TCM4400E Datasheet, PDF (40/64 Pages) Texas Instruments – GSM/DCS BASEBAND AND VOICE A/D AND D/A RF INTERFACE CIRCUIT

TCM4400E
GSM/DCS BASEBAND AND VOICE A/D
AND D/A RF INTERFACE CIRCUIT
SLWS082A – JULY 1999 – REVISED MARCH 2000
PRINCIPLES OF OPERATION
voltage references
Voltage and current generators are integrated inside the GSM converter. Some additional components are
required for the decoupling and regulation of the internal references. In addition, the internal buffers are
automatically shut down with the corresponding functions being powered down.
There are six terminals reserved for voltage references decoupling and use: VGAP, IBIAS, VREF, MICBIAS,
and VMID (see Table 1):
VGAP:
This terminal is connected to the internal band gap reference voltage. It must be externally
connected to a 0.1-µF capacitor. The band gap drives the current generator and the voltage
reference. This bandgap may be powered down by the PWRDN pin, depending on bit VGAPPN
of register PWDNRG2.
IBIAS:
This terminal is connected to the current reference. It must be externally connected to a 100 kΩ
resistor. As this block is connected to the AFC function, the power down is controlled with similar
means. The current generator is shut down with the same bits as the band gap – one bit for the
power down selection of a hardware solution (with the external PWRDN terminal).
VREF:
This terminal is connected to the internal reference voltage. It must be externally connected to a
0.1-µF capacitor. This band gap may be powered down with the control of the bits VREFPN and
VREFPD of the register PWDNRG2. This voltage reference is internally connected to three
buffers corresponding to the blocks of speech downlink, speech uplink, and GMSK downlink. The
two first blocks are powered down with the inactivity of the corresponding speech blocks. This last
block is shut down outside the radio downlink activations.
VMID:
This buffer gives the V_DD/2 or 1.35 V common-mode output voltage of the baseband uplink path.
This voltage value is selected with the SELVMID bit.
MICBIAS: This buffer is designed to drive an electret-type microphone. The output voltage can be chosen
by software (bit MICBIAS of VBCTL1 register) between the value 2 V to 2.5 V.
ADCMID:
For decoupling purposes, the ADCMID terminal is connected to the internal comparison threshold
of the ADC. Setup time before the ADC is powered on is dependent on the value of the external
decoupling capacitor.
Table 1. Voltage References
REFERENCE
VGAP
VREF
VMID
MICBIAS
ADCMID
VOLTAGE
DEFINITION
1.22 V Band gap used for all other references
1.75 V Voltage reference of GMSK internal ADC and DAC
AVDD2/2 Common-mode reference for uplink/downlink GMSK
2 V / 2.5 V Microphone-driving voltage
AVDD3/2 Voltage dc biasing of the auxiliary ADCs
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