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X9470
Figure 3. Bias Adjustment Block Diagram
V BIAS (unbuffered)
RW BIAS
RH BIAS
RL BIAS
2
Gate Bias
Op Amp
–
+
R BIAS
10k ?
256-tap
V BIAS
to LDMOS gate
SHDN
Legend
External pin/signal
Internal node/signal
3
XDCP
Memory and Control
WCR (Rbias)
Bias Register
non-volatile
INC/DEC is logic HIGH or LOW
from Sense/Scale Block
and is used to increment or
decrement the Rbias resistor
(XDCP) to adjust the gate voltage.
Power-On Recall
(POR)
U/D
INC/DEC
1
SCL
INC
CS
Note:
1) WCR = Wiper Control Register
CS
NON-VOLATILE STORE OF THE BIAS POSITION
The following procedure will store the values for the
Rref and Rbias wiper positions in Non-Volatile mem-
ory. This sequence is intended to be performed after a
BiasLock calibration sequence to simplify storage. If
BiasLock has not been achieved, then the Rbias wiper
position may change when the CS pin is brought high
and SCL begins clocking. See Figure 4 for the actual
sequence.
1. Set the WEL bit with a write command (02h to reg-
ister 0Fh)
2. Peform a calibration and achieve BiasLock. Leave
CS pin high.
3. Write the address byte only (START, followed by
device/slave address and a 0 for a write, see page
19).
4. Perform a STOP command.
5. With SCL still low, bring the CS low. The falling
edge of the CS will initiate the NV write.
The WEL bit may be reset afterwards to prevent fur-
ther NV writes.
INC/DEC FUNCTION
The INC/DEC pin is an open-drain logic output that
tracks the activity of the increment/decrement compar-
ator. A logic HIGH at INC/DEC indicates that the I DQ
did not rise up to the desired setting indicated by V REF
while a logic LOW at the INC/DEC pin indicates that
the IDQ is higher than the desired setting.
14
INC/DEC is used as an internal control signal as well.
As an example, when INC/DEC is LOW, the Bias
Adjustment Circuit Block will start to move the Rbias
resistor wiper towards the RL BIAS terminal end when
CS is HIGH and SCL is clocking. Consequently, the
V BIAS voltage will decrease, and the I DQ decreases to
meet the desired V REF setting.
The INC/DEC signal can also be used to detect a
damaged RF power FET. For instance, If INC/DEC
stays HIGH during and after a calibration sequence it
may indicate that the RF power FET has failed. This
indicator can also be used with a level sense on the
V OUT pin to perform diagnostics.
SHUTDOWN MECHANISM
This hardware control shutdown pin (SHDN) will pull
the voltage of V BIAS to VSS with an internal pull down
resistor. When shutdown is disabled (V BIAS is active
when SHDN is LOW), the V BIAS voltage will move to
the previous desired bias voltage.
It will take less than a microsecond to enable the inter-
nal output buffer depending on the loading condition at
the V BIAS pin.
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.
FN8204.0
March 8, 2005
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