AN1385: LDMOS Transistor Bias Control in Basestation RF Power Amplifiers Using Intersil's ISL21400

LDMOS Transistor Bias Control in Basestation RF Power
Amplifiers Using Intersil’s ISL21400
Application Note
February 27, 2007
Introduction
AN1385.0
The ISL21400 Programmable Output Temperature
Sensor IC
LDMOS transistors are used for RF Power Amplification in
numerous applications from point-to-multipoint communications
to Radar. The most pervasive application is in cell phone
basestations. These RF Power Amplifiers (RFPA) provide from
5W to over 200W of output power per channel, and require very
good linearity to maximize the data throughput in a given
channel. The main point to consider is that linearity is the DC
biasing of the LDMOS transistor for optimal drain current for a
given power output. This bias needs to be held constant over
temperature and time. Typically the target accuracy for bias
current over temperature is ±5% but ±3% is much more
desirable for a high performance design.
The ISL21400 is an analog output temperature sensor,
which is programmable for both DC output voltage and
temperature slope (see Figure 2). Two voltage reference
blocks produce both temperature compensated and
proportional to temperature outputs. There are two DACs
inside the device which are programmed via the I2C bus
interface. One DAC is for the voltage reference, the other is
for the temperature sensor, and they provide 8-bit control to
scale either output. The resulting output is summed and then
a variable gain stage provides for a gain of 1, 2 or 4.
The DC voltage reference output is 1.20V nominal, and
considering the DAC scaling and the gain available, this
gives a DC output range of 0V to 4.8V (with a 5.0V supply).
The nominal temperature slope is -2.1mV/°C, and this is
scaled for both positive and negative slopes by the DAC.
Including the gain stage, this provides for up to ±8.4mV/°C
temperature slope.
A simplified circuit of an LDMOS amplifier bias circuit is shown
in Figure 1. The DC Bias on these amplifiers is set by applying
a DC voltage to the gate (VGS) and monitoring the Drain current
(IDD). Ideally, this IDD will be constant over temperature, but
since the VGS of LDMOS amplifier devices varies with
temperature, some type of temperature compensation is
required. One method of setting this DC bias involves using an
adjustable reference, DAC, or Digital potentiometer combined
with a temperature compensation source, such as a transistor
VBE multiplier. This solution can work well, but getting tight
temperature compensation can be problematic since the VBE
junction temperature characteristic for production transistors
will vary. Also, the VGS tempco for LDMOS amplifiers will vary
with IDD. The result is that there are variations in VBE junction
characteristics as well as the LDMOS characteristics. For
optimal temperature compensation, in-circuit adjustments need
to be made for both the temperature compensation as well as
the VGS bias itself.
VDD
RF OUT
LDMOS
TRANSISTOR
RF IN
BIAS
GENERATOR
FIGURE 1. RFPA SIMPLIFIED SCHEMATIC
A new way to bias an LDMOS amplifier is presented in the
following, which involves digitally converting temperature
information.
VCC
TEMP
SENSE
VTS(n)
DAC
VREF(m)
VREF
VOUT
A
S
DAC
GAIN
SELECT
AV = 1, 2, 4
BIAS
EEPROM
5 BYTES
n = 0 to 255
m = 0 to 255
VSS
SCL
COMMUNICATIONS
AND
REGISTERS
A0
A1
SDA
A2
FIGURE 2. ISL21400 BLOCK DIAGRAM
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 1385
The ISL21400 is especially suited to temperature
compensation functions due to the slope programmability
and 2% accuracy of the temperature sensing function. The
bias voltage produced by the device has both a
programmable DC component and a temperature slope
component. The voltage output is capable of driving nominal
resistive loads with up to 0.5mA of DC output current, and
can handle up to 500pF of capacitive loads. These
characteristics are well suited to LDMOS applications where
the output voltage is isolated from any capacitive load with a
small resistor, and the bias current required is negligible.
U2 VCC supply. An LC filter is added to the U2 VCC supply to
insure no RF energy is present on that supply line.
Hardware Design using the ISL21400
The ISL21400 SCL and SDA lines can be tied to a local
microcontroller or to an I/O connector for external PC control
and programming. The A0, A1, A2 pins are all tied to ground
giving an I2C slave address of 0101000x, where x is the
read/write bit.
RFPA bias control using the ISL21400 is very
straightforward. The dashed rectangle highlights the RFPA
circuit using an MRF9080 from NXP (formerly
Freescale)1.The basic schematic is shown in Figure 3. The
maximum supply voltage for the ISL21400 (U2) is 5.5V and
U1 drops the LDMOS VDD supply from +26V to +5.0V for the
The ISL21400 output is connected to the LDMOS gate (VGG)
through a lowpass filter, which blocks any RF energy from
reaching the ISL21400. A series 100 output resistor (R2)
isolates the filter capacitor from the VOUT pin to insure
stability. Also, R2 allows a simple shutdown circuit to be
added with Q2 and R3, which will provide a soft VGG clamp
when the gate of Q2 is brought high (>2V). An open drain
gate can be used as well as long as the leakage current at
high temperatures is not excessive.
FIGURE 3. RFPA BIAS CONTROL WITH THE ISL21400
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AN1385.0
February 27, 2007
Application Note 1385
Calculating the ISL21400 Register Values
The ISL21400 data sheet2 contains guidelines for calculating
temperature slope using the three control registers: Offset,
Slope and Gain control. We will use the equations given in the
following sections to calculate the register values for this
design.
In this circuit, the N-channel LDMOS transistor gate has
approximately a -2.8mV/°C temperature coefficient from
-10°C to +85°C. A constant bias drain current is desired, with
a target VGS range derived from the data sheet of 2.5V to
3.5V at +25°C.
OFFSET SETTING
Using Equation 1 for setting VOUT offset and targeting
VOUT = 3.0VDC:
V OUT  DC  = A V  V REF  A REF = 3.00V
V REF = 1.20V
(EQ. 1)
A V  A OS = 2.50
Note that since VGS drift is not perfectly linear with
temperature, that the IDS bias error will increase at the
temperature extremes due to this nonlinearity.
Results
The amplifier platform was powered up with the VGG voltage
clamped in shutdown mode until the ISL21400 was powered
up and programmed. The initial setting for VGS = 3.0V was
too low for the target value of IDD = 600mA so the value for n
was increased until a suitable IDD close to the target was
reached. The final register setting was n = B0h.
The amplifier platform including the ISL21400 bias control
circuit was placed in a temperature chamber and tested from
-10°C ambient to +65°C. This resulted in board temperatures
from -10°C to +90°C. The bias current was monitored (RF
power OFF and input/output terminated in 50) and results
are shown in Figure 4. VGG bias voltage was monitored with a
voltmeter tied to the drain of Q2 to limit parasitic effects on the
LDMOS gate. The result is plotted in Figure 4 as well. Error
from Ideal is shown in Figure 5.
Figure 5 includes ±5% range, and the amplifier stays within
these limits fairly consistently, meeting the design goals. The
initial setting for bias appears somewhat high at 620mA
(compared to target of 600mA) but this is limited by the
resolution of the ISL21400 at the gain = 4 setting. The next
lowest offset level results in a bias of 568mA, which is too low.
690
Note that AREF varies from 0 to 1, so to get 2.40, AV = 4, use
Equation 2.
2.50
n
A  REF  = ----------- = 0.625 = ---------4
255
n = 159 decimal
670
IDD (mA)
(EQ. 2)
= 9F hex
The variable n corresponds to register address 0h, and the
variable AV is the gain register, which is address 02h.
TEMPERATURE SLOPE SETTING
Using Equation 3 for temperature slope, we can solve for
Slope directly:
V OUT  TS  = A V  K  A PTAT = – 2.8mV  C
– 2.8
A PTAT = -------------------4  – 2.1
3.70
VGG
650
3.65
630
3.60
610
3.55
IDD
590
3.50
570
3.45
550
-10
10
30
50
3.40
90
70
TEMPERATURE (°C)
FIGURE 4. MEASURED DRAIN CURRENT vs TEMPERATURE
5
(EQ. 3)
 2  m  – 255
A PTAT = 0.333 = ---------------------------------255
3.75
VDD (V)
This entire circuit was implemented on the RFPA evaluation
board with the MRF9080. The ISL21400 is placed adjacent
to the LDMOS device to get best temperature tracking. The
register programming is done using a LabVIEW PC tool for
software control and parallel port interface board, which has
lines for SCL/SDA. The board is disconnected for testing in a
temperature chamber.
The variable m corresponds to address 01h.
The ISL21400 device is then programmed with these
parameters for initial testing. Temperature chamber testing is
then performed to verify performance, and if needed,
adjustments to the register settings are implemented to
optimize performance.
PERCENT ERROR
3
m = 170 decimal
= A9 hex
1
-1
-3
-5
-10
0
10
20 30 40 50 60
TEMPERATURE (°C)
70
80
90
FIGURE 5. IDD BIAS ERROR VS TEMPERATURE
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AN1385.0
February 27, 2007
Application Note 1385
One thing to note in this design or any that requires
temperature compensation is the mechanical properties of
the board mounting and the cooling system. In this example,
airflow over the LDMOS device and the temperature sensor
was limited, which enhanced the resulting compensation.
Also, the sensor was surface mounted with conductive
grease next to the LDMOS device. In many designs, precise
control over placement and airflow is not possible, but since
calibration takes place after the assembly of the unit, these
effects can be minimized as long as the final installation is
similar to the calibration conditions.
References
1. NXP (formerly Freescale) Wireless Infrastructure
Division
2100 East Elliot Road
Tempe, AZ 85284
(800) 521-6274
http://www.nxp.com/
2. Intersil Corporation
1001 Murphy Ranch Road
Milpitas, CA 95035
http://www.intersil.com/
LDMOS amplifiers also have a characteristic IDD drift over
time (drain current reduces for a given VGS), as well as
temperature. This can be addressed with either recalibration
or purposely setting the IDD bias high, knowing the drift will
be in the negative direction.
Intersil Corporation reserves the right to make changes in circuit design, software and/or specifications at any time without notice. Accordingly, the reader is cautioned to
verify that the Application Note or Technical Brief is current before proceeding.
For information regarding Intersil Corporation and its products, see www.intersil.com
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AN1385.0
February 27, 2007
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