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August ’01 Chapter 2. Electrical Description
inputs (RF1, RF2, RF3) and GND is recommended to keep maximum VCL.
Figure 7: Schematic for Battery Backup Supply
2.2.2 VCL Supply
Circuit parts, which are only used during Backup Mode, are supplied by the charge voltage
VCL in the Charge Capacitor CL. CL is charged during Wake-up Time (tWake) in Backup
Mode or is fed externally during Test and Trimming Function.
2.2.2.1 Backup Mode
The capacitor value depends on the current consumption during response and the Response
Time (tTX). Because of high current consumption a supply of the UHF transmitter can typi-
cally not be considered. Therefore the Microcomputer uses LF Transmission in Battery Back-
up Mode. This voltage supply is only available in case of strong activation fields and
extended Wake Times. For calculation of the capacitor value the following information must
be available:
- Supply current of TMS37122 in Transmit Mode (Itx)
This current depends on the quality factor of the resonant circuit. At minimum quality
factor necessary for Backup Mode (Q=10), the current consumption Itx is the sum of
IVCL (~110µA) and Iact (~30µA).
- Supply current of the Microcomputer and supervision circuit (IµC)
This current is customer specific and depends on many factors. Currents of several
hundred microampere are practical.
- Required time for response over LF Channel (tTX).
For typical TIRIS frequency-scheme the maximum high bit duration is 135,6 µs.
Transmission of 96 bits therefore require maximum 13 ms. For security purpose it
should be calculated with 15 ms.
- Available VCL voltage range (dVCL).
VOLT.
REG.
VBAT
3DSUP03A.CH4
CR
LR
GND
VCL
RF
µC
Cbat = 22nF
Battery
VCCO
Rectifier
Diode
CL > = 2.2µF
VCL
RF
VBAT
BAT42
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