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Load
A
TDMA StarTAC
TM
P8190:
Analog/Digital Switch
A
20
Description
The switching circuits illustrated above are used to define the proper PA loads when operating in analog or in digital mode.
Q1501 is a FET switching circuit which allows SW_QMOD_KEY to be sourced from the TX_2.75 supply. During non-transmit states, Q1501 is switched off by pulling QMOD_KEY
high. This state will not allow SW_QMOD_KEY to be sourced from TX_2.75, thus reducing unnecessary current consumption in numerous TX circuits that use the SW_QMOD_KEY
supply.
When transmitting in analog mode, LOAD A is used and LOAD B is bypassed. LOAD B is bypassed by allowing CR822 to be forward biased. Forward biasing CR822 will provide a
ground state at the anode side, thus allowing only LOAD A(C823) to be present.
CR822 is forward biased by having a supply voltage present at the AD_SW line. When the A_D line is pulled down, Q505 is switched on, forcing the base of Q821 low. PNP transistor
Q821 will then be switched on, allowing the output(AD_SW) to be pulled high from SW_QMOD_KEY.
In digital mode, CR821 is not forward biased, thus having CR821 in an open state. During this state LOAD A and LOAD B are used for the PA load. To have CR822 in an open state,
AD_SW is not pulled high by SW_QMOD_KEY. The AD_SW state is determined by the A_D line. Having a high state at A_D would switch Q505 off, thus not switching Q821 on.
When a high PA supply voltage is present, the PA may begin dissipating more heat. This may cause overheating to the Call Processor(U1000). The protect the U1000 from overheating,
power is reduced by switching in LOAD C and LOAD D. Under normal voltage supplies LOAD C is used and LOAD D is bypassed by forward biasing CR821. When CR821 is for-
ward biased, the anode side is in a ground state. This will not allow LOAD C to be used.
CR821 is forward biased by suppling a voltage at HI_BAT_SW. HI_BAT_SW is controlled by the LOAD_SW line. When HI_BAT_SW is pulled low, PNP transistor Q820 is switched
on, allowing HI_BAT_SW to be supplied by SW_QMOD_KEY.
When LOAD C and LOAD D are used CR821 is not forward biased or in an open state condition. To bring CR821 in an open state condition HI_BAT_SW is not supplied. LOAD_SW
is pulled high causing Q820 to be switched off, thus not allowing HI_BAT_SW to be supplied by SW_QMOD_KEY.
QMOD_KEY
TX_2.75
SW_QMOD_KEY
Q1501
A_D
AD_SW
HI_BAT_SW
LOAD_SW
TX_800
FL450
Load
B
AD_SW
Load
D
Load
C
HI_BAT_SW
CR822
CR821
Q505
Q821
Q820
Содержание Timeport P8190
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