6

LTC4402-1/LTC4402-2

4402f

This closes the RF power control loop and the RF power is

then controlled during ramp down.

LTC4402-1 Description

The LTC4402-1 is identical in performance to the

available. The LTC4402-1 can drive a single band (300MHz

to 2400MHz) or a dual RF channel module with an

internal multiplexer. Several manufacturers offer dual RF

channel modules with an internal multiplexer.

General Layout Considerations

The LTC4402-X should be placed near the coupling com-

ponents. The feedback signal line to the RF pin should be

a 50

Ω transmission line.

Capacitive Coupling

An alternative to a directional coupler is illustrated on the

first page of this data sheet. This method couples RF from

the power amplifier to the power controller through a

0.4pF

±0.05pF capacitor and 50Ω series resistor, com-

pletely eliminating the directional coupler.

LTC4402-X Timing Diagram

Application Note AN91 describes the capacitive coupling

scheme in full detail. Demo boards featuring this coupling

method are available upon request.

Power Ramp Profiles

The external voltage gain associated with the RF channel

can vary significantly between RF power amplifier types.

Frequency compensation generally defines the loop dy-

namics that impact the power/time response and possibly

(slow loops) the power ramp sidebands. The LTC4402-X

operates open loop until an RF voltage appears at the RF

pin, at which time the loop closes and the output power

follows the DAC profile. The RF power amplifier will

require a certain control voltage level (threshold) before an

outputs must quickly rise to this threshold voltage in order

to meet the power/time profile. To reduce this time, the

LTC4402-X starts at 450mV. However, at very low power

outputs may take several microseconds to reach the RF

power amplifier threshold voltage. To reduce this time, it

may be necessary to apply a positive pulse at the start of

threshold voltage. This can generally be achieved with

DAC programming. The magnitude of the pulse is depen-

dent on the RF amplifier characteristics.

Power ramp sidebands and power/time are also a factor

when ramping to zero power. For RF amplifiers requiring

high control voltages, it may be necessary to further adjust

the DAC ramp profile. When the power is ramped down,

the loop will eventually open at power levels below the

LTC4402-X detector threshold. The LTC4402-X will then

stop falling. If this voltage is high enough to produce RF

output power, the power/time or power ramp sidebands

may not meet specification. This problem can be avoided

by starting the DAC ramp from 200mV (Figure 1). At the

end of the cycle, the DAC can be ramped down to 0mV.

This applies a negative signal to the LTC4402-X thereby

200mV ramp step must be applied at least 2

µs after SHDN

is asserted high to allow the autozero to cancel the step.

11

µs28µs

2

µs

28

µs

543

µs

T1

T2 T3

T4

T6

T5

T8

T7

SHDN

PCTL

4402 TD

T1: PART COMES OUT OF SHUTDOWN 11

µs PRIOR TO BURST.

T2: INTERNAL TIMER COMPLETES AUTOZERO CORRECTION, TYPICALLY 9

µs.

T3: BASEBAND CONTROLLER STARTS RF POWER RAMP UP AT LEAST 11

µs AFTER

SHDN IS ASSERTED HIGH.

T4: BASEBAND CONTROLLER COMPLETES RAMP UP.

T7: BASEBAND CONTROLLER STARTS RF POWER RAMP DOWN AT END OF BURST.

T8: RETURNS TO SHUTDOWN MODE BETWEEN BURSTS.

AM MODULATION PERIOD