TCM4400EPET datasheet(40/64 Pages) TI

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TCM4400E

GSM/DCS BASEBAND AND VOICE A/D

AND D/A RF INTERFACE CIRCUIT

SLWS082A – JULY 1999 – REVISED MARCH 2000

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

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 k

Ω

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 VDD/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

VOLTAGE

DEFINITION

VGAP

1.22 V

Band gap used for all other references

VREF

1.75 V

Voltage reference of GMSK internal ADC and DAC

VMID

AVDD2/2 Common-mode reference for uplink/downlink GMSK

MICBIAS

2 V / 2.5 V

Microphone-driving voltage

ADCMID

AVDD3/2 Voltage dc biasing of the auxiliary ADCs

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