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Analog Front End IC TMS37122 - Reference Guide
August ’01
In case of very high field strength VCL decreases due to IC characteristic in the very near
distance range. To keep the maximum VCL it is recommended to connect a Schottky diode
(e.g. BAT42) between every RF input (RF1,RF2,RF3) and GND.
Figure 6: Schematic for Rechargeable Battery Supply
2.2.1.3
Battery Backup Supply
If you have a Battery Backup Supply option the non-rechargeable battery is de-coupled from
Microcomputer and 3D-AFE by a diode (see Figure 7). A Schottky-diode should be used to
minimize voltage drop.The charge capacitor CL must be increased to the µF range to be able
to supply the microcontroller during FSK response phase. Recommendations for calculation
of the capacity are given in section 2.2.2.1
A Low-Power Reset Circuit should supervise the Microcomputer supply voltage. If the min-
imum supply voltage (for example: 2 V) is reached, the Microcomputer will reset and the 3D-
AFE will go into Off-Mode because WDEEN is set to low. If the user of the Identification
Device tries to get into the car in Keyless Entry Mode by pulling the door handle, he will be
not successful, if he is too far away from the Base Station Antenna. This, because the Body
Controller will not get a response on his challenge because the 3D-AFE and Microcomputer
is off. After several trials the Body Controller can increase the Wake-up Time in order to
charge up the charge capacitor to a higher voltage. He reports this action also to the Identifi-
cation Device by using a special Battery Backup Command with the Challenge Data. The
User has to support the process by bringing the Identification Device nearer to the Body Con-
troller Antenna. When the field strength is so high that VCL reaches limitation, also VBAT
has reached via VCCO output a sufficient high value so that the Microcomputer starts. It will
initialize and enable the 3D-AFE via WDEEN.
Because of the received special command a transmission via UHF Channel will be avoided.
Response will be handled via LF Channel, which requires less current.
Also in case of Battery Backup function the use of Schottky diodes (e.g. BAT42) between RF
VOLT.
REG.
VBAT
3DSUP02A.CH4
CR
LR
GND
VCL
RF
µC
Cbat = 22nF
Rechargeable
Battery
VCCO
Rectifier
Diode
CL >= 22nF
VCL
RF
VBAT
BAT42