Rainbow Electronics MAX11008 - Manual (Page 41)

MAX11008
Dual RF LDMOS Bias Controller with
Nonvolatile Memory
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quantities. But to avoid the possibility of mathematical
overflow, the magnitude of the values should be limited
to 12 bits (-4096 to +4095, which allows full movement
over the range of the 12-bit DAC).
Temperature/APC LUT Configuration Registers
The LUT Configuration register (see Table 5) specifies
the location and the size of the temperature and auto-
matic power control (APC) LUTs. The EEPROM can be
configured to have a total of four LUTs (one tempera-
ture LUT for each temperature-sensor channel and one
APC LUT for each DAC channel). These registers can
only be programmed when the device is in LUT stream-
ing mode and are set while data is being streamed into
the LUT. The data contained in the LUT Configuration
registers is stored in the EEPROM.
When V
GATE_
calculations are made using temperature
and/or APC LUT values, the MAX11008 uses a LUT
pointer to retrieve the correct values for the calculation.
The LUT pointer value is derived from the most recent
12-bit ADC measurement or directly transferred from
the APC Parameter register (see Table 16). The source
of the LUT pointer value depends on the settings of the
Software Configuration register (see Table 13). PSIZE
determines the size of the LUT pointer (see Table 5a).
TSIZE specifies the size of the table (see Table 5c). It is
permissible to use an LUT pointer that is larger than the
table indexed. An 8-bit pointer functions properly with a
LUT of 32 data locations. The LUT pointer values that
extend beyond the table are limited to the upper (or
lower) bound of the table. This technique increases the
effective table resolution when the dynamic range of
ADC samples is limited.
The POFF bits set the offset value that is added to the
resulting LUT pointer value. POFF is a signed 6-bit value
that is used to apply both positive and negative offset
values to the LUT pointer. The range of acceptable off-
set values depends on PSIZE (see Table 5a). POFF is
typically used for temperature LUTs that have LUT data
for 0°C measurements located at the center of the LUT.
For example, if a temperature LUT has 64 data locations
(locations 0 through 63), the data for 0°C is located at
the center of the LUT (location 31). If a temperature
measurement is made at 0°C, the resulting ADC conver-
sion is 0, which instructs the LUT pointer to retrieve data
from the first location (location 0) in the LUT. To retrieve
the correct data for 0°C (location 31), a pointer offset of
31 needs to be added to the LUT pointer.
To increase the accuracy of V GATE_ calculations, the
MAX11008 can linearly interpolate intermediate temper-
ature and APC compensation values from the two clos-
est LUT data locations. To accomplish this, fractional
bits are added to the LUT pointer by setting the INT bits
(see Table 5b). When INT = 00 the LUT pointer has no
fractional bits and no interpolation is performed. When
INT = 00, every LUT pointer corresponds directly to a
table entry. If INT = 01, the LUT pointer has 1 fractional
bit, which represents a fractional 1/2. This represents
an LUT pointer that falls midway between two table
entries, and the MAX11008 performs a linear interpola-
tion between those two entries. Similarly, INT = 10 pro-
vides 2 fractional bits (1/4 resolution or 4:1
interpolation), and INT = 11 provides 3 fractional bits
(1/8 resolution or 8:1 interpolation). See the
Calculating
an LUT Pointer from an ADC Sample/APC Parameter
section for a detailed description and examples on cal-
culating LUT pointer values.
The SOT bits set the starting addresses of each corre-
sponding LUT in the EEPROM (see Table 5d). Each
table starts at one of six possible locations within the
EEPROM memory space. It is also possible to make
several LUT tables occupy the same memory space
within the EEPROM by simply setting identical SOT val-
ues. This is useful when temperature or APC data is
common to both channels. This allows a single shared
table of double the resolution to be implemented
instead of two separate identical tables.
Tables 5e and 5f contain examples on how to configure
the LUTs in EEPROM using the TSIZE, SOT, and
PSIZE bits.
Calculating an LUT Pointer from an ADC
Sample/APC Parameter
Calculate the LUT pointer value using the following
steps:
1) The 12-bit ADC value is first shifted to the right by
the number of bits as determined by the following
equation:
12-bit ADC value right shift = 7 - PSIZE - INT
where PSIZE and INT are the decimal values of
PSIZE and INT in the LUT Configuration register. The
LUT pointer is interpreted as a fixed-point fractional
number where PSIZE specifies the number of inte-
ger bits and INT specifies the number of fractional
bits.
2) The pointer offset value is left-shifted in the following
manner:
If PSIZE = 00 or 01, no shifting is performed.
If PSIZE = 10, POFF is shifted to the left by 1 bit.
If PSIZE = 11, POFF is shifted to the left by 2 bits.
POFF is interpreted as a signed number.
3) The resulting POFF value is added to the LUT point-
er value.