RF6886 Datasheet, PDF (6/16 Pages) RF Micro Devices – 3.6V, 100MHz TO 1000MHz LINEAR POWER AMPLIFIER

RF6886
In each case, VSWR was tested over phase, with device on/off cycle done several times at phase angle where current was max-
imum. Test showed that the best off/on sequence for RF6886 is as follows:
Turn on:
1. Apply V_CC
2. Apply V_REF1/2
3. Apply drive at RF input
Turn off:
1. Remove drive at RF input
2. Bring down voltage at V_REF1/2
3. Bring down V_CC (not necessary in system of course)
Many systems will use closed loop power control. When taking output VSWR variation into account, the limits in table above
still apply, with same practical maximum limit on RF drive. At some phase angles, higher output powers will not be attainable.
Thus, a limit on maximum drive should be taken into account to prevent overdrive of the device by power control circuit.
The VSWR limits set here apply to the most demanding case, where input drive is set for maximum output power. For example,
Pout >36dBm, V_CC=3.6V, Pin=10dBm. It is entirely conceivable that the amplifier be used in a linear application at backed off
power. In that case, it follows that a higher VSWR could be tolerated. As an example, consider 32dBm output power with
V_CC=3.6V. Test showed that power control loop would achieve 32dBm from 865MHz to 955MHz over phase into 5:1 VSWR. The
increased VSWR specification as compared to the 3:5:1 limit in table comes about for the following reason:
The harshest condition is encountered at phase angle where 10dBm drive results in forward power >38.5dBm and current >
>3000mA. A power control loop sensing forward (coupled) power would back input drive down in this case and prevent dam-
age. That provided it reacts quickly enough. The more limiting factor in this case, phase angle for lowest power presents a situ-
ation where target power cannot be achieved. That even if drive is allowed to go beyond practical maximum. But because the
amplifier was seen to achieve 32dBm over phase along with ruggedness, the increased VSWR specification becomes reason-
able in the presence of power control and lower output power requirement. So, a multitude of scenarios could exist, with test
being required to determine allowable VSWR specification.
Power control can be implemented via several different methodologies, using circuitry external to RF6886. One method
already touched upon, sampling forward coupled output power and feedback to adjust at one of two points in the system:
1. With constant drive level at RF6886 input, adjust voltage level at V_REG1 and/or V_REG2. V_REG1/2 can be tied together,
or one of the two can be kept constant with the other adjusted.
2. With V_REF1/2 constant, RF drive at device input can be adjusted via feedback to a system control point behind
RF6886.
Two RF6886 output pins are also available for use in a power control scheme, PWR SENSE (pin 23) and PWR REF (pin 24).
Viewing the evaluation board schematics, it can be seen that both pins are tied to V_CC through 390 resistors. Both pins sink
current, resulting in following voltages at respective board connectors:
V_PWR REF = V_CC - 390*I_PWR_REF
V_PWR SENSE = V_CC - 390*I_PWR_SENSE
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