Ds1870 ldmos rf power-amplifier bias controller – Rainbow Electronics DS1870 User Manual
Page 13
DS1870
LDMOS RF Power-Amplifier Bias
Controller
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13
the full-scale (FS) value. The expected FS value is also
found by multiplying an all-ones digital answer by the
LSB weight.
Example: Since the FS digital reading is 65,528 (FFF8
hex) LSBs, if the LSB’s weight is 50µV, then the FS
value is 65,528 x 50µV = 3.2764V.
A binary search is used to calibrate the gain of the con-
verter. This requires forcing two known voltages to the
input pin. It is preferred that one of the forced voltages
is the null input and the other is 90% of FS. Since the
LSB of the least significant bit in the digital reading reg-
ister is known, the expected digital results can be cal-
culated for both the null input and the 90% of full scale
value.
An explanation of the binary search used to scale the
gain is best served with the following example pseudo-
code:
/* Assume that the null input is 0.5V */
/* Assume that the requirement for the LSB is 50µV */
FS = 65528 * 50e-6;
/*3.2764V */
CNT1 = 0.5 / 50e-6;
/* 1000 */
CNT2 = 0.9 X FS / 50e-6;
/* 58981.5 */
/* So the null input is 0.5V and 90% of FS is 2.949V */
Set the input’s offset register to zero
gain_result = 0h;
/* Working register for gain
calculation */
CLAMP = FFF8h;
/* This is the max ADC value*/
For n = 15 down to 0
begin
gain_result = gain_result + 2
n
;
Write gain_result to the input’s
gain register;
Force the 90% FS input (2.949V);
Meas2= ADC result from DS1870;
If Meas2 ≥ CLAMP
Then
gain_result = gain_result - 2
n
;
Else
Force the null input (0.5V)
Meas1 = ADC result from DS1870
If [(Meas2-Meas1)>(CNT2-CNT1)]
Then
gain_result = gain_result - 2
n
;
end;
Write gain_result to the input’s gain
register;
The gain register is now set and the resolution of the
conversion matches the expected LSB. Customers
requiring non-zero null values (e.g., 0.5V) must next
calibrate the input’s offset. If the desired null value is
0V, leave the offset register programmed to 0000h and
skip this step.
To calibrate the offset register, program the gain regis-
ter with the gain_result value determined above. Next,
force the null input voltage (0.5V for the example) and
read the digital result from the part (Meas1). The offset
value can be calculated using the following formula:
Temperature-Monitor Offset Calibration
The DS1870’s temperature sensor comes precalibrated
and requires no further adjustment by the customer for
proper operation. However, it is possible for customers
to characterize their system and add a fixed offset to
the DS1870’s temperature reading so it is reflective of
another location’s temperature. This is not required for
biasing because the temperature offset can be
accounted for by adjusting the data’s location in the
LUTs, but this feature is available for customers who
see application benefits.
To change the temperature sensor’s offset: write the
temperature offset register to 0000h, measure the
source reference temperature (T
ref
), and read the tem-
perature from the DS1870 (T
DS1870
). Then, the follow-
ing formula can be used to calculate the value for the
temperature offset register.
Once the value is calculated, write it to the temperature
offset register.
Power-Up and Low-Voltage Operation
During power-up, the device is inactive until V
CC
exceeds the digital power-on-reset voltage (V
POD
). At
this voltage, the digital circuitry, which includes the I
2
C-
compatible interface, becomes functional. However,
EEPROM-backed registers/settings cannot be internally
read (recalled) until V
CC
exceeds the analog power-on
reset (V
POA
), at which time the remainder or the device
becomes fully functional. Once V
CC
exceeds V
POA
, the
Rdyb bit in byte 74h is timed to go from a 1 to a 0 and
indicates when ADC conversions begin. If V
CC
ever
dips below V
POA
, the Rdyb bit reads as a 1 again.
Once a device exceeds V
POA
and the EEPROM is
recalled, the values remain active (recalled) until V
CC
falls below V
POD
.
TempOffset
T
T
XOR
BB
h
ref
DS
bitwise
=
× −
+
−
(
)
(
)
64
275
40
1870
Offset
Meas
= − Ч
1
1
4