TH72032_12 Datasheet, PDF (5/20 Pages) Melexis Microelectronic Systems – ASK Transmitter

TH72032
868/915MHz
ASK Transmitter
2.4 Output Power Selection
The transmitter is provided with an output power selection feature. There are four predefined output power
steps and one off-step accessible via the power selection pin PSEL. A digital power step adjustment was
chosen because of its high accuracy and stability. The number of steps and the step sizes as well as the
corresponding power levels are selected to cover a wide spectrum of different applications.
The implementation of the output power control
logic is shown in figure 3. There are two
matched current sources with an amount of
about 8 µA. One current source is directly
applied to the PSEL pin. The other current
source is used for the generation of reference
voltages with a resistor ladder. These reference
voltages are defining the thresholds between
the power steps. The four comparators deliver
thermometer-coded control signals depending
on the voltage level at the pin PSEL. In order to
have a certain amount of ripple tolerance in a
noisy environment the comparators are
provided with a little hysteresis of about 20 mV.
With these control signals, weighted current
sources of the power amplifier are switched on
or off to set the desired output power level
(Digitally Controlled Current Source). The
LOCK, ASK signal and the output of the low
voltage detector are gating this current source.
RPS
PSEL
ASKDTA _&
&
&&&
OUT
Fig. 3: Block diagram of output power control circuitry
There are two ways to select the desired output power step. First by applying a DC voltage at the pin PSEL,
then this voltage directly selects the desired output power step. This kind of power selection can be used if
the transmission power must be changed during operation. For a fixed-power application a resistor can be
used which is connected from the PSEL pin to ground. The voltage drop across this resistor selects the
desired output power level. For fixed-power applications at the highest power step this resistor can be
omitted. The pin PSEL is in a high impedance state during the “TX standby” mode.
2.5 Lock Detection
The lock detection circuitry turns on the power amplifier only after PLL lock. This prevents from unwanted
emission of the transmitter if the PLL is unlocked.
2.6 Low Voltage Detection
The supply voltage is sensed by a low voltage detect circuitry. The power amplifier is turned off if the supply
voltage drops below a value of about 1.85 V. This is done in order to prevent unwanted emission of the
transmitter if the supply voltage is too low.
3901072032
Rev. 011
Page 5 of 20
Data Sheet
Jun/12