AN1446: ISL21400 Programmable Temperature Slope Voltage Reference

ISL21400 Programmable Temperature Slope
Voltage Reference
®
Application Note
January 16, 2009
AN1446.0
Introduction
Why the ISL21400?
In the real world, many physical processes and
measurements have a strong dependence on their local
temperature. For example, in ultrasonic distance
measurements the speed of the acoustic wave in air varies
as a function of temperature (°C) by the relationship:
It is not difficult to find voltage references with a fixed output
voltage; likewise, there are many different types of
temperature sensors. What is unique about the Intersil
ISL21400, Programmable Temperature Slope Voltage
Reference is the combination of a voltage reference and
temperature sensor that allows programming both the output
voltage and the temperature slope.
S = 13,044 * (1 + T/273) inches/sec
Therefore, the ultrasonic distance measurement system
must provide a compensating factor based on the system’s
ambient temperature. Different transmission media other
than air require a different amount of temperature
compensation.
Likewise, temperature measurements with a thermocouple
are difficult because a thermocouple provides an output
voltage that is a function of the difference between the
measuring end (“hot end”) and the terminal end (“cold end”).
Thermocouple output voltage vs temperature tables are
based on maintaining the terminal end at 0 °C with an ice
bath. Since it is difficult to mount an ice cube on a terminal
block, most thermocouple couple measurement systems
measure the terminal block temperature, a.k.a. “cold
junction”, and add a compensating term based on the cold
junction temperature. This technique is known as “cold
junction compensation”. Each thermocouple type (E, J, T, K,
S, etc.) requires a different slope of the temperature
compensation term.
Voltage references can range from the inexpensive, such as
the ISL60002 to high accuracy (0.5%) and low drift
(2ppm/°C), such as the ISL21007. However, the output
voltage is fixed to a factory set initial value (1.2, 2.5, 3.3,
etc), and there is no way to change the output voltage. By
their very nature and design, the output voltage variations
are extremely low because that is the job of a voltage
reference.
There are many different types of temperature sensors; four
of the most popular available to the design engineer are
summarized in the following chart.
SENSOR
Thermocouple
Operate over a very wide temperature
range; difficult to interface due to their low
level output voltage and need for cold
junction compensation; poor initial
accuracy; non-linear output voltage
RTD
Operate over a wide temperature range;
requires signal conditioning circuits to
obtain an output voltage; very accurate;
non-linear resistance vs temperature
curve
Thermistor
Inexpensive, small size, narrow
temperature range; requires signal
conditioning circuits to obtain an output
voltage; non-linear resistance vs
temperature curve
IC Based
Very accurate; direct output voltage,
output current, or digital bus (I2C); narrow
temperature range; fixed output format
Another example is a system which requires a reference
voltage (or current) with an accurate temperature coefficient:
VOUT = VREF * (1 ± X * Ta)
The Intersil ISL21400, Programmable Temperature Slope
Voltage Reference, provides the user the ability to generate
a software programmable voltage reference (VREF) with a
software programmable temperature slope (VTS). The VREF
and VTS terms are programmed via an I2C bus with 8 bits of
resolution and stored on non-volatile registers. Non-volatile
memory storage assures the programmed settings are
retained on power-down, eliminating the need for software
initialization at device power-up. Low supply current of
200µA and small package size (8 Ld MSOP) make the
ISL21400 ideal for small battery operated systems.
ADVANTAGES/DISADVANTAGES
Not shown on this chart is the fact that each of these
temperature sensors have a fixed output format (mV/ °C,
mV/ °F, Ω/°C, digital code/°C, etc.), and in order to change
the format, a different device must be selected (ie, J type
thermocouple vs K type, or LM35 vs LM34, etc.).
Notice that most of the temperature sensors exhibit a
non-linear output response vs temperature which require
complex linearizing techniques to obtain a usable output
signal.
1
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
1-888-INTERSIL or 1-888-468-3774 | Intersil (and design) is a registered trademark of Intersil Americas Inc.
Copyright Intersil Americas Inc. 2009. All Rights Reserved
All other trademarks mentioned are the property of their respective owners.
Application Note 1446
Block Diagram
FIGURE 1.
ISL21400 Theory of Operation
T = Device temperature, °C
Note: Refer to the Block Diagram.
N = 8 to 255, Register 0, Programmed via the I2C serial bus
The Intersil ISL21400, Programmable Temperature Slope
Voltage Reference provides a programmable output voltage,
VOUT which combines both a temperature independent term
(VREF) and a temperature dependent term VTS. The
temperature independent term uses a bandgap voltage
reference, and the temperature dependent term uses a
Proportional To Absolute Temperature (PTAT) reference for
the temperature sensor. Both VREF and VTS voltage sources
are scalable using two 8 bit non-volatile DACs via the I2C
serial bus and summed together for the output voltage,
VOUT. The output of the summer circuit goes into a
Programmable Gain Amplifier (PGA) which provides
programmable gain of 1, 2, 4 via the I2C serial bus. The
resulting output voltage can programmed from 0V to 4.8V
and has a programmable Temperature Slope (TS).
M = 0 to 255, Register 1, Programmed via the I2C serial bus
The output voltage, VOUT can be programmed as shown in
Equation 1:
AV = 1, 2, 4 Register 2, Programmed via the I2C serial bus
The DAC registers are non-volatile such that the values are
restored during the VCC power-up cycle of the ISL21400.
There are two additional 8 bit non-volatile registers (Register
4, 5) and one 6 bit non-volatile register (Register 2) in the
ISL21400 for general purpose storage needs such as board
serial number. The register table is shown in Table 1:
TABLE 1. REGISTER TABLE
REGISTER
VREF = 1.200v
ASSIGNED BITS
0
VREF setting (N value)
D0 to D7
1
VTS setting (M value)
D0 to D7
2
AV setting, 1, 2, 4
D0, D1
General purpose storage
D2 to D7
3
General purpose storage
D0 to D7
4
General purpose storage
D0 to D7
VOUT = AV × VREF × ( N ⁄ 255 ) + AV × VTS × ( 2M – 255 ) (EQ. 1)
where:
DESCRIPTION
Communications for programming the ISL21400 is provided
via the I2C serial bus and supports a bidirectional bus
VTS = -2.1mV/°C * (T - 25°C)
2
AN1446.0
January 16, 2009
Application Note 1446
oriented protocol. The protocol defines any device that
sends data onto the bus as a transmitter and the receiving
device as the receiver. The device controlling the transfer is
the master and the device being controlled is the slave. The
master always initiates data transfers and provides the clock
for both transmit and receive operations. Therefore, the
ISL21400 operates as a slave device in all applications. A
complete technical description and programming information
of the ISL21400 I2C serial bus can be found in the ISL21400
data sheet.
Features
Programmable Output Voltage (VOUT) up to 4.8V or
VCC - 0.1V
The internal voltage reference, VREF of the ISL21400 is fixed
at 1.200V with a bandgap reference. Under software control
via the I2C serial bus, the VREF DAC programs its output
voltage from 0.100V to 1.200V such that:
VoDAC = VREF * N/255,
where N is programmed from 0 to 255
(EQ. 2)
VoDAC = 1.200 * N/255
The output of the VREF DAC, VoDAC goes into the
programmable gain amplifier which can be set for a gain of
1, 2, or 4 via the I2C serial bus. Therefore, without
considering the programmable temperature slope
contribution, the overall output voltage, VOUT for the
ISL21400 is shown in Equation 3:
VOUT1 = AV × VREF × ( N ⁄ 255 )
(EQ. 3)
The ISL21400 output voltage, VOUT, is the summation of the
programmable reference voltage, VOUT1 and the
programmable temperature slope, VOUT2 such that:
VOUT = VOUT1 + VOUT2
VOUT = Av * VREF * (N/255) + Av * VTS * (2M – 255) / 255
(EQ. 7)
VOUT = Av * 1.2 * (N/255) + Av * (-2.1mV/ °C) * (T - 25) * (2M
- 255) / 255
ThIS discussion assumes the ISL21400 is operating with a
+5V supply (VCC) and no output load current. When
operating on a lower power supply voltage such as 3.3V
and/or increased load current, the maximum VOUT voltage is
limited by VCC and the load current as discussed in
Paragraph 5, “Output Voltage Considerations” on page 4.
8 Bit Resolution For VREF and VTS
Each of the DACs for the VREF programming and the VTS
programming feature 8 bits of resolution which are
programmed via the I2C serial bus.
Non-Volatile Storage of Programming Registers
The DAC, programmable gain amplifier, and storage
registers are non-volatile such that the values are restored
during the VCC power-up cycle of the ISL21400.
Two Uncommitted Registers of 8 Bits of
Non-Volatile Storage
There are two 8-bit non-volatile registers (Register 4, 5) and
one 6-bit non-volatile register (Register 2) in the ISL21400
for general purpose storage needs, such as board serial
number.
Bidirectional Bus Oriented Protocol, I2C Interface,
Slave Device
Programmable Temperature Slope (VTS) of
±8.4mV/ °C
The internal voltage sensor, VTS of the ISL21400 is fixed at
-2.1mV/ °C * (T - 25). Under software control via the I2C
serial bus, the VTS DAC programs its output voltage from
-2.1mV/ °C to +2.1mV/ °C such that:
Communication for programming the ISL21400 is provided
via the I2C serial bus and supports a bidirectional bus
oriented protocol. The ISL21400 operates as a slave device
in all applications.
Operating Voltage (VCC) of +2.7V to +5.5V
VoDAC = VTS * (2M – 255) / 255,
(EQ. 4)
where M is programmed from 0 to 255
VoDAC = -2.1mV/ °C * (T - 25) * (2M - 255) / 255
(EQ. 5)
The output of the VTS DAC, VoDAC goes into the
programmable gain amplifier which can be set for a gain of
1, 2, or 4 via the I2C serial bus. Therefore, without
considering the VREF contribution, the overall output voltage,
VOUT for the ISL21400 is shown in Equation 6:
Low Supply Current of 500µA
The active supply current is only 500 µA (maximum)
operating on a +5V supply, and is reduced to 400µA
(maximum) operating on a 2.7V supply. This allows the
ISL21400 to be used in a battery operated system and
achieve long battery life.
• 2% total accuracy over the complete VCC and temperature
range
• Industrial temperature range, -40°C to +85°C, operating
• Very small surface mount MSOP, 8 Lead MSOP package
VOUT2 = AV × ( – 2.1mV/°C ) × ( T – 25 ) × ( 2M – 255 ) ⁄ 255
3
(EQ. 6)
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January 16, 2009
Application Note 1446
Design Equations, Procedure and
Programming
As shown in Table 2 dVOUT/dT can range from ±2.1 mV/ °C
to ±8.4 mV/ °C, depending on the values of AV and M.
TABLE 2.
VOUT and dVOUT /dT Design Equations
The output voltage, VOUT can be programmed as shown in
Equation 8:
AV
MIN dVOUT/dT
MAX dVOUT/dT
M=0
M = 255
VOUT = AV × VREF × N ⁄ 255 ) + AV × VTS × ( 2M – 255 ) ⁄ 255
(EQ. 8)
1
+2.1mV/°C
-2.1mV/°C
2
+4.2mV/°C
-4.2mV/°C
where:
4
+8.4mV/°C
-8.4mV/°C
VREF = 1.200V
T = Device temperature, °C
For example, a 2.052V general purpose voltage reference at
+25°C with a temperature coefficient of +3.6 mV/°C can be
obtained by setting AV = 2, N = 218, and M = 18.
N = 8 to 255, Register 0, Programmed via the I2C serial bus
Output Voltage Considerations
M = 0 to 255, Register 1, Programmed via the I2C serial bus
The output drive current capability of the ISL21400 is limited
to ±500µA at the rated accuracy, and the output resistance is
5Ω (maximum) so care must be given when driving loads.
Additionally, the maximum load capacitance capability is
5000 pF so a buffer amplifier should be used if driving large
load capacitance.
VTS = -2.1 mV/ °C * (T – 25 °C )
Av = 1, 2, 4 Register 2, Programmed via the I2C serial bus
or,
VOUT = AV × 1.2 × ( N ⁄ 255 ) + AV × ( – 2.1mV ⁄ °C ) × ( T – 25 ) × ( 2M – 25
(EQ. 9)
This equation can be broken down into two terms, fixed
programmable voltage output and a programmable
dependant voltage output. The fixed programmable voltage
reference with M = 128 (ie, no temperature slope) is shown
in Equation 10:
VOUT = AV × 1.2 × ( N ⁄ 255 )
(EQ. 10)
or,
N = VOUT × 255 ⁄ ( AV × 1.2 )
(EQ. 11)
For example, a 2.052V general purpose voltage reference
can be obtained by setting AV = 2, N = 218, and M = 128.
The programmable temperature dependant voltage output
with N = 0 is:
VOUT = AV × ( – 2.1mV ⁄ °C )x ( T – 25 )x ( 2M – 255 ⁄ 255 )
(EQ. 12)
Taking the first derivative:
dVOUT /dT = AV * (2M – 255) / 255
While it may seem obvious, it must be noted that the
ISL21400 will not generate a proper output voltage that is
outside the Output Voltage Swing values shown in Table 3
even though the VOUT design equation (Equation 1) is
satisfied.
TABLE 3. OUTPUT VOLTAGE CONSIDERATIONS
Output Resistance, ROUT
5Ω maximum
Output Current, IOUT
500µA
Short Circuit Output Current, Isc
±9mA
Load Capacitance
5000pF, maximum
Output Voltage Swing - Unloaded
VCC - 100mV, Gnd + 100mV
Output Viltage Swing - 500 µA
VCC - 250mV, Gnd + 250mV
The ISL21400 exhibits a non-zero output voltage (VOS) at
N = 0 due to the saturation voltage of output amplifier and
offset voltages in the DAC and Summer stages as shown in
Table4. In a closed loop system these are not important
since the VOS error is calibrated out. However, in a high
accuracy open loop system, the VOS error can be significant
and should be considered as discussed in Appendix A.
where VS = -2.1 mV/ °C
TABLE 4.
Solving for M:
M – 255 × ( dVOUT ⁄ dT + AV × VS ) ⁄ ( 2 × AV × VS )
4
AV
TYP VOX
(mV)
1
76
2
110
3
191
(EQ. 13)
AN1446.0
January 16, 2009
Application Note 1446
TABLE 5. ISL21400 REGISTER BIT MAP
ADDR
D7
(MSB)
D6
D5
D4
D3
D2
D1
D0
(LSB)
0
VREF7
VREF6
VREF5
VREF4
VREF3
VREF2
VREF1
VREF0
1
TS7
TS6
TS5
TS4
TS3
TS2
TS1
TS0
2
D7
D6
D5
D4
D3
D2
GAIN 1
GAIN 0
3
D7
D6
D5
D4
D3
D2
D1
D0
4
D7
D6
D5
D4
D3
D2
D1
D0
Application Circuits
Output Noise Filtering
The ISL21400 output voltage noise is typically 90µVP-P in a
0.1Hz to 10Hz bandwidth with AV = 1. Adding load
capacitance up to 5000pF will only result in marginal
improvements in output noise. For high impedance loads, a
low pass filter with an R-C network can be added to filter the
high frequency noise and preserve DC accuracy.
Temperature Compensated Current Source
A simple, yet effective, 2mA current source can be made
with a transistor and resistors as shown in the following
diagram.
Vcc = +5V
Open Loop vs Closed Loop Operating Systems
Many of the applications for the ISL21400 are compensating
voltages in a closed loop system where in-circuit calibration
allows for most accurate operation. From initial calculation to
programming to hardware operation, there is usually
some error that can be re-calibrated out of a system with the
use of production test equipment or an embedded
microprocessor. Any initial ISL21400 error source should be
calibrated out using this technique.
R2
100
IOUT
Vx = 2.65V
However, there may be systems where open loop operation
and more precision is required. Appendix A describes the
calculations for improving the accuracy of the ISL21400 in
open loop systems.
Q1
2N4401
R1
1000
Register Descriptions
FIGURE 2.
TABLE 6.
REGISTER
DESCRIPTION
For this circuit, IOUT = (Vx – Vbe) / R1
ASSIGNED BITS
0
VREF setting (N value)
D0 to D7
1
VTS setting (M value)
D0 to D7
2
AV setting, 1, 2, 4
D0, D1
General purpose storage
D2 to D7
3
General purpose storage
D0 to D7
4
General purpose storage
D0 to D7
I2C Programming (Reference Data Sheet
Information
For applications information on I2C Serial Interface, refer to
the ISL21400 data sheet.
5
IOUT = (2.65 - 0.65) / 1000 (assuming Vbe = 0.65V at +25°C)
IOUT = 2.0mA
Since a transistor Vbe has a temperature coefficient of -2.1
mV/ °C, the output current will also be temperature
dependant.
dIout/dT = dVbe/dT / R1 = -2.1 mV/ °C / 1000
= - 2.1 µa/ °C
At +75 °C, IOUT = 2.0 mA – 2.1 µA/ °C * (75°C – 25°C )
= 2.0mA - 0.105 mA = 1.895mA
The ISL21400 can be used to provide Vx = 2.65V with a
temperature coefficient of -2.1 mV/C to compensate for the
Vbe temperature coefficient as shown in the following
diagram.
AN1446.0
January 16, 2009
Application Note 1446
IOUT = 2.0mA independent of temperature
+5V
Temperature Controlled Current Source
R2
100
+5V
IOUT
ISL21400
I2C BUS
8 VCC
VOUT 7
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
VOUT
A much more accurate current source can be made by using
an op amp in a feedback loop to tightly regulate the output
current controlled by a input voltage source as shown in
Figure 4.
Q1
2N4401
IOUT
+5V
R1
1000
LOAD
VOLTAGE
Vc
IOUT = Vc/Rs
FIGURE 3.
To compensate the temperature drift of Q1’s Vbe of
-2.1mV/°C, the ISL21400 provides a 2.65V reference with a
temperature coefficient of -2.1mV/ °C by appropriate settings
of AV, N, and M as follows:
VOUT = AV × 1.2 × ( N ⁄ 255 )
Rs
FIGURE 4.
(EQ. 14)
N = VOUT * 255/(AV * 1.2)
Set Av = 4 since a 2.65V output is required
If the ISL21400 is used for the source of the control voltage,
Vc, the output current and output current temperature
coefficient can be programmed via the I2C bus with the N
and M registers as shown on the schematic in Figure 5.
N = 2.65 * 255/(4 * 1.2)
N = 140.78
Vx = Av * VREF * (N/255) + Av * VTS * (2M – 255) / 255
Use N = 141
Set Av = 1
dVOUT/dT = AV * Vs * (2M – 255)/255
Vx = 1.2 * (N/255) - 2.1mV/ °C * (T – 25 °C) * (2M – 255)/255
where Vs = -2.1mV/ °C
Resistors R1 and R2 divide down the ISL21400 output
voltage to set the full scale output current based on the value
of Rs.
Solving for M:
(EQ. 15)
M = 255 * (dVOUT /dT * AV * Vs) / (2 * Av * Vs)
M = 255 * (( -2.1mV/ °C * 4 * -2.1mV/ °C ) / (2 * 4 *
-2.1mV/°C)
M = 159.4
Vy = Vx * R2/(R1+R2)
(EQ. 16)
Vy = Vx * 13.7k /(52.3k + 13.7k)
Vy = Vx * .208
Use M = 159
The op amp feedback loop forces IOUT * Rs = Vy
VOUT = AV * 1.2 * (N/255) + AV * (-2.1mV/ °C) * (T – 25) *
(2M – 255)/255
VOUT = 4 * 1.2 * (141/255) + 4 * (-2.1mV/ °C) * (T – 25) *
(2 * 159 – 255)/255
VOUT = 2.65 - 2.1mV/ °C * (T – 25 °C)
For this circuit, IOUT = (VOUT -Vbe) / R1
Vbe = 0.65 + (-2.1mV/ °C )*(T – 25 °C)
IOUT = [2.65 - 2.1mV/ °C * (T – 25 °C) - (0.65 - 2.1mV/ °C *
(T – 25 °C))] / R1
IOUT = Vy / Rs = Vx * 0.208 /Rs
IOUT = Vx * 0.208/.05
IOUT = 4.16 Vx
(EQ. 17)
IOUT = 4.16 * [1.2 * (N/255) - 2.1 mV/ °C * (T – 25 °C) * (2M –
255) / 255 ]
IOUT = 5.0 * (N/255) - 8.75 mA/ °C * (T– 25 °C) *
(2M – 255)/255
IOUT = (2.65 - 0.65) / R1
6
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Application Note 1446
IOUT = 5 (N/255) - 8.75 MA/C * (T - 25 °C) * (2M - 255) / 255
+5V
A1
+5V
ISL21400
I2C BUS
8 VCC
VOUT 7 VX
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
R1
52.3k
VY
3
6
A2
ISL28146
5
4
R2
13.7k
1
2
R3
10
C1
1000pF
LOAD VOLTAGE
Q1
IRLZ44
(1V - 50V)
HEATSINK REQUIRED
R4
1k
Rs
0.05
2W
FIGURE 5. 8-BIT ADJUSTABLE CURRENT SOURCE W/ADJUSTABLE TEMP. CONTROL
The output current at +25 °C and the output current
temperature coefficient can both be programmed via the I2C
bus from values of 150mA to 5A (N = 8 to 255) with a
temperature slope ranging from - 8.75 mA/ °C to + 8.75 mA/
°C (M = 0 to 255).
RFPA LDMOS Bias
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 base stations. 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.
A simplified circuit of an LDMOS amplifier bias circuit is
shown in Figure 6.
VDD
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.
A new way to bias an LDMOS amplifier is presented in
Intersil Application Note AN1385, “LDMOS Transistor Bias
Control in Base Station RF Power Amplifiers Using Intersil
ISL21400” which is summarized in the following. This
Application Note shows using the ISL21400 to set both the
DC bias level and temperature compensation for the VGS
bias, and is shown in the following schematic in Figure 7.
RF OUT
RF IN
LDMOS
TRANSISTOR
BIAS
GENERATOR
FIGURE 6. SIMPLIFIED CIRCUIT
7
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January 16, 2009
Application Note 1446
effects can be minimized as long as the final installation is
similar to the calibration conditions. 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 recalibration of the ISL21400 bias
generator via system level software via the I2C bus.
VDD = 26V
L1
C1
3.9pF
C3
3.9pF
RFIN
Q1
MRF9080
Thermocouple Input Cold Junction
Compensation
R1
1k
+5V
I2C BUS
8
6
5
4
A1
ISL21400
VCC VOUT 7
SDA
A0 3
SCL
A1 2
VSS
A2 1
R2
100
Thermocouples are the industry standard temperature
sensor for measuring a wide range of temperatures from
-250°C to +2300°C. The four most popular thermocouple
types are shown in the table below; however, any time two
dissimilar metals are placed in contact, a thermocouple is
created via the Seebeck Effect.
C4
1000pF
BIAS GENERATOR
TABLE 7.
TEMPERATURE
RANGE
Minimum
Maximum
E
-200°C
+900°C
-8.83
68.79
61.00
-328°F
+1652°F
0°C
+750°C
0.00
42.30
51.70
+32°F
+1382°F
-200°C
+1250°C
-5.89
50.64
40.50
-328°F
+2282°F
-250°C
+350°C
-5.60
17.82
40.70
-328°F
+662°F
J
To determine the N register value:
K
VOUT = Av * 1.2 * (N/255)
dVo/dT
from 0° to
+50°C
(µV/°C)
TYPE
FIGURE 7.
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. The ISL21400 bias generator sets the DC
bias level to 3.0VDC which is the midpoint of the VGS range.
VO at
Tmax
(mV)
VO at
Tmin
(mV)
AV must be 4 for an output voltage of 3.0V.
(EQ. 18)
N = VOUT * 255/(4 * 1.2)
T
N = 3.0 * 255/(4 * 1.2)
N = 159 decimal
Thermocouples present several unique challenges when
interfacing them to a real world measurement system.
To determine the M register value:
dVOUT/dT = -2.8 mV/ °C
dVOUT /dT = AV * (-2.1mV/ °C) * (2M – 255) / 255
(EQ. 19)
-2.8mV/ °C = 4 * (-2.1mV/ °C) * (2M – 255) / 255
M = 170 decimal
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
8
1. Thermocouples generate a very low output voltage that
must be amplified with a high gain amplifier. Each
thermocouple type requires a different gain when
interfacing to a A/D Converter with a fixed full scale
voltage, VFS/VoMAX.
2. Thermocouples do not generate an absolute voltage that
is proportional to temperature. Instead, they generate a
voltage that is a relative voltage that is the proportional to
the temperature difference between the “hot” end and the
“cold” end. All thermocouple tables showing output
voltage vs temperature are for the “cold” end placed in an
ice bath at 0°C. Since it is very impractical to place an ice
bath on a PCB, electronic cold junction compensation is
used. Each thermocouple type requires a cold junction
compensation rate, dVcjc/dT.
3. The output voltage of a thermocouple is non-linear, and is
dependant on the type of thermocouple. Linearization is
most often done with diode break-point techniques or via
AN1446.0
January 16, 2009
Application Note 1446
A1
Y?
EL8173
E, J, K, T TYPE
8 Hz INPUT FILTER
R1
1k
THERMOCOUPLE
C1
10µF
R2
1k
(VTC)
R3 (VCJC)
1M
+5V
A5
1 EN
VS+
7
3 IN+
VS-
4
10 MHZ
16
HI7190 17
OSC1
OSC2
+5V
DVDD 15
+5V
7 AVSS
SCLK 1
CLOCK
(VOUT)
12 VINHI
SDIO 3
DATA I/O
R6
150.0k
11 VINLO
SDO 2
DATA OUT
S
2 INVOUT 6
8 FB+
+5V
13 AVDD
-5V
10 VCM
5 FB-
6 SDA
3 A0
SLAVE
ADDRESS
BUS
(010100 0X)
VOUT 7
R4
357k
2 A1
1 A2
VSS
4
A2
R7
4.53k
1. Hz LPF
ISL21400 S
6 DGND
+5V
8
S
7 RH
VCC
WP-L 1
5 RW
SCL 2
50k
R5
10.7k
SDA 3
6 RL
4
R8
S
1.05k
S
S
DATA READY
RST 18
RESET
MODE 20
9 VRHI
WRITE
PROTECT
I2C BUS
+5V
GND
COLD JUNCTION COMPENSATION
CS
DRDY 5
8 VRLO
C2
10µF
SYNC
CS 4
14 AGND
5 LSC VCC
I2C BUS
SYNC 19
S
8
A4
A3
2 VIN
6
VOUT
GND
R9
2k
1
C4
10µF
4
ISL95810
ISL21009-25
PROGRAMMABLE GAIN
C3
0.01µF
GAIN = 30 - 150
S
FIGURE 8.
microprocessor software, and is not covered in this
Application Note.
The circuit shown in Figure 8 uses the unique features of the
Intersil ISL21400 to provide a programmable cold junction
compensation voltage for each of the standard thermocouple
types.
An Intersil EL8173 Instrumentation Amplifier is used to
simplify the Thermocouple interface to a high resolution A/D
Converter (A5). A programmable gain digital pot (A3) and
the ISL21400 programmable temperature sensor (A2) allows
digital selection of the four most popular thermocouple
types; E, J, K, and T.
Cold junction compensation is programmed via the I2C bus
by the ISL21400 programmable reference and temperature
sensor (A2) and resistor divider network R4 and R5
according to Table 8 with AV = 1 and N register = 0.
TABLE 8.
TC TYPE
Vcjc (µV/ °C)
M REGISTER
E
61.0
0
J
51.7
20
K
40.5
43
T
40.7
43
digital pot for each of the thermocouple types as shown in
Table 9.
TABLE 9.
TC Type
MAXZ VOUT
GAIN
D-POT
CODE10
E
68.97mV
36.34
195
J
42.30mV
59.10
094
K
50.64mV
49.37
126
T
17.82mV
140.3
000
Low pass filters (R1, R2, C1) provide noise filtering with a
8 Hz cut-off frequency. R3 is used for a return current path
for the EL8173’s input bias current. An additional low pass
filter (R4, R5, C2) attenuates the ISL21400’s output noise
voltage with a 1.6Hz cut-off frequency.
A high resolution (24-bit) Sigma-Delta A/D Converter,
HI7190, converts the output of the instrumentation amplifier,
EL8173, with a full scale input voltage of 2.5V set by the
ISL21009 -2.5 voltage reference.
The non-zero output voltage (VOS) of the ISL21400 at N = 0
results in an offset in the temperature reading since for cold
junction compensation the compensating voltage must be
zero voltage at 0°C.
The programmable gain amplifier (A1, A3) provides a gain
from 30 to 150 that is programmed via the I2C bus with the
9
AN1446.0
January 16, 2009
Application Note 1446
In this thermocouple input application circuit, the output of
the ISL21400 is divided by 34.4 by a resistor divider (357k,
10.7k) which divides the VOS by the same amount.
VFB+ = VOS / 34.4
VOS = 76mV (Measured value)
Verror = 76mV/34.4 = 2.21mV
Terror = Verror/dV/dT of TC
(EQ. 20)
Terror = 2.2mV/ 51.7 µV/C For J-type TC
Terror =
+42.5°C
Table 10 shows the temperature error for each TC type and
a VOS typical value of 76mV and maximum value of
100mV. Microprocessor software must subtract the
Temperature Error from the actual reading by measuring the
actual ambient temperature.
TABLE 10.
TEMPERATURE ERROR
TC Type
dV/dT
(µV/C)
VOS = 76mV VOS = 100mV
(C)
(C)
E
61.0
36.2
47.7
J
51.7
42.7
56.2
K
40.5
54.6
71.8
TC TYPE
40.7
54.3
71.4
Using the Intersil ISL21400,
Programmable Temperature Slope
Voltage Reference to Optimize DVD Write
A DVD writer uses one of two laser colors - traditional red
(650nm) and newer blue (405nm) known as Blue-ray. The
laser is used to melt small marks into the underside of the
DVD to record the information. The smaller the marks, the
more marks (and information) can be stored. The writing
process generates a sizable amount of heat that makes the
laser energy less effective at creating distinct marks, which
10
creates signal jitter. Therefore, thermal feedback with a
simple thermistor to monitor temperature is included in DVD
writers to optimize write time. The drawback to this design
choice is that the output voltage of a thermistor is not linear
with respect to temperature and the feedback system must
account for the nonlinearities to be able to operate at a wider
range of temperatures.
Whether a red or Blue-ray optical unit is employed,
continuous recording times for a television episode or a
movie can be sizable. As expected, the current trends
crunch more data in smaller spaces, so the Blue-ray system,
where the wavelength is 37% smaller, is preferred. With
more data to store (HDTV), Blue-ray systems commonly
require longer write times which cause the temperature to
increase more than +60°C, which can harm the laser. In
addition, there will be shift in the laser output wavelength
due to the change in temperature. The system must adapt to
maintain quality marking and protect the laser with this
increase in temperature by temporarily disabling the laser,
changing the laser current, or activating a cooling fan. A
better solution can minimize the disabled time, as well as the
need for a cooling fan by accurately monitoring the
temperature, adjusting the laser current to maintain the
proper wavelength, and intelligently controlling the fan as
needed.
Figure 9 shows the Intersil ISL21400, Programmable
Temperature Slope Voltage Reference in place of a
thermistor in a typical DVD writer application. Since the
Intersil ISL21400, Programmable Temperature Slope
Voltage Reference is a programmable device, the output
voltage range can be linearized as well as matched to the
operating temperature range of the system. The
programmable nature also provides other system design
aids, such as matching the full scale voltage input range of
the ADC (a typical load in DVD systems, see Figure 9) and
maximizing sensitivity. In addition to the advantages listed
previously, the ISL21400 includes 2 bytes of EEPROM that
can be used for storage of laser diode calibration, serial
numbers, manufacturing codes or model numbers. This
would eliminate the need for assemblers to manually enter
this information.
AN1446.0
January 16, 2009
Application Note 1446
ISL21400
DAC
VREF
DAC
+
8x8
EEPROM
+
VOUT
ADC
AV
DSP
SCLK
SDA
ADDR
SEN
INTERFACE
TEMP
SENSOR
COOLING FAN
DSP BLOCK
PMIC
LASER DIODE DRIVER
VHI
BLUE LASER
FIGURE 9. BLOCK DIAGRAM FOR LASER-WRITING USING THE INTERSIL ISL21400 PROGRAMMABLE TEMP - SLOPE VREF
Variable DAC Reference
The Intersil ISL21400, Programmable Temperature Slope
Voltage Reference can easily be used to provide a
programmable reference voltage for either a Digital to
Analog Converter (DAC) or an Analog to Digital Converter. In
the case of the DAC, the full scale output voltage (or current)
could be digitally programmed over a wide range with 8 bits
of resolution. Likewise, the full scale input voltage of an ADC
could be programmed which could be used in place of a
programmable gain amplifier (PGA).
The circuit shown in Figure 10 demonstrates using the
ISL21400 to program the full scale output current of a high
speed 260MHz), 12-bit ISL5857 DAC over the
recommended output current operating range of 2mA to
20mA; operation below 2mA is possible with performance
degradation. In this example, the temperature slope is fixed
at zero by setting M = 128, but it could be adjusted if desired
depending on the needs of the application.
11
AN1446.0
January 16, 2009
Application Note 1446
3.3V
10µH
10µF
27
A2
0.1µF
13, 14, 26
D
3.3V
DVDD
A
IOUTA
10µH
10µF
ISL5857
DCOM
0.1µF
24
AVDD
16
REFLO
20
ACOM
22
50
IOUTB
I2C BUS
8 VCC
VOUT 7
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
17
REFIO
FS ADJ
18
CO PM
23
SLEEP
15
N/C
A1
50
a
21
A
VOUT
1:1
RSET
1.91k
a
0.1µF
19, 25
ISL21400
DATA IN
12 - 1
D0 - D11
CLK
28
A
CLK IN
50
d
FIGURE 10.
By biasing REFLO to AVDD, the ISL5857 internal reference
voltage is disabled, and the ISL21400 output voltage
(N-register value) connected to REFIO determines the full
scale output current (IFS) at IOUTA and IOUTB.
IFS = VREFIO / Rset * 32
Setting M = 128 for zero temperature slope,
ISL21400 output voltage, VOUT = AV * 1.2 * N/255
Setting AV = 1,
ISL21400 output voltage, VOUT = 1.2 * N/255
IFS = 1.2 * N * 32/(255 * Rset)
IFS = 0.15 * N/Rset
In this example with N = 255, Rset = 1.91kΩ
IFS = 20mA
For 2mA full scale output current, N = 2mA * 1.91k/0.15
N = 25
12
Sealed Lead Acid (SLA) Battery Charging
Temperature Compensation
A circuit that is set for the maximum allowable charge
voltage, but has a constant current limit to control the initial
absorption current can produce a very nice charger. This
type of charger can both charge at a reasonable rate and
maintain the battery at full charge without damage. However,
the maximum voltage following the constant charge current
is a function of temperature. A temperature compensated
charger is a little more expensive, and should be used where
the temperature varies significantly from room temperature.
The ISL21400 can be used to program both the charger type
(Cyclic Use or Standby Use) and the temperature
compensation as shown in the following section,
“Temperature Sensor with Programmable Custom Scaling”
on page 14. It must be noted that this section describes the
temperature compensated maximum allowable charge
voltage for a general purpose voltage regulator with either a
linear regulator or switching regulator. The constant charge
current circuit is not shown in this section, and would need to
be included for a complete SLA battery charger.
AN1446.0
January 16, 2009
Application Note 1446
Table 11 is taken from data for a 12V SLA battery from
Applications Information shown on the PowerStream web
site; www.powerstream.com.
TABLE 12.
BATTERY
USE
TABLE 11.
CHARGE VOLTAGE (V)
BATTERY
TEMPERATURE
(°C)
CYCLIC USE
STANDBY USE
0
15.30 to 15.90
13.80 to 14.10
10
14.94 to 15.54
13.68 to 13.98
20
14.58 to 15.18
13.56 to 13.86
25
14.40 to 15.00
13.50 to 13.80
30
14.22 to 14.82
13.44 to 13.74
40
13.86 to 14.46
13.32 to 13.62
50
13.50 to 14.10
13.20 to 13.50
dv/dt
(mV/°C)
BATTERY CHARGING VOLTAGE
Cycle Use
36
VBAT = 14.70 - 36mV/°C * (T - 25°C)
Standby Use
12
VBAT = 13.65 - 12mV/°C * (T - 25°C)
Assuming a general purpose voltage feedback loop typical
of a linear or switching regulator with the ISL21400, refer to
Figure 12.
Note: Figure 12 only shows the voltage regulation loop; there
must be an additional charging current control loop.
Summing currents:
(VBAT -VREF)/R1 + (0 -VREF)/R2 + (Vx - VREF)/R3 = 0(EQ. 21)
Solving for VBAT:
Plotting the midpoint of each Battery Charging Voltage and
calculating the slope (dV/dT) for each battery use:
VBAT = VREF * (1 + R1/R2 + R1/R3) - Vx * R1/R3
16.00
BATTERY VOLTAGE
15.50
CYCLE
USE
Set R1 and R2 for 14 V which is approximately midpoint
between the two +25 °C charging voltages. The actual
+25°C battery voltage will be programmed with the ISL21400
via the N register value.
15.00
14.50
STANDBY
USE
14.00
(EQ. 22)
Let’s assume that the charge must be programmable for
either Cycle Use charging or Standby Use charging for a
12V SLA battery.
If VREF = 1.2V, and we let R2 = 10k, then R1 = 107k for a
14V battery voltage.
13.50
To determine the value for the ISL21400 output voltage, Vx:
13.00
0
10
20
30
40
50
60
Vx = [VREF * (1 + R1/R2 + R1/R3) – VBAT ] * R3/R1 (EQ. 23)
TEMP (°C)
FIGURE 11. 12V SLA BATTERY CHARGING vs TEMPERATURE
Let R3 = 5k (general approximation)
VOLTAGE REGULATOR
VOUT
VBAT
12V
SLA BATTERY
R1
ERROR AMPLIFIER
R3
VX
VREF
R2
A1
+5V
ISL21400
I2C BUS
8 VCC
VOUT 7
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
FIGURE 12.
13
AN1446.0
January 16, 2009
Application Note 1446
For VBAT = 14.70V at +25°C (Cycle use charging),
Vx = 1.169V
VOUT1 = AV * VREF * (N/255) + AV * VS * (T1 - 25) * (EQ. 24)
(2M - 255)/255
For VBAT = 13.65V at +25°C (Standby use charging),
Vx = 1.216V
VOUT2 = AV * VREF * (N/255) + AV * VS * (T2 - 25) * (EQ. 25)
(2M - 255)/255
To determine the ISL21400 N register value:
ISL21400 output voltage Vx = AV * 1.2 * N/255
For example, suppose your Turboencabulator1 project
requires temperature compensation for the flux gate
capacitors required to stabilize the unilateral phase
detectors. Due to the extreme difficulty of obtaining the flux
gate capacitor, three vendors are selected to assure
production inventory. However, each vendor’s flux gate
capacitor requires a much different compensating voltage as
shown in the following Summary table.
Set AV = 2 since the ISL21400 output voltage must be
greater than 1.2V
For Vx = 1.169, N = 124(Cycle use charging)
For Vx = 1.218, N = 129(Standby use charging)
To determine the ISL21400 M register value:
VBAT = VREF * (1 + R1/R2 + R1/R3) – Vx * R1/R3
TABLE 14. SUMMARY TABLE
dVBAT/dT = -dVx/dT * R1/R3
Solving for dVx/dT:
VENDOR
VOUT1
at
T1
(°C)
VOUT2
at
T2
(°C)
dVx/dT = -dVBAT/dT * R3/R1
MaxCap
0.75
+25
1.15
+75
Fox Capacitors
2.15
+25
2.50
+75
I2CGet
3.765
+25
4.000
+75
For Cycle Use charging, dVBAT/dT = 36mV/ °C
dVx/dT = -(36mV/°C ) * 5k/107k = -1.682mV/ °C
The Intersil ISL21400, Programmable Temperature Slope
Voltage Reference can easily be used to provide a
programmable compensation voltage for the flux gate
capacitor that can be programmed via the I2C bus for each
of the vendors shown in Table 14.
ISL21400 output voltage, dVx/dT = AV * (-2.1mV/ °C ) *
(2M - 255)/255
-1.682mV/ °C = 2 * (-2.1mV/ °C ) * (2M - 255)/255
M = 179
For Standby Use charging, dVBAT/dT = 12mV/ °C
+5V
A1
ISL21400
dVx/dT = -(12mV/ °C ) * 5k/107k = .561mV/ °C
ISL21400 output voltage, dVx/dT = Av * (-2.1mV/ °C )*
(2M - 255)/255
I2C BUS
-0.561mV/ °C = 2 * (-2.1mV/ °C ) * (2M - 255)/255
8 VCC
VOUT 7
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
FLUX GATE
CAPACITOR
M = 145
FIGURE 13.
TABLE 13. SUMMARY TABLE
BATTERY
USE
VBAT @
+25°C
dV/dT
(mV/°C)
AV
N
REGISTER
M
REGISTER
Cycle Use
14.70
36
2
124
179
Standby
Use
13.65
12
2
129
145
Temperature Sensor with Programmable
Custom Scaling
Often it is necessary to generate an output voltage based on
a linear relationship between two fixed points for output
voltage vs temperature; i.e., VOUT1 at T1, VOUT2 at T2. This
is easily accomplished with the ISL21400 by applying the
following technique and solving two simultaneous equations
for N and M values.
For the MaxCap flux gate capacitor, the values for VOUT1,
T1, VOUT2, and T2 can be inserted into the Equations 24 and
25.
0.75 = AV * VREF * (N/255) + AV * VS * (25 - 25) *
(2M - 255)/255
(EQ. 26)
1.15 = AV* VREF * (N/255) + AV * VS * (75 - 25) *
(2M - 255)/255
(EQ. 27)
where
AV = 4
VREF= 1.200V
1. General Electric product catalog, Dec. 31, 1962
14
AN1446.0
January 16, 2009
Application Note 1446
Vs = -2.1mV
From the ICL7663S data sheet Equation 1 and 2:
Solving these equations simultaneously either by hand
calculations or a math solving program (MathCAD, TK
Solver) shows that N = 39.8 and M = 6.07 such that:
VOUT = VSET * (1 + R2/R1) + R2/R3 * (VSET - VTC)
(EQ.
VOUT = 1.3 * (1 + 1.8M/2.7M) + 1.8M/300k * (1.3 - VTC
) 28)
N = 40 and M = 6 (Integer values)
VOUT = 10 - 6 * VTC
Applying the same calculations for the other flux gate
capacitor vendors yields the Table 15 for the N and M
values.
Where VTC is .9V with a temperature coefficient of +2.5
mV/C so that:
TABLE 15. SUMMARY TABLE
T1
(°C)
VOUT2
at
T2
(°C)
N
M
MaxCap
0.75
+25
1.15
+75
40
6
Fox
Capacitors
2.15
+25
2.50
+75
114
21
I2CGet
3.765
+25
4.000
+75
200
56
VENDOR
Voltage Regulator Output Voltage
Programming
Often it desirable to design a programmable voltage
regulator such that its output voltage and output voltage
temperature slope can be adjusted under software control. A
common application is a multiplexed LCD display where
temperature has an important effect in the variation of
threshold voltage, as shown in Figure 14.
+5V
V+IN
VOUT1
GND
VOUT2
(EQ. 29)
VTC = 0.9 * (1 + (T - 25) * 2.5mV/C)
VOUT1
at
VOUT = 10 - 6 * .9 * (1 + (T - 25) * 2.5mV/C)
(EQ. 30)
VOUT = 4.6 * (1 + (T - 25) * 15mV/C)
The drawback of this circuit is that the output voltage and
temperature coefficient is fixed by resistor values, and there
is no way to program the output voltage.
The circuit shown below adds the ISL21400 to provide
temperature sensing and the ability to program both the
output voltage and temperature coefficient with the I2C bus
such that:
VOUT = VSET * (1 + R2/R1) + R2/R3 * (VSET – VX)
where VX = Av * VREF * (N/255) + Av * VS * (T-25) *
(2M - 255) / 255
(EQ. 31)
VOUT = 4.6 *
(1 - (T-25) *15mv/C
R2
1.8M
VSET
VTC
R3
301k
ICL7663S
R1
2.7M
FIGURE 14.
15
AN1446.0
January 16, 2009
Application Note 1446
+5V
V+IN
VOUT1
GND
VOUT2
VOUT = 4.6 * (1 - (T-25) * 15 m
v/C)
R1
1.8M
VSET
VTC
ICL7663S
R2
2.7M
R3
301k
+5V
I2C Bus
8 VCC
VOUT 7
6 SDA
A0 3
5 SCL
A1 2
Av = 4
4 VSS
A2 1
N = 48
Vx
M = 90
ISL21400
FIGURE 15.
1, 2 VIN
INPUT VOLTAGE
(2.5V to 5.5V)
LX 13,14,15
10
2 x 22µF
IC
100 k
PGND 11,12
4 SYNC
R2
100k
EP
7 PG
s
FB 8
5 EN
ENABLE
47pF
p
SGND 9,10
s
POWER GOOD
VOUT = 1.8V @ 3.0 A
2 x 22 µF
3 VDD
1µF
1.5 µH
R3
49.9k
R1
32.4k
ISL8013IRZ
S
+5V
8 VCC VOUT 7
I2C BUS
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
VX
ISL21400
FIGURE 16.
In Figure 15, the VTC output is replaced by the ISL21400,
and with the N and M values shown, is identical to the
ICL7663S VTC output. However, the output voltage can be
programmed over a range of 1.3V to 10.0V (Note: The V+IN
voltage supply must be increased to >10.5V), and the
temperature coefficient programmed up to ±50mV/C by
programming the N and M values of the ISL21400.
16
This concept can be applied at any voltage regulator by
summing the output voltage (VX) from the ISL21400 into the
feedback summing node with a resistor (R3). Figure 16uses
the ISL21400 to set the output voltage via the I2C bus of a
ISL8013, 3A switching regulator, from 1.20V to 3.3V as
shown in Table 16.
AN1446.0
January 16, 2009
Application Note 1446
TABLE 16.
VOUT
AV
N
M
3.3
1
161
128
2.5
1
251
128
1.8
2
158
128
1.5
2
175
128
1.2
2
192
128
The basic equation for the ISL21400 output voltage is:
VOUT = AV * 1.2 * (N/255) + AV * (-2.1 mV/ °C) * (T - 25) *
(EQ. 32)
(2M -255) / 255
This equation can be modified to include the effect of VOS
by adding it to the VOUT equation.
NOTE: Since this is an open loop application, in Table16, the
N values have been adjusted to account for the ISL21400
non-zero output voltage, as described in Appendix A. For
additional accuracy, production test measurements can be
used to determine a final N value; at most this might require
a ±1 change in the N value.
VOUT = (VSAT + AV*V0) + ( AV * VREF - VSAT - AV*V0) *
(EQ. 33)
N / 255
The graph in Figure 18 shows the error in the ISL21400
output voltage with the standard VOUT equation and the
modified VOUT equation.
7.0
6.0
Appendix A
NO ZERO
CORRECTION
Improving Accuracy in Open-Loop
Applications
The ISL21400 exhibits a non-zero output voltage at N = 0
due to the output amplifier saturation voltage and input offset
voltage. Lab measurements have shown the output voltage
at N = 0 to be 76mV, 110mV, and 191mV at Av = 1, 2, and 4.
A simple circuit model of the output at N = 0 can be
described by the following equation:
% FS ERROR
5.0
4.0
3.0
ZERO
CORRECTION
2.0
1.0
0.0
-1.0
0
50
100
150
CODE
VOS = VSAT + V0 * Av
250
300
FIGURE 18.
where VSAT is the output amplifier saturation voltage;
approximately 38mV V0 is the offset voltage of the output
amplifier, DAC, and summer circuit; approximately 38mV.
The graph in Figure 17 shows the ISL21400 output voltage,
VOUT, vs Code In; notice the output voltage VOS (76mV) at
N = 0.
To demonstrate the advantage of applying the zero
correction to a real circuit, the ISL21400 was connected to a
linear voltage regulator to program its output voltage over a
range of 1.20V to 5.0V as shown in Figure 19 with
M = 128.
Av = 1
1.40
+6V
VIN
VOUT
1.20
VOUT
ADJ
1.00
VOUT
200
R2
100k
GND
0.80
(VR = 1.22)
0.60
R3
20k
0.40
0.20
+5V
R1
49.9k
0.00
0
50
100
150
CODE IN
200
FIGURE 17. EFFECT OF ZERO CORRECTION, AV=1
250
I2C BUS
8 VCC
VOUT 7
6 SDA
A0 3
5 SCL
A1 2
4 VSS
A2 1
VX
ISL21400
FIGURE 19.
17
AN1446.0
January 16, 2009
Application Note 1446
Where:
VOUT Desired is the desired output voltage
AV is the ISL21400 programmed gain
N is the programmed N value with no zero correction applied
NADJ is the programmed N value with zero correction applied
VOUT Actual is the measured output voltage from the
regulator.
It is interesting to note that the slight output voltage error is a
result of the quantizing error due to the integer values of N;
i.e., for a 2.5V output, the calculated value for N is 149.4 that
must be rounded down to the integer value of 149.
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
18
AN1446.0
January 16, 2009
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