PCF5079 datasheet(8/28 Pages) PHILIPS

8 / 28 page

2001 Nov 21

Philips Semiconductors

Product speciﬁcation

Dual-band power ampliﬁer controller for

GSM, PCN and DCS

PCF5079

7.5

Home position voltage

Internally, a forward voltage of an on-chip silicon diode is

provided as a default home position. This voltage matches

the requirements at the control input of most PAs and

exhibits the same temperature coefficient.

7.6

End of burst

The ramp-down should drive the PA from conduction to

shut off in a controlled way (see Fig.5). To get this result,

correct DAC programming is required, so that the last code

of the DAC ramp-down (CODEEND) is lower than the initial

code of the ramp-up (CODESTART). In this way, the energy

corresponding to the difference between start and end

codes, applied for a certain number of Quarter-Bits (QB),

is used to balance the energy stored in the summing node

during the time interval between the start of control voltage

on pin VCD or VCG ramping-up and the feedback of a

detected ramp to the sensor input. Also a very slow

ramp-down is avoided when the PA switches off and the

loop gain becomes zero.

The amount of energy required at the end of the

ramp-down depends on the overall loop gain and on the

time needed to reach PA conduction from the home

position. At the end of a burst, when pin PU is set to

logic 0, control voltage on pin VCD or VCG is forced

to VSS.

handbook, full pagewidth

VSS

VVCD, VVCG

time

time (QB)

time

open

closed

open

closed

time

CODESTART

CODEEND

VVDAC

S1, S3

S4, S5

MGT328

(1)

td5

<1 µs

. . . i

−8i−6i−4i−2i

Fig.5 End of burst timing diagram.

(1) The exact duration of tA depends on both PCF5079 and the application loop characteristics. The contribution of PCF5079 is due mainly to the group

delay of the low-pass filter on the VDAC input (see Fig.11).

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