LTC1042
Window Comparator
FEATURES
■
■
■
DESCRIPTIO
■
■
■
■
Micropower 1.5µW (1 Sample/Second)
Wide Supply Range — 2.8V to 16V
High Accuracy
Center Error
±1mV
Max
Width Error
±0.15%
Max
Wide Input Voltage Range
V
+
to Ground
TTL Outputs with 5V Supply
Two Independent Ground-Referred Control Inputs
Small Size 8-Pin MiniDIP
The LTC
®
1042 is a monolithic CMOS window comparator
manufactured using Linear Technology’s enhanced
LTCMOS
™
silicon gate process. Two high impedance
voltage inputs, CENTER and WIDTH/2, define the middle
and width of the comparison window. Whenever the input
voltage, V
IN
, is inside the window the WITHIN WINDOW
output is high. The ABOVE WINDOW output is high
whenever V
IN
is above the window. By interchanging V
IN
and CENTER, the ABOVE WINDOW output becomes
BELOW WINDOW and is high if V
IN
is below the window.
Sampling techniques provide high impedance voltage
inputs that can common mode to both supply rails
(V
+
and GND). An important feature of the inputs is their
non-interaction. Also the device is effectively “chopper
stabilized,” giving it extremely high accuracy over
all conditions of temperature, power supply and input
voltage range.
Another benefit of the sampling techniques used to design
the LTC1042 is the extremely low power consumption.
When the device is strobed, it internally turns on the power
to the comparators, samples the inputs, stores the outputs
in CMOS latches and then turns off power to the compara-
tors. This all happens in about 80µs. Average power can be
made small, almost arbitrarily, by lowering the strobe rate.
The device can be self-strobed using an external RC
network or strobed externally by driving the OSC pin with
a CMOS gate.
APPLICATIO S
■
■
■
Fault Detectors
Go/No-Go Testing
Microprocessor Power Supply Monitor
, LTC and LT are registered trademarks of Linear Technology Corporation.
LTCMOS is a trademark of Linear Technology Corp.
TYPICAL APPLICATIO
V
+
Battery-Powered Remote Freezer Alarm
10000
150k
150k
1
2
3
3V TO 16V
R1*
7.5k
T
4
LTC1042
8
7
6
5
0.05µF
R2*
576Ω
Total Supply Current
vs Sampling Frequency
V
+
= 6V
TOTAL SUPPLY CURRENT, I
T
(µA)
I
T
“HI” = TEMPERATURE
BETWEEN
10M
±5%
26°F AND 31°F
±1°F
“HI” = TEMPERATURE
ABOVE 31°F
±1°F
1000
I
T
100
10
LTC1042 SUPPLY
CURRENT
1
0.1
0.01
FOR THIS APPLICATION
f
S
≈
1HZ
0.1
1
10
100
1000
SAMPLING FREQUENCY, f
S
(Hz)
10000
T = YELLOW SPRINGS INSTRUMENT CO., INC. P/N 44007
ALL RESISTORS
±1%
UNLESS OTHERWISE SPECIFIED
*OTHER TEMPERATURE BANDS MAY BE SELECTED BY CHOOSING APPROPRIATE VALUES FOR R1 AND R2
LTC1042 • A01
U
LTC1042 • TA02
U
U
1042fa
1
LTC1042
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
WITHIN
WINDOW 1
CENTER 2
V
IN
GND
3
4
8
7
6
5
V
+
OSC
ABOVE
WINDOW
WIDTH / 2
Total Supply Voltage (V
+
to GND) ............................ 18V
Input Voltage ..................................... V
+
+0.3V to –0.3V
Operating Temperature Range
LTC1042C ......................................... –40°C to 85°C
LTC1042M
(OBSOLETE)
................. –55°C to 125°C
Storage Temperature Range ................. –55°C to 150°C
Lead Temperature (Soldering, 10 sec).................. 300°C
Output Short Circuit Duration ....................... Continuous
ORDER
PART NUMBER
LTC1042CN8
N8 PACKAGE
8-LEAD PDIP
T
JMAX
= 110°C,
θ
JA
= 150°C/W
J8 PACKAGE
8-LEAD CERDIP
LTC1042MJ8
OBSOLETE PACKAGE
Consider the N8 Package as an Alternate Source
LTC1042 • POI01
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL
PARAMETER
Center Error (Note 3)
TEST CONDITIONS
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
MIN
●
TYP
±0.3
+
±0.05
±1
+
±0.05
±0.6
+
±0.1
±2
+
±0.1
±0.3
MAX
±1
+
±0.15
±3
+
±0.15
±2
+
±0.3
±6
+
±0.3
UNITS
mV
% WIDTH/2
mV
% WIDTH/2
mV
% WIDTH/2
mV
% WIDTH/2
nA
MΩ
V
+
= 2.8V to 6V (Note 2)
V
+
= 6V to 15V (Note 2)
●
Width Error (Note 4)
V
+
= 2.8V to 6V (Note 2)
●
V
+
= 6V to 15V (Note 2)
●
I
BIAS
R
IN
Input Bias Current
Average lnput Resistance
Input Voltage Range
V
+
= 5V, T
A
= 25°C, OSC = GND
V
IN
, CENTER and WIDTH/2 Inputs
f
S
= 1kHz (Note 5)
●
●
●
10
GND
2.8
15
V
+
16
1.2
0.001
0.001
80
3
0.5
5.0
100
PSR
I
S(ON)
I
S(OFF)
T
D
V
OH
V
OL
Power Supply Range
Power Supply ON
Current (Note 6)
Power Supply OFF
Current (Note 6)
Response Time (Note 7)
Output Levels
Logic 1 Output
Logical 0 Output
V
+
= 5V
V
+
= 5V, LTC1042C
LTC1042M
V
+
= 5V
V
+
= 4.75V, l
OUT
= –360µA
V
+
= 4.75V, l
OUT
= –1.6mA
●
●
●
●
●
2.4
4.4
0.25
0.45
2
U
V
V
mA
µA
µA
µs
V
V
1042fa
W
U
U
W W
W
LTC1042
ELECTRICAL CHARACTERISTICS
SYMBOL
R
EXT
f
S
PARAMETER
External Timing Resistor
Sampling Frequency
TEST CONDITIONS
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C.
MIN
●
TYP
MAX
10,000
UNITS
kΩ
Hz
Resistor connected between V
+
and OSC Pin
V
+
= 5V, T
A
= 25°C
R
EXT
=1MΩ, C
EXT
= 0.1µF
100
5
Note 1:
Absolute Maximum Ratings are those values beyond which the life
of a device may be impaired.
Note 2:
Applies over input voltage range limit and includes gain
uncertainty.
Note 3:
Center error = [(V
U
+V
L
)/2 – CENTER] (where V
U
= upper band
limit and V
L
= lower band limit).
Note 4:
Width error = (V
U
–V
L
– 2 • WIDTH/2) (where V
U
= upper band
limit and V
L
= lower band limit).
Note 5:
R
IN
is guaranteed by design and is not tested. R
IN
= 1/(f
S
x 66pF).
Note 6:
Average supply current = T
D
• l
S(ON) •
f
S + (1 –
T
D
f
S)
I
S(OFF)
.
Note 7:
Response time is set by an internal oscillator and is independent
of overdrive voltage. T
D
is guaranteed by correlation test and is not directly
measured.
TYPICAL PERFOR A CE CHARACTERISTICS
I
S(ON)
vs V
+
20
NORMALIZED SAMPLING FREQUENCY
(f
S
AT 5V, 25°C)
18
16
14
T
A
= 125°C
1.6
1.4
1.2
1.0
0.8
0.6
T
A
= – 55°C
0
2
8
10 12
4
6
SUPPLY VOLTAGE, V
+
(V)
14
16
T
A
= 25°C
SAMPLE RATE, f
S
(Hz)
IS(ON) (mA)
12
10
8
6
4
2
0
2
4
10
8
6
12
SUPPLY VOLTAGE, V
+
(V)
125°C
–55°C
Response Time vs Supply Voltage
110
T
A
= 25°C
100
130
120
V
+
= 5V
RESPONSE TIME, t
D
(µs)
RESPONSE TIME, t
D
(µs)
110
100
90
80
70
60
50
AVERAGE INPUT RESISTANCE, R
IN
(1/F
S
• 66pF)
(Ω)
90
80
70
60
50
2
4
10
14
8
12
6
+
SUPPLY VOLTAGE, V (V)
16
U W
25°C
14
Normalized Sampling Frequency
vs V
+
, Temperature
2.2
2.0
1.8
R = 1M, C = 0.1µF
Sampling Rate vs R
EXT
C
EXT
10
3
C
EXT
= 1000pF
10
2
C
EXT
= 0.01µF
10
C
EXT
= 0.1µF
1
C
EXT
= 1µF
0.1
100k
C
EXT
= 0.05µF
16
1M
R
EXT
(Ω)
10M
LTC1042 • TPC03
LTC1042 • TPC01
LTC1042 • TPC02
Response Time vs Temperature
10
11
R
IN
vs Sampling Frequency
10
10
10
9
10
8
40
–50
10
7
1
0
25
–25
50
75 100
AMBIENT TEMPERATURE, T
A
(°C)
125
10
10
2
10
3
SAMPLING FREQUENCY, f
S
(Hz)
10
4
LTC1042 • TPC04
LTC1042 • TPC05
LTC1042 • TPC06
1042fa
3
LTC1042
APPLICATIO S I FOR ATIO
The LTC1042 uses sampled data techniques to achieve its
unique characteristics. It consists of two comparators,
each of which has two differential inputs (Figure 1). When
the sum of the voltages on a comparator’s inputs is
positive, the output is high; when the sum is negative, the
output is low. The inputs are interconnected such that
when (CENTER – WIDTH/2)
≤
V
IN
≤
(CENTER + WIDTH/2)
both comparator outputs are low. In this condition V
IN
is
within the window and the WITHIN WINDOW output is
high. When V
IN
> CENTER + WIDTH/2, V
IN
is above the
window and the ABOVE WINDOW output is high.
An important feature of the LTC1042 is the non-interaction
of the inputs. This means the center and width of the
window can be changed without one affecting the other.
Also note that the width of the window is set by a ground
referred signal WlDTH/2).
Strobing
An internal oscillator allows the LTC1042 to strobe itself.
The frequency of oscillation sets the sampling rate and is
set with an external RC network (see typical curve, OSC
frequency vs R
EXT
, C
EXT
). To assure oscillation, under all
conditions, R
EXT
must be between 100kΩ and 10MΩ.
There is no limit to the size of C
EXT
.
A sampling cycle is initiated on the positive going transi-
tion of the voltage on the OSC pin. When this voltage is
near the positive supply, a Schmitt trigger trips and
initiates the sampling cycle. A sampling cycle consists of
applying power to both comparators, sampling the inputs,
WINDOW
CENTER
(V
IN
)
2
+
–
+
–
+
–
+
–
COMP A
4
WINDOW
CENTER
V
+
WITHIN
WINDOW
ABOVE
WINDOW
V
IN
(WINDOW
CENTER)
WIDTH/2
1 WITHIN WINDOW
3
ABOVE WINDOW
(BELOW WINDOW)
COMP B
6
5
OUTPUT VOLTAGE (V)
GND
4
TIMING
GENERATOR
4
0V
POWER ON
V
L
INPUT VOLTAGE, V
IN
POWER OFF
80µs
V
U
OSC
7
(A)
Figure 1. LTC1042 Block Diagram
4
U
storing the results in CMOS output latches and turning the
power off. This whole process takes approximately 80µs.
During the 80µs “active” time, the LTC1042 draws
typically 1.2mA (l
S(ON)
) at V
+
= 5V. Because power is
consumed only during the “active” time, extremely low
average power consumption can be achieved at low sample
rates. For example, at a sample rate of 1 sample/second
the average power consumption is:
Power = (V
+
) (I
S(AVG)
) = 5V • 1.2mA • 80µs/1sec
= 0.48µW
At low sampling rates, R
EXT
dominates the power con-
sumption. R
EXT
consumes power continuously. The aver-
age voltage at the OSC pin is approximately V
+
/2. The
power consumed by R
EXT
is:
P(R
EXT
) = (V
+
/2)
2
R
EXT
Example: Assume R
EXT
= 1MΩ and V
+
= 5V. Then:
P(R
EXT
) = (2.5)
2
/1MΩ = 6.25µW
This is more than ten times the typical power consumed by
the LTC1042 at V
+
= 5V and 1 sample/second. Where
power is a premium, R
EXT
should be made as large as
possible. Note that the power dissipated by R
EXT
is
not
a
function of the sampling frequency or C
EXT
.
If high sampling rates are needed and power consumption
is of secondary importance, a convenient way to get the
maximum possible sampling rate is to make R
EXT
= 100kΩ
and C
EXT
= 0. The sampling rate, set by the LTC1042’s
active time, will nominally be
≈
10kHz.
8 V
+
–WIDTH/2
WIDTH/2
W
U U
(B)
LTC1042 • AI01
1042fa
LTC1042
APPLICATIO S I FOR ATIO
To synchronize the sampling of the LTC1042 to an external
frequency source, the OSC pin can be driven by a CMOS
gate. A CMOS gate is necessary because the input trip
points of the oscillator are close to the supply rails and TTL
does not have enough output swing. Externally driven,
there will be a delay from the rising edge of the OSC input
and the start of the sampling cycle of approximately 5µs.
Input Impedance
The input impedance of the LTC1042 does not look like a
classic linear comparator; CMOS switches and a precision
capacitor array form the dual differential input structure.
Input impedance characteristics can be determined from
the equivalent circuit shown in Figure 2. The input
capacitance will charge with a time constant of R
S
• C
IN
. It
is critical, in determining errors caused by the input
charging current, that C
IN
be fully charged during the
“active” time.
For R
S
≤
10kΩ
For Rs less than or equal to 10kΩ, C
IN
fully charges and no
error is caused by the charging current.
For R
S
> 10kΩ
For source resistances greater than 10kΩ, C
IN
cannot fully
charge, causing voltage errors. To minimize these errors
an input bypass capacitor, C
S
should be used. Charge is
shared between C
IN
and C
S
causing a voltage error. The
magnitude of this error is
∆V
= V
IN
x C
IN
/(C
IN
+ C
S
). This
error can be made arbitrarily small by increasing C
S
.
The averaging effect of the bypass capacitor C
S
causes
another error term. Each time the input switches cycle
between the plus and minus inputs, C
IN
is charged and
discharged. The average input current due to this is
R
S
V
IN
C
S
Figure 2. Equivalent Input Circuit
1042fa
U
l
AVG
= V
IN
x C
IN
x f
S
, where f
S
is the sampling frequency.
Because the input current is directly proportional to the
differential input voltage, the LTC1042 can be said to have
an average input resistance of R
IN
= V
IN
/I
AVG
= 1/(f
S
x C
IN
).
Since two comparator inputs are connected in parallel, R
IN
is one half this value (see typical curve of R
IN
vs Sampling
Frequency). This finite input resistance causes an error
due to voltage divided between R
S
and R
IN
.
The input error caused by both of these effects is
V
ERROR
= V
IN
[2C
IN
/(2C
IN
+ C
S
) + R
S
/(R
S
+ R
IN
)].
EXAMPLE: Assume f
S
= 10Hz, R
S
= 1MΩ, C
S
= 1µF and
V
IN
= 1V. Then V
ERROR
= 1V(66µV + 660µV) = 726µV. If the
sampling frequency is reduced to 1Hz, the voltage error
from input impedance effects is reduced to 136µV.
Input Voltage Range
The input switches of the LTC1042 are capable of
switching either to the V
+
supply or ground. Consequently,
the input voltage range includes both supply rails. This is
a further benefit of the input sampling structure.
Error Specifications
The only measurable errors on the LTC1042 are the
deviations from “ideal” of the upper and lower window
limits [Figure 1(B)]. The critical parameters for a window
comparator are the width and center of the window. These
errors may be expressed in terms of V
U
and V
L
.
center error = [(V
U
+ V
L
)/2] – CENTER
width error = (V
U
– V
L
) – 2 x (WIDTH/2)
The specified error limits (see Electrical Characteristics)
include error due to offset, power supply variation, gain,
time and temperature.
C
IN
~
(~33pF)
S1
W
U U
+
S2
–
V
–
LTC1042 DIFFERENTIAL INPUT
LTC1042 • AI02
5
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