X9470 Datasheet, PDF (15/25 Pages) Xicor Inc. – RF Power Amplifier (PA) Bias Controller

Preliminary Information
X9470
OUTPUT (V_BIAS)
V_BIAS is a buffered output of RW_BIAS (wiper output). It
can deliver a high current for driving up to typically 1nF
capacitive loading with stable performance and fast
settling time.
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
Calibration
Set
Stop
and Bias Lock Address Byte
Set WEL
bit
CS
Initiates
high voltage write
1
2
3
4
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 amplifiers
to account for V_GS drift and temperature variations.
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 cal-
ibrate 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]
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 normal PA
power monitoring, and over-current sensing using an
external comparator. The INC/DEC pin can be moni-
tored 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 regis-
ter 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).
REV 11.16 3/20/03
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