MAX11014 Datasheet, PDF (16/69 Pages) Maxim Integrated Products – Automatic RF MESFET Amplifier Drain-Current Controllers

Automatic RF MESFET Amplifier
Drain-Current Controllers
Detailed Description
The MAX11014/MAX11015 set and monitor the bias con-
ditions for dual MESFET power devices found in cellular
base stations and point-to-point microwave links. The
internal DAC sets the voltage across the current-sense
resistor by controlling the GATE voltage. These devices
integrate a 12-bit ADC to measure voltage, internal and
external temperature, and communicate through a 4-wire
20MHz SPI-/MICROWIRE-compatible serial interface or
2-wire 3.4MHz I²C-compatible serial interface
(pin-selectable).
The MAX11014/MAX11015 operate from an internal
+2.5V reference or individual ADC and DAC external
references. The external current-sense resistors moni-
tor voltages over the 0 to (V_DACREF / 4) range. Two cur-
rent-sense amplifiers with a preset gain of four monitor
the voltage across the sense resistors. The
MAX11014/MAX11015 accurately measure their inter-
nal die temperature and two external remote diode tem-
perature sensors. The remote pn junctions are typically
the base-emitter junction of an npn transistor, either
discrete or integrated on a CPU, FPGA, or ASIC.
The MAX11014/MAX11015 also feature an alarm output
that can be triggered during an internal or external
overtemperature condition, an excessive current-sense
voltage, or an excessive GATE voltage. Figure 4 shows
the MAX11014’s functional diagram.
The MAX11014 integrates complete dual analog
closed-loop drain-current controllers for Class A
MESFET amplifier operation. See the MAX11014 Class
A Control Loop section. The analog control loop sets
the drain current through the current-sense resistors.
The MESFET gate-drive amplifier can vary the DAC
code accordingly if the temperature or other system
variables change.
Implement Class A amplifier operation with the follow-
ing three steps:
1) Characterization
Characterize the MESFET over temperature to deter-
mine the amplifier’s set of drain-current values,
assuming the part-to-part calibration curve is consis-
tent. There may be an offset shift, but no important
change in the shape of the function. Load these val-
ues into the MAX11014 LUTs at power-up. In opera-
tion, there is a linear interpolation between the
values stored in the LUTs.
Adjust the drain current for other variables such as
output power or drain voltage by loading values into
the numerical K LUTs.
2) Calibration
In production of the power amplifier, measure the
quiescent drain current at a fixed calibration temper-
ature (probably room) and adjust the V_SET(CODE)
value until the drain current is within the specified
limits for that temperature. The V_SET(CODE) value is
stored for loading after power-up. Prior to operation,
command a PGA calibration after powering up by
writing to the PGA calibration control register, setting
the TRACK bit to 0 and the DOCAL bit to 1 (see
Table 18).
3) Operation
Upon request, the MAX11014 measures the temper-
ature of the MESFET and compares it with the previ-
ous reading. If the temperature reading has
changed, the MAX11014 reads the LUTs with the
characterization data and updates the DAC to cor-
rect the drain current. Setting the TRACK, DOCAL,
and SELFTIME bits to 1 in the PGA calibration con-
trol register starts automatic monitoring and adjust-
ment of drain current for variations in temperature.
Also, if the K LUTs are used, their values are monitored
for changes. A DAC correction is then made if necessary.
For Class AB operation with the MAX11015, measure
the MESFET temperature and set the GATE_ voltage
through the LUTs and DAC to control the drain current.
See the MAX11015 Class AB Control section.
Implement Class AB amplifier operation with the same
three steps as Class A operation, with the exception
that the LUTs set the GATE_ voltage for constant drain
current with varying temperature.
Power-On Reset
On power-up, the MAX11014/MAX11015 are in full
power-down mode (see the SHUT (Write) section). To
change to normal power mode, write two commands to
the shutdown register. Set the FULLPD bit to 0 (other
bits in the shutdown register are ignored) on the first
command. A second command to this register then
activates the internal blocks.
MAX11014 Class A Control Loop
The MAX11014 is designed to set and continuously
control the drain current for MESFET power amplifiers
configured to operate in Class A. Set the DAC code to
control the voltage across the RCS_+ and RCS_- cur-
rent-sense resistor connections. The MAX11014 inter-
nal control loop automatically keeps the voltage across
the current-sense resistor to the value set by the DAC.
See the 12-Bit DAC section.
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