Rainbow Electronics DS1870 - Manual (Page 10)

DS1870
LDMOS RF Power-Amplifier Bias
Controller
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Detailed Description
The DS1870 is a dual-channel LDMOS bias controller.
It is intended to replace traditional bias control solu-
tions that are limited by a constant temperature-coeffi-
cient correction. This IC offers lookup table correction
that is programmable as a function of temperature as
well as drain supply voltage or current. The flexibility to
use a nonlinear bias correction improves efficiency sig-
nificantly. This is a direct consequence of the ability to
lower the bias current, particularly in class AB opera-
tion, since the bias correction no longer requires a con-
stant temperature coefficient. In addition, correcting the
bias as a function of drain supply voltage, or drain cur-
rent in class AB, assists in distortion reduction and gain
management.
Two outputs (W1 and W2), each controlled by a dedi-
cated two-dimensional lookup table as shown in the
functional diagram, drive two LDMOS gates. The two
degrees of freedom are temperature and either drain
supply voltage or drain current. The lookup tables are
programmed during power-amplifier assembly and
test. After calibration, the IC automatically recalls the
proper control setting for each output, based on tem-
perature and drain characteristics.
A 13-bit ADC samples and digitizes the chip tempera-
ture, V
CC , the drain supply voltage, and two drain cur-
rents. These digitized signals are stored in memory
ready to be accessed by the look up table controls.
The digitized values are also compared to alarm
thresholds generating high or low alarm flags. The
FAULT output can be configured to assert high based
any alarm’s assertion, or the alarms can be masked to
prevent unwanted fault assertions. The ADC readings
as well as the alarm flags and fault status are accessi-
ble through the I
2 C-compatible interface.
Voltage/Current Monitor Operation
The DS1870 monitors four voltages (V CC , V D , I D1 , and
I D2 ) plus the temperature in a round-robin fashion using
its 13-bit ADC. The converted voltage values are stored
in memory addresses 62h–69h as 16-bit unsigned
numbers with the ADC result left justified in the register.
The three least significant bits of the ADC result registers
are masked to zero. The round-robin time is specified by
t
frame
in the analog voltage-monitoring characteristics.
The default factory-calibrated values for the voltage
monitors are shown in Table 1.
To calculate the voltage measured from the register
value, first calculate the LSB weight of the 16-bit regis-
ter that is equal to the full-scale voltage span divided
by 65,528. Next, convert the hexadecimal register
value to decimal and multiply it times the LSB weight.
Example: Using the factory default V CC trim, what volt-
age is measured if the V CC register value is C347h?
The LSB for V CC is equal to (6.553V - 0V) / 65,528 =
100.00µV. C347h is equal to 49,991 decimal, which
yields a supply voltage equal to 49,991 x 100.00µV =
4.999V. Table 2 shows more conversion examples
based on the factory trimmed ADC settings.
By using the internal gain and offset calibration regis-
ters, the +FS and -FS signal values shown in Table 1
can be modified to meet customer needs. For more
information on calibration, see the Voltage/Temperature
Monitor Calibration section.
Note: The method shown above for determining the
input voltage level only works when the offset register is
set to zero.
SIGNAL +FS SIGNAL
+FS
(hex)
-FS
SIGNAL
-FS
(hex)
V CC 6.553V FFF8 0V 0000
V D 2.5V FFF8 0V 0000
I D1 0.5V FFF8 0V 0000
I D2 0.5V FFF8 0V 0000
Table 1. Voltage-Monitor Factory Default
Calibration
SIGNAL
LSB
WEIGHT (µV)
REGISTER
VALUE (hex)
INPUT
VOLTAGE (V)
V CC 100.00 8080 3.29
V CC 100.00 C0F8 4.94
V
D
38.152 C000 1.875
V D 38.152 8080 1.255
I D1 7.6303 8000 0.2500
I D2 7.6303 1328 0.0374
Table 2. Voltage-Monitor Conversion
Examples