AS3900 datasheet(53/59 Pages) AMSCO

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AS3900

Data Sheet - Detail ed Description

Figure 25. Battery Level Indicator

8.10.4 Battery Level Indicator

The AS3900 has an integrated battery level detection, which is realized with an analog comparator. The actual battery level is compared to a

reference voltage that can be set via the SDI interface. The reference voltage can be selected between 2.6V to 2.0V in REG 0x21<1:0>, and has

a hysteresis of ±40mV. The output of the comparator can be selected inverted analog or digital faked output signal via lowbatsel_tx in REG

0x07<7>. Depending on this setting the comparator output bit is routed to lowbatval_tx in REG 0x07<6> or Lbat_ok in REG 0x23<6>.

The comparator can only indicate if the reference level is lower or higher than the actual battery level. In order to get the correct level of the

battery, the reference voltage needs to be set to a lower value via the SDI interface, if the output signal of the comparator indicates battery =

LOW.

With this algorithm the reference level is always updated and reflects the actual battery level that can be read out in REG 0x21<1:0>.

8.10.5 Output Power Selection

The AS3900 has a kind of integrated class D power amplifier that output power can be changed by changing the load impedance seen by the PA.

Furthermore the output power can be varied by the switching from differential output to single ended mode.

8.10.5.1 Single Ended / Differential

In single ended mode the second output RF2OUT is always tight to GND. Only RF1OUT delivers an RF-signal close to VDD with a frequency of

the crystal oscillator. This adjustment can be set via the single bit in the REG 0x21<2>.

In differential mode RF1OUT is switched to VDD and RF2OUT is switched to GND for the first cycle. For the second cycle RF1OUT is switched

to GND and RF2OUT is switched to VDD. In differential mode the output voltage between RF1OUT and RF2OUT is approximately twice the

VDD.

Note: It

is important to note that the PA switches their output automatically to GND during the AS3900 is in receive mode (LNA is turned on).

It is possible to set the PA outputs manually to GND via the bit fzero in REG 0x21<0>.

8.10.5.2 Output Impedance

The output impedance Rout can be set to 25Ω or 12.5Ω. Rout describes the sum of both output stages (RF1OUT and RF2OUT) and limits the

current that drives the antenna and matching network. The output impedance can be changed by the bit half in REG 0x21<1>. Rout was

implemented to prevent the AS3900 from burning, if the outputs of the PA are shorted.

8.10.5.3 Calculation of the load impedance for a desired output power

The output power can be calculated as following using the PA differentially:

PdBm

()

10,

UANT MATCH

RMS

()

RANT MATCH

1mW

---------------------------------------------------

⎝⎠

⎜⎟

⎛⎞

log

Where:

UANT MATCH

RMS

VDD

–

---------------------------

U 0,1V

≅

CLIENT2

CLIENT3

CLIENT4

CLIENT5

CLIENT6

CLIENT7

CLIENT8

CYCLE TIME = 125ms

CLIENT1

Pseudo Random Pointer

Cycle Time -30%

Cycle Time +30%

Time Slot max = 15.6ms

Example: 105kbps

2.9ms

CLIENT1

Pseudo Random Pointer

CLIENT2

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