BB XTR501

®
XTR501
HIGH CURRENT BRIDGE DRIVER
and 4-20mA Transmitter
FEATURES
DESCRIPTION
● SENSOR EXCITATION OF 1W
● VARIABLE EXCITATION VOLTAGE:
1.5V to 5.0V
● SINGLE SUPPLY: 11.4V to 30VDC
● INRUSH CURRENT LIMITING
The XTR501 contains a high efficiency DC/DC converter and 4-20mA three wire current transmitter. It
provides regulated bridge excitation, optional half
bridge, differential inputs and current transmitter necessary for the excitation and signal conditioning of
low impedance bridge sensors and high integrity signal transmission.
● 4-20mA TRANSMITTER
The DC/DC converter is capable of supplying 1W into
a regulated bridge voltage of 1.5V to 5.0V from a
supply of 11.4V to 30V. The combination of a low
startup current and high efficiency current step-up
allows for a combined supply line resistance of up to
100Ω when exciting low impedance sensors.
APPLICATIONS
● GAS DETECTION SENSORS
● PELLISTOR CATALYTIC DETECTORS
● STRAIN GAGES
● HIGH CURRENT BRIDGES
● LOAD CELLS
The instrumentation amplifier of the current transmitter can be used over a wide range of gains, accommodating a variety of input signals and sensors.
● HOT-WIRE ANEMOMETERS
The XTR501 is particularly suited to excitation of
high current/low impedance sensors used in bridge
applications allowing the use of lighter cabling leading to considerable savings on cabling costs.
RSPAN
+24V
GA
GB
+VS
Long Cables
XTR501
+VIN
–VIN
IOUT
VBB
4-20mA
VOUT
+VB
RSENSE
DC/DC
Com
VBSET
RSET
International Airport Industrial Park • Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Tel: (520) 746-1111 • Twx: 910-952-1111 • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132
©
1994 Burr-Brown Corporation
PDS-1212
Printed in U.S.A. October, 1994
SPECIFICATIONS
ELECTRICAL
TA = +25°C, VS = 24V, VBRIDGE = 2V, ILOAD = 300mA unless otherwise specified.
XTR501
PARAMETER
CONDITIONS
MIN
INSTRUMENTATION AMPLIFIER/CURRENT TRANSMITTER
SIGNAL OUTPUT
Output Current Equation
RG in Ω, VIN in V
Output Current
Linear Operating Range
Over-scale Limit
Under-scale Limit
ZERO
Output Current
Offset Error
vs Temperature
vs Supply Voltage
SPAN
Span Equation
Untrimmed Error
4
±50
0.2
0.5
VS = 11.4V to 30V
G = 1, IO = 4mA to 20mA
INPUT
Common-Mode Range
Offset Voltage
vs Temperature
vs Supply Voltage
Common-Mode Rejection
Impedance; Differential, Common-Mode
DC/DC CONVERTER
BRIDGE EXCITATION VOLTAGE SOURCE
Output Voltage
vs Temperature
vs Long Term Stability
Output Power
Line Voltage Regulation
Load Voltage Regulation
Output Voltage Ripple
Output Voltage Ripple Frequency
Output Short-Circuit Current
Input Current
MAX
IO = 0.004 + 0.016 [(1 + 50kΩ/RG)/4.94] VIN
4
20
25
27
0
RG in Ω,VIN in V
G=1
G = 250
vs Temperature
Nonlinearity
TYP
±100
2
UNITS
A
mA
mA
mA
mA
µA
µA/°C
µA/V
Span = 0.016[(1 + 50kΩ/RG)/4.94]
±0.2
±2.5
±1.5
±10
50
±0.025
A/V
%
%
ppm/°C
%
4.94(1)
V
mV
µV/°C
dB
dB
Ω || pF
5
0
16
50
75
85
1010 || 6
VIN = 0V, G= 1
1.5
200
100
VS = 11.4V to 30.0V
Load Current 160mA to 340mA
Load Voltage 2V
Load Current 300mA
Load Voltage 2V
0.25
0.25
V
ppm/°C
ppm/1000hrs
W
%
%
150
mV
100
2.6
150
kHz
A
mA
1
Limited Duration
Output Short-Circuit
POWER SUPPLY
Supply Voltage, VS
Supply Current
11.4
TEMPERATURE
Operating
Storage
–40
–40
24
See Typical Curve
30
V
+70
+85
°C
°C
NOTE: (1) Common-Mode Range is based on a multiple of a bandgap reference of 1.235V.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
®
XTR501
2
ABSOLUTE MAXIMUM RATINGS
ORDERING INFORMATION
Supply Voltage, VS ............................................................................... 30V
Input Voltage ....................................................................................... 10V
Output Power ........................................................................................ 1W
Operating Temperature Range ............................................. 0°C to +70°C
Storage Temperature Range ............................................ –40°C to +85°C
MODEL
PACKAGE
TEMPERATURE
RANGE
XTR501
24-Pin Plastic Module
–40°C to +70°C
PACKAGE INFORMATION
PIN CONFIGURATION
Top View
MODEL
PACKAGE
PACKAGE DRAWING
NUMBER(1)
XTR501
24-Pin Plastic Module
902
NOTE: (1) For detailed drawing and dimension table, please see end of data
sheet, or Appendix D of Burr-Brown IC Data Book.
GBSET
1
24 GASET
GBSENSE
2
23 GASENSE
–VIN
3
22 +VIN
PIN DESCRIPTION
PIN
NAME
DESCRIPTION
1
GBSET
Connect to RSPAN to set transconductance.
2
GBSENSE
3
–VIN
Inverting input to transmitter.
10
IOUT
Output Current connect through RSENSE to
common.
XTR501
18 VBB
17 –VB
16 +VB
IOUT 10
15
+VBSENSE
Common 11
14
VBSET
VS 12
11
Common
12
VS
14
VBSET
15
+VBSENSE
Sense pin for GBSET connect to GASET.
Supply return for sense and VB connectors.
Supply to XTR501 +11.4V to 30.0V.
Single resistor to common sets the bridge
excitation voltage.
Positive bridge sense input connect to positive
excitation voltage at bridge.
16
+VB
Positive bridge excitation voltage.
17
–VB
Negative bridge excitation voltage.
18
VBB
Output from internal half bridge connect to –VIN.
22
+VIN
Non-Inverting input to transmitter.
23
GASENSE
24
GASET
Sense pin for GASET connect to GASET.
Connect to RSPAN to set transconductance.
®
3
XTR501
TYPICAL PERFORMANCE CURVES
TA = +25°C, VS = 24V, VBRIDGE = 2V, ILOAD = 300mA unless otherwise specified.
NON-LINEARITY (Low Gain)
NON-LINEARITY (High Gain)
25
25
20
VBRIDGE = 2.0V, ILOAD = 300mA, VS = 24V
15
10
5
5
m%
10
0
0
–5
–5
–10
–10
–15
–15
–20
–20
–25
–25
0
1
2
3
4
0
5
5
10
15
20
25
VIN (V)
VIN (mV)
SMALL SIGNAL TRANSIENT RESPONSE
LARGE SIGNAL TRANSIENT RESPONSE
RLOAD = 250Ω
RLOAD = 250Ω
VIN
IO
IO
VIN
IO = 4-20mA, VIN = 27mV
IO = 4-20mA, VIN = 5V
5ms/Div
2ms/Div
OUTPUT NOISE
FFT of OUTPUT NOISE
NOTES: IO into 250Ω.
(1) No smoothing. (2) 5µF capacitor across 250Ω.
2mV/Div
310kHz
100mV/Div
m%
20
VBRIDGE = 2.0V, ILOAD = 300mA, VS = 24V
15
(1)
(2)
1µs/Div
100kHz/Div
®
XTR501
4
30
TYPICAL PERFORMANCE CURVES (CONT)
TA = +25°C, VS = 24V, VBRIDGE = 2V, ILOAD = 300mA unless otherwise specified.
ZERO CURRENT TC (µA/°C)
MAXIMUM CURRENT LOOP RESISTANCE
20
30
15
Number of Units
Supply Voltage (V)
Operating point must
be within this region
20
Example:
24V Supply
RMAX = 700Ω
10
10
5
0
0
300
400
500
600
700
800
0
900
0.05
0.1
0.15
0.2
0.25
Zero Current Drift (µA/°C)
Average = 0.100µA/°C
3σ = 0.115µA/°C
Resistance (Ω)
UNTRIMMED ERROR TC G = 1 (ppm/°C)
BRIDGE VOLTAGE LINE REGULATION
14
0.1
12
0.08
10
0.06
8
0.04
%
Number of Units
VBRIDGE = 2.0V, ILOAD = 300mA, VS = 12V - 30V
6
0.02
4
0
2
–0.02
0
0
10
20
30
40
–0.04
50
12
18
0.1
VBRIDGE = 2.0V, ILOAD = 300mA, VS = 24V
0.04
VBRIDGE = 2.0V, VS = 24V
Bridge Voltage Change (%)
Bridge Voltage Change (%)
30
BRIDGE VOLTAGE REGULATION vs TEMPERATURE
BRIDGE VOLTAGE LOAD REGULATION
0.05
0.03
0.02
0.01
0
–0.01
0
0.1
0.2
0.3
0.4
0.5
–0.02
–0.03
0.06
24
VS (V)
Untrimmed Error TC Drift (ppm/°C)
Average = 21ppm/°C
3σ = 25ppm/°C
0.6
0.12
0.18
–40
0.24
–10
25
55
85
Temperature (°C)
Output Current (A)
®
5
XTR501
TYPICAL PERFORMANCE CURVES (CONT)
TA = +25°C, VS = 24V, VBRIDGE = 2V, ILOAD = 300mA unless otherwise specified.
SUPPLY CURRENT vs EXCITATION LOAD CURRENT
110
14
100
12
Supply Current (mA)
Number of Candidates
BRIDGE EXCITATION VOLTAGE TC (ppm/°C)
16
10
8
6
4
Bridge Voltage (VEX) = 2.0V
90
0V
VS
=3
80
4V
VS
70
=2
18V
VS =
V
13.5
VS =
60
50
40
2
30
20
0
0
30
60
90
120
0
150
100
200
300
400
500
ILOAD (mA)
Bridge Excitation Voltage Temperature Drift (ppm/°C)
Average = 70ppm/°C
3σ = 70ppm/°C
METHOD OF OPERATION
APPLICATIONS
The XTR501 consists of a high efficiency DC/DC converter
with current and voltage mode control and a current loop
transmitter.
The pulse-width modulation controller monitors the current
and voltage control signals and varies the conduction period
to regulate the bridge excitation voltage, VB.
The XTR501 is designed to be used with a wide range of
pellistor catalytic gas detectors. The pellistor gas detector
consists of a matched pair of elements; an active bead which
is the sensing element, and an inactive bead which is the
compensating element. These elements form one side of a
Wheatstone bridge arrangement. The bridge serves a dual
purpose: to raise the temperature of the elements to about
500°C, which is their working temperature, and to allow
detection of the presence of combustible gases through imbalance of the bridge. This happens as the pellistor increases
temperature due to oxidation of the flammable gas and thus
increases its resistance.
A soft-start feature is provided to negate problems caused by
high in-rush currents and lead resistances, thus allowing the
XTR501 to be driven through cables of up to 100Ω of supply
line resistance with no reduction in performance.
A single resistor, RSET, determines the regulated bridge
excitation voltage which may be in the range 1.5V to 5.0V.
The gain of the transconductance amplifier, which forms the
current loop transmitter, is again determined by a single
resistor RSPAN and allows input voltages from 25mV to drive
the 4-20mA current loop.
LEL Methane
Bridge Voltage VEX
RSET
IO
RSPAN
=
=
=
=
=
5% = VIN = 50mV
2.0V
21.858kΩ
4-20mA
510Ω
470µH
+24V
Compensator
10kΩ
12
XTR501
16 +V
B
15 +VBSENSE
RT
In general, pellistor catalytic gas detectors are limited in use
to monitoring up to 100% of the LEL (lower explosive
limit). Beyond this point ambiguous results can occur due to
the inability of the pellistor to oxidize the gas as the available oxygen decreases (see Figure 3).
+
VS
GASET
23
GASENSE
100Ω
18 VBB
RSPAN
SENSE
(1)
Detector
17 – V
B
11
24
COM
–VIN
3
10µF
Tantalum
1
GB
100Ω
2
GBSET
IO 10
14
VBSET
VOUT = IO RLOAD
IO = 4 to 20mA
RSET
VOUT
RLOAD
250Ω
+
5µF
Tantalum
+VIN
22
NOTE: (1) VQ 21 Gas Sensor - EEV Gas Sensors.
FIGURE 1. Basic Connection
®
XTR501
6
1V to 5V
100
RT
Bridge Output (mV)
100µH
Compensator
For additional bridge
smoothing, 100µH
inductors may be
inserted as shown.
Note that inductors
must be able to handle
high bridge current.
Detector
50
100µH
0
10
20
30
40
50
60
70
80
90
% Methane In Concentration
FIGURE 3. Typical Pellistor Response.
FIGURE 2. Bridge Smoothing.
An interesting feature of pellistor elements, and those tested
in the applications, is that they create a similar bridge output
at LEL for almost all hydrocarbons. This feature allows a
comparative measurement to be made of one gas when an
instrument has been calibrated for another.
10
X
T
R
5
0
1
e.g. Instrument calibrated for Methane (K=112) measuring
Hydrogen (K=85.8).
Actual % of LEL will be 1.31 x meter reading. (112/85.8).
This is an approximation and it is recommended that for
exact conversions the instrument should be calibrated using
the relevant gases.
Tables of these constants can be sourced through the manufacturers of gas sensing products.
An example of the XTR501 used with a pellistor catalytic
gas detector is shown in Figure 1.
To Calculate RSET
See Figure 4. Point (a) will be maintained at 1.235V.
(10kΩ ) (1. 235)
=
5 ( V B – 1. 235)
1. 235
–1=
R SET =
Example:
11
(a)
+VBSET
RSET
50kΩ
To Calculate RSPAN
I O = 0. 004 + 0. 016
 1 + 50kΩ / R SPAN 
V

 IN
4. 94
For VIN = 10mV and IO = 20mA
0. 02 = 0. 004 + 0. 016
 1 + 50kΩ / R SPAN 
0. 01


4. 94
1 + ( 50kΩ / R SPAN )
0. 016
=
4. 94
( 0. 016 ) ( 0. 01)
50kΩ + R SET
R SET
= 1+
10kΩ
FIGURE 4. Internal Circuit +VBSET.
V B – 1. 235 1. 235 ( 50kΩ + R SET )
=
10kΩ
R SET 50kΩ
50kΩ ( V B – 1. 235)
+VBSENSE
50kΩ
R SET
494 = 1 + 50kΩ / R SPAN
R SPAN =
50kΩ
R SET
= 101. 4
(50kΩ ) (1. 235)
5 ( V B − 1. 235) – 1. 235
VB
RSET Calculated
1.5
2.0
3.0
4.0
5.0
Open
21.858kΩ
7.891kΩ
4.815kΩ
3.464kΩ
50kΩ
493
i.e
4. 94
50kΩ
= 1+
V IN
R SPAN
R SPAN =
50kΩ
4. 94
–1
V IN
®
7
XTR501