Freescale Semiconductor, Inc.
PLL Tuned UHF Transmitter Module
Power Management
16.8 Power Management
To improve the battery lifetime, the output power is gradually reduced
when the battery voltage falls below the soft power down threshold,
V
V
. If the battery voltage subsequently falls to the shutdown voltage
SPD
, all of the circuit except those parts needed to generate the
SDWN
microprocessor clock will switch off silently (for instance, no feedback to
the microprocessor that V has been passed). The microprocessor
SDWN
clock will continue to be supplied as long as PLLEN (or MODE) = 1 and
as long as sufficient V is available (about 1.8 V at 25°C).
CC
Independant of what now happens on V (which will normally rise due
CC
to reduced current drain), the UHF part of the circuit remains latched off
until PLLEN has been reset to 0 by the microprocessor. It may then try
again. In the case where the circuit has passed from a cold environment
to a warmer one, subsequent tries may be successful.
RF POWER
P
RED
V
BATT
V
V
SPD
SDWN
Figure 16-6. Battery Life Timing
MC68HC908RFRK2
MOTOROLA
AdvanceInformation
229
PLL Tuned UHF Transmitter Module
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