DN123 - Ultralow Power Comparators Include Reference

Ultralow Power Comparators Include Reference
Design Note 123
Robert Reay
With the explosion of battery-powered products has
come the need for circuits that draw as little supply
current as possible in order to extend battery life. Linear
Technology’s new family of micropower comparators
with built-in references is designed to meet that need.
Drawing only 1μA of supply current per comparator, the
LTC ®1440–LTC1445 family provides the perfect solution to battery-powered system monitoring problems.
7
3 IN+
+
8
4 IN –
OUT
–
5 HYST
6 REF
2.4M
5%
The LTC1440–LTC1445 family features 1μA comparators, adjustable hysteresis, TTL/CMOS outputs that sink
and source current and a 1μA reference that can drive
a bypass capacitor of up to 0.01μF without oscillation.
The parts operate from a 2V to 11V single supply or a
±1V to ±5V dual supply. Each comparator’s input voltage
range swings from the negative supply rail to within
1.3V of the positive supply. The comparator propagation
delay is 12μs with a 10mV overdrive, and the supply
current glitches that commonly occur when comparators change logic states have been eliminated. Table 1
summarizes the features of each member of the family.
Voltage Reference
The internal bandgap voltage reference has an output
voltage of 1.182V above V– for the LTC1440–LTC1443
and 1.22V ±1% for the LTC1444 and LTC1445. The
reference output is capable of sourcing up to 200μA
and sinking 15μA. The reference output can directly
drive an external bypass capacitor up to 0.01μF without oscillation. By placing a resistor in series with the
V+
LTC1440
51k
5%
+
430Ω
5%
1μF
V–
2
GND
DN123 F01
Test Circuit
8V
V+
5V
VREF
OUT
2mV/DIV
DN123 F01b
Figure 1. Reference Settling
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Table 1
PART NUMBER
NUMBER OF
COMPARATORS
LTC1440
1
LTC1441
2
LTC1442
2
Single
LTC1443
4
Dual
5.0μA
No
1.182V ±1%
CMOS
LTC1444
4
Single
5.0μA
Yes
1.221V ±1%
Open Drain
LTC1445
4
Single
5.0μA
Yes
1.221V ±1%
CMOS
08/96/100_conv
SUPPLY
CURRENT
ADJUSTABLE
HYSTERESIS
Dual
2.5μA
Yes
1.182V ±1%
CMOS
Single
3.5μA
No
1.182V ±1%
CMOS
3.5μA
Yes
1.182V ±1%
CMOS
SUPPLY
REFERENCE
COMPARATOR
OUTPUT
bypass capacitor, ringing at the reference output can
be eliminated and a greater capacitance value can be
used. The bypass capacitor prevents reference load
transients or power supply glitches from disturbing
the reference voltage, which helps eliminate false triggering of the comparators when they are connected
to the reference. Figure 1 shows the reference voltage
settling during a power supply transient.
VCC
A
1/4 LTC1444
R2
82.5k
1%
1 CELL
LITHIUMION
BATTERY
+
4
R3
392k
1%
R4
2.4M
5%
6
+
–
R7
51k
5%
–
B
1/4 LTC1444
7 + HYST
R3
1M
1%
14
NC
NC
R5
51k
5%
R4
2.4M
5%
+
3
1
5
2
7
SW
LT1300
ILIM
+
8
R6
430Ω
5%
C1
1μF
8 REF
GND
1
2
DN123 F02
2, 4
Q1
MMFT2955ETI
C3
100μF
R9
267k
1%
R8
732k
1% 11
+
C
1/4 LTC1444
10
LTC1444
REF
9 V
LTC1442
V–
1
3
SEL
C1
0.01μF
4
SHDN
PWR GND
6 REF
R5
10k
5%
Comparators C and D provide the reset input to the
microprocessor. As soon as the boost converter output rises above the 4.65V threshold set by R8 and R9,
comparator C turns off and R10 starts to charge C4.
After 200ms, comparator D turns off and the Reset pin
is pulled high by R12.
6
SENSE
–
5 HYST
microprocessor to warn of a low-battery condition. To
protect the battery from overdischarge, the output of
comparator B is pulled high by R7 when the battery voltage falls below 2.4V. P-channel Q1 and the LT1300 are
turned off, dropping the quiescent current to 20μA. Q1
is needed to prevent the load circuitry from discharging
the battery through L1 and D1.
VIN
C2
100μF
OUT B 8 OVERVOLTAGE
(5.5V)
Figure 2. Undervoltage/Overvoltage Detector
D1
1N5817
2
–
4 IN – B
L1
10μH
SUMIDA
CD54-100
3
+
+
The voltage monitoring circuitry takes advantage of the
LTC1444’s open-drain outputs and low supply voltage
operation. Comparators A and B, along with R1, R2
and R3, monitor the battery voltage. When the battery voltage drops below 2.6V, comparator A’s output
pulls low to generate a nonmaskable interrupt to the
V+
3 IN+ A
R2
84.5k
1%
Single Cell Lithium-Ion Battery Supply
Figure 3 shows a single cell lithium-ion battery to 5V
supply with the low-battery warning, low-battery shutdown and reset functions provided by the LTC1444. The
LT®1300 micropower step-up DC/DC converter boosts
the battery voltage to 5V using L1 and D1. Capacitors
C2 and C3 provide input and output filtering.
5
V+
OUT A 1 UNDERVOLTAGE
(4.5V)
Undervoltage/Overvoltage Detector
The LTC1442 can be easily configured as an undervoltage
and overvoltage detector as shown in Figure 2. R1, R2
and R3 form a resistive divider from VCC so that comparator A goes low when VCC drops below 4.5V, and
comparator B goes low when VCC rises above 5.5V. A
10mV hysteresis band is set by R4 and R5 to prevent
oscillations near the trip points.
R1
1.1M
5%
7
R1
1.33M
1%
16
–
R10
3.37M
5%
R11
51k
5%
13
R12
51k
15 5%
+
D
1/4 LTC1444
12
VCC
NMI
μP
RESET
–
C4
0.22μF
–
DN123 F03
C2, C3: AUX TPSD107M010R0100 OR
SANYO OS-CON 16SA100M
Figure 3. Single Cell to 5V Supply
Data Sheet Download
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Linear Technology Corporation
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call (408) 432-1900
dn123f_conv LT/GP 0296 155K • PRINTED IN THE USA
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© LINEAR TECHNOLOGY CORPORATION 1996
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