAMP pin’s sinking capability is
equivalent to a 10 k pull-down resistor.
Vhi = 3 V when Vdd > 3.3 V. When Vdd < 3.3 V, the Vhi will be closely following the Vdd, and ramping time will be
smaller also.
Vlo = 0 V when NO current needed to be sunk into TXRAMP pin. If 10uA need to be sunk into the chip, Vlo will be
10 µA x 10k = 100 mV.
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Rev 1.0