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X9470
A single pole filter should be placed in between the
V BIAS output and the RF input signal to isolate any
high frequency noise.
Figure 4. Non-Volatile Store of the Bias Position
CS
Set WEL
bit
1
Calibration Set
and Bias Lock Address Byte
2 3
Stop
4
Initiates
high voltage write
cycle
5
SCL
SDA
R BIAS non-volatile register
t WR
Non-volatile Write of R BIAS and R REF value Using SDA, SCL and CS pins
Stored in
Non-volatile
memory
X9470 PRINCIPLES OF OPERATION
The X9470 is a Bias Controller that contains all the
necessary analog components for closed-loop DC
bias control of LDMOS Transistors in RF Applications.
The X9470 provides a mechanism to periodically set
DC bias operating points of Class A or AB-type ampli-
fiers to account for V GS drift and temperature varia-
tions. The following is an example of X9470 operation.
The X9470 incorporates an instrumentation amplifier,
comparator and buffer amplifier along with resistor
arrays and their associated registers and counters. The
serial interface provides direct communication between
the host and the X9470. This section provides a
detailed example of how the X9470 can be used to cali-
brate and dynamically set the optimum bias operating
point of an RF power amplifier (see Figure 5):
– State 0: Power-on Monitor Mode
– State 1: DC-bias Setting When No RF is Present
[Calibration]
– State 2: Calibration Disable When RF is Present
– State 3: PA Standby Mode. Dynamic Adjustment for
V GS drift and Temperature variation
– State 4: Power Off (Shutdown) Mode [Turn off the
Power Amplifier]
15
State 0: Monitor Mode
The V OUT and INC/DEC outputs of the X9470 can be
used for monitoring and diagnostic purposes. Since
V OUT has a lower gain (20x, default) than the internal
IA output, it can handle higher drain sense current
while keeping the output below the rail. This allows nor-
mal PA power monitoring, and over-current sensing us-
ing an external comparator. The INC/DEC pin can be
monitored during calibration to see if there is no
change, which indicates LDMOS functional problems.
Note that the INC/DEC status is also available in the
status register for software status reads.
State 1: DC-bias Setting When No RF is Present
[Calibration]
At calibration, the DC bias operating point of the
LDMOS Power Amplifier must be set. As soon as the
Bias Adjustment Circuit Block is enabled (CS enabled,
SDA high, and SCL pulse provided), the X9470 will
automatically calibrate the external Power Amplifier by
continually sampling the drain current of the external
Power Amplifier and make adjustments to the gate
voltage of the amplifier (See Figure 6).
FN8204.0
March 8, 2005
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