PCF5078 datasheet(5/20 Pages) PHILIPS

5 / 20 page

1999 Apr 12

Philips Semiconductors

Product speciﬁcation

Power ampliﬁer controller for GSM and

PCN systems

PCF5078

After this preset, the control voltage is free to increase

according to the control loop if RF input is present

(see Fig.3).

For higher DAC ramp steps the delay time of opening

switch S3 (and closing switch S4) is reduced. On the

contrary, the delay time between opening switch S2 with

respect to opening switch S3 (and closing switch S4)

remains unchanged.

For a correct start-up it is required that the rising time of the

supply voltage is maximum 20

µs.

End of a burst

For a proper down ramp, the final value of the DAC input

voltage should be below the value at the beginning of the

burst and so be able to really shut-off the PA (see Fig.5).

This means the code programmed for the last bit of the

DAC down ramp (CODEEND) has to be lower than the

initial value of the up ramp (CODESTART). Moreover, the

last code must be maintained until the supply voltage has

been switched off.

When the voltage on pin VC is detected to be lower than

VVHOME a built-in mechanism forces the voltage on pin VC

to Vprebias by closing switches S1, S2 and S3 and by

opening switches S4 and S5.

For proper operation, the supply voltage should be

switched off at least 15

µs later with respect to the end of

the down ramp on pin VDAC.

PA protection against mismatching

A second sensor amplified input is integrated into the

PCF5078 for measuring the reflected wave of the

directional coupler. The signal is added to the measured

RF power signal (see Fig.3). When mismatching at the

output of the PA occurs the power is reduced. A high

Voltage Standing Wave Ratio (VSWR) at the output of the

PA often occurs in systems where the PA is connected to

the antenna via switches with low attenuation instead of

using a duplex filter.

Home position voltage

A forward voltage of an on-chip silicon diode is provided as

the default home position voltage Vhome. This voltage

matches the requirements at the control input of most PAs

and exhibits the same temperature coefficient.

However, if another value is needed for a certain PA the

level can be adjusted by connecting external components

to pin VHOME (see Figs 10 and 11). The home position

voltage can be set between 200 and 1000 mV when using

a capacitor of 50 pF connected between pins VINT

and VC.

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