This integrated circuit contains a complete signal conditioning sys-
tem to interface low-level variable impedance transducers to a
digital system. A pair of matched, temperature-compensated cur-
rent sources are provided for balanced transducer excitation fol-
lowed by a precision, high-gain comparator. The output of this
comparator can be delayed by a user-selectable duration, after
which a second comparator will switch complementary outputs
separately activated for diagnostic operation and has an optional
latch with external reset capability. An added feature is a high cur-
rent power source useful as a heater driver in differential tempera-
ture sensing applications. The UC3704 is designed for 0C to
+70C environments.
Bridge Transducer Switch
UC3704
SENSOR TYPE
ACTIVATION SOURCE
Temperature
Pressure
Force
Position
Displacement
Velocity
Shock
Thermistor
X
X
Sensistor
X
X
Thermocouple
X
Semiconductor
X
X
X
Photo Voltaic
X
X
X
Photo Resistive
X
X
X
Strain Gage
X
X
X
X
X
X
Piezoelectric
X
X
X
X
X
Magneto Resistive
X
X
Inductive
X
X
X
X
Hall Effect
X
X
Capacitive
X
Dual Matched Current Sources
High-gain Differential Sensing Circuit
Wide Common-mode Input Capability
Complementary Digital Open-collector
Outputs
Externally Programmable Time Delay
Optional Output Latch with Reset
Built-in Diagnostic Activation
Wide Supply Voltage Range
High Current Heater Power Source Driver
UC3704 COMPATIBLE SENSORS
BLOCK DIAGRAM
6/93
FEATURES
DESCRIPTION
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (+V
IN
). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40V
Output Current (each output) . . . . . . . . . . . . . . . . . . . . . . 50 mA
Buffer Power Source Current . . . . . . . . . . . . . . . . . . . . . 200mA
Comparator 1 Inputs. . . . . . . . . . . . . . . . . . . . . . . -0.5V to V
REF
Comparator 2 Inputs. . . . . . . . . . . . . . . . . . . . . . . . . . . 0 to 5.5V
Remote Activation and Reset Inputs . . . . . . . . . . . . . . 0 to 5.5V
Power Dissipation at T
A
= 25C . . . . . . . . . . . . . . . . . . 1000mW
Operating Junction Temperature . . . . . . . . . . . -55C to +150C
Storage Temperature Range . . . . . . . . . . . . . . -65C to +150C
Lead Temperature (Soldering, 10 Seconds) . . . . . . . . . +300C
Note: Unless otherwise specified, all voltages are with respect
to ground (Pin 1).
Currents are positive into, negative out of the specified
terminal.
Consult Packaging section of Databook for thermal
imitations and considerations of package.
ELECTRICAL CHARACTERISTICS:
PARAMETERS
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Power Inputs
Supply Voltage Range
4.2
36
V
Supply Current
V
IN
= 36V
5
10
mA
Reference Section (with respect to V
IN
)
V
REF
Value |V
IN
- V
REF
|
T
J
= 25C
2.1
2.2
2.3
V
V
REF
Temperature Coefficient
Note 1
-1
-2
-3
mV/C
Line Regulation
V
IN
= 4.2 to 25V
2
10
mV
Load Regulation
I
O
= 0 to 4mA
2
10
mV
Short Circuit Current
V
IN
= 36V, V
REF
= V
IN
or Ground
25
mA
Current Source (Q1 and Q2)
Output Current (Note 2)
Current Set = 10
A
-9
-9.5
-10
A
Current Set = 200
A
-180
-195
-200
A
Output Offset Current
R
E6
= R
E8
= 20k
0
1
A
Comparator One
Input Offset Voltage
1
4
mV
Input Bias Current
-100
-300
nA
Input Offset Current
60
nA
CMRR
V
CM
= 0 to 12V
60
70
dB
Voltage gain
R
L
>150k
70
85
dB
Delay Current Source
34
40
52
A
Output Rise Time
Overdrive = 10mV, C
D
= 15pF, T
J
= 20C
2
V/
s
UC3704
DIL-16 (TOP VIEW)
J or N Package
Unless otherwise stated, these specifications apply for T
A
= 0C to +70C for the
UC3704: V
IH
= 15V, T
A
=T
J.
CONNECTION DIAGRAM
2
UC3704
ELECTRICAL CHARACTERISTICS:
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNITS
Comparator Two (Q
OUT
and Q
OUT
)
Threshold Voltage
2.2
3.0
3.8
V
Threshold Resistance
To Ground
14
20
24
k
Input Bias Current
V
IN
(Pin 12) = 5V
1
3
A
Remote Activate Current
Pin 14 = 0V
0.2
0.5
mA
Reset Current
Pin 13 = 0V
0.2
0.5
mA
Remote Activate Threshold
T
A
= 25C
0.8
1.2
V
Reset Threshold
T
A
= 25C
0.8
1.2
V
Output Saturation
I
OUT
= 16mA
0.2
0.5
V
I
OUT
= 50mA
0.7
2.0
V
Output Leakage
V
OUT
= 40V
0.2
10
A
Output Response
Comp. Overdrive = 1V
R
L
= 5k to V
IN
Turn-on
0.4
s
Turn-off
1.0
s
Buffer
Set Voltage (V
IN
-V
S
)
T
J
= 25C, I
S
= 100mA
1.9
2.1
2.3
V
Drive Current
T
J
= 25C, R
S
= 200
, V
D
= 0V
90
100
120
mA
Note 1: Parameter guaranteed by design, not tested in production.
Note 2: Collector output current =
V
IN
-
V
REF
-
V
BE
R
E
1.5
V
R
E
APPLICATIONS INFORMATION
Sensor Section
The input portion of the UC3704 provides both excitation
and sensing for a low-level, variable impedance
transducer. This circuitry consists of a pair of highly
matched PNP transistors biased for operation as constant
current sources followed by a high gain precision compa-
rator.
The reference voltage at the bases of the PNP transistors
has a TC to offset the base-emitter voltage variation of
these transistors resulting in a constant voltage across
the external emitter resistors and correspondingly con-
stant collector currents for balancing, offsetting, or to pro-
vide unique temperature characteristic.
With the PNP transistor's optimum current ranging from
10 to 200
A, and the common-mode input voltage of the
comparator usable from ground to (V
IN
- 3V), a wide
range of transducer impedance levels is possible.
The sensor comparator has a current source pull-up at
the output so that an external capacitor from this point to
ground can be used to provide a programmable delay be-
fore reaching the second comparator's threshold. The
low-impedance on-state of Comp 1's output provides
quick reset of this capacitor. This programmable delay
function is useful for providing transient protection by re-
quiring that Comp 1 remain activated for a finite period of
time before Comp 2 triggers. Another application is in
counting repetitive pulses where a missing pulse will al-
low Comp 1's output to rise to Comp 2's threshold. This
time delay function is:
Delay
=
Comp
2
Threshold
Delay Current
X
C
D
175
ms
/
F
If hysteresis is desired for Comparator 1, it may be ac-
commodated by applying positive feedback from the de-
lay terminal to the non-inverting input on Pin 7. This will
aid in providing oscillation-free transitions for very slowly
changing inputs.
Unless otherwise stated, these specifications apply for T
A
= 0C to +70C for the
UC3704: V
IH
= 15V, T
A
=T
J.
3
Output Section
The output portion of the UC3704 is basically a second
comparator with complimentary, open-collector outputs.
This comparator has a built-in, ground-referenced thresh-
old implemented with a high-impedance current source
and resistor so that it may be easily overridden with an
external voltage source if desired. Comp 2's input tran-
sistors are NPN types which require at least 1V of com-
mon-mode voltage for accurate operation and should not
see a differential input voltage greater than 6V.
For diagnostic or latching purposes, the output logic is
equipped with a Remote Activate and Reset function.
These pins have internal pull-ups and are only active
when pulled low below a threshold of approximately 1V. A
low signal at the Remote Activate Pin causes the outputs
to change state in exactly the same manner as if Comp
2's input is raised above the threshold on Pin 11. If Pin
16 is connected to Pin 14, positive feedback results and
the outputs will latch once triggered by Comp 2's input.
Pulling the Reset terminal low overrides the Remote Acti-
vate Pin releasing the latch.
Reference Buffer
This circuit is designed to provide up to 100mA to drive a
high current external PNP transistor useful for powering
a heater for differential temperature measurements. Care
must be taken that power dissipation in Q6 does not
cause excessive thermal gradients which will degrade
the accuracy of the sensing circuitry.
Using a heating element attached to a temperature sen-
sitive resistor, RS1, in one leg of the input bridge imple-
ments a flow sensor for either gasses or liquids. As long
as there is flow, heat from the element is carried away
and the sensor voltage remains below threshold. Using
an identical sensor, RS2, without a heater to establish
this threshold compensates for the ambient temperature
of the flow.
UC3704
TYPICAL APPLICATION FOR MONITORING LIQUID OR GAS FLOW
UNITRODE INTEGRATED CIRCUITS
7 CONTINENTAL BLVD.
MERRIMACK, NH 03054
TEL. (603) 424-2410
FAX (603) 424-3460
APPLICATIONS INFORMATION (cont.)
4
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1999, Texas Instruments Incorporated
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