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LTC1540CMS8#PBF

P1-P3P4-P6P7-P9P10-P12
7
LTC1540
sn1540 1540fas
APPLICATIONS INFORMATION
WUU
U
Hysteresis
Hysteresis can be added to the LTC1540 by connecting a
resistor (R1) between the REF and HYST pins and a
second resistor (R2) from HYST to V
–
(Figure 5).
The difference between the upper and lower threshold
voltages, or hysteresis voltage band (V
HB
), is equal to twice
the voltage difference between the REF and HYST pins.
When more hysteresis is added, the upper threshold
increases the same amount as the low threshold de-
creases. The maximum voltage allowed between REF and
HYST pins is 50mV, producing a maximum hysteresis
voltage band of 100mV. The hysteresis band may vary by
up to 15%. If hysteresis is not wanted, the HYST pin
should be shorted to REF. Acceptable values for I
REF
range
from 0.1µA to 5µA. If 2.4M is chosen for R2, then the value
of R1 (kΩ) is equal to the value of V
HB
(mV).
6
R2
HYST
V
–
REF
5
R1
1540 • F05
LTC1540
2
I
REF
R1 =
V
HB
(2)(I
REF
)
R2 =
I
REF
1.182V –
()
V
HB
2
Figure 5. Programmable Hysteresis
Figure 6. Glitch-Free Level Detector with Hysteresis
TYPICAL APPLICATIONS
U
Level Detector
The LTC1540 is ideal for use as a nanopower level detector
as shown in Figure 6. R1 and R2 form a voltage divider
from V
IN
to the noninverting comparator input. R3 and R4
set the hysteresis voltage, and R5 and C1 bypass the
reference output. The following design procedure can be
used to select the component values:
1. Choose the V
IN
voltage trip level, in this example 4.65V.
2. Calculate the required resistive divider ratio.
Ratio = V
REF
/V
IN
Ratio = 1.182V/4.65V = 0.254
3. Choose the required hysteresis voltage band at the
input V
HBIN
, in this example 60mV. Calculate the hyster-
esis voltage band referred to the comparator input V
HB
.
V
HB
= (V
HBIN
)(Ratio)
V
HB
= (60mV)(0.254)
V
HB
= 15.24mV
4. Choose the values for R3 and R4 to set the hysteresis.
R4 = 2.4M
R3 (kΩ) = 15k, V
HB
(mV) = 15mV
5. Choose the values for R1 and R2 to set the trip point.
R1 =
R2 = R1
– 1
= = 1.18M
V
REF
I
BIAS
V
REF
+
V
IN
V
HB
2
1.182V
1µA
R2 = 1.18M
R2 = 3.40M
– 1
1.182V +
4.65V
15mV
2
V
+
R5
430Ω
5%
R3
15k
1%
C1
1µF
OUT
V
–
21
7
5V
V
IN
8
GND
LTC1540
1540 F06
IN
+
IN
–
HYST
REF
3
4
5
6
–
+
R2
3.4M
1%
R1
1.18M
1%
R4
2.4M
1%
8
LTC1540
sn1540 1540fas
TYPICAL APPLICATIONS
U
3.3V Output Low Dropout Linear Regulator
The LTC1540 can be connected as a micropower (I
Q
=
5.5µA at V
IN
= 5V) low dropout linear regulator (Figure 7).
When the output is low, Q1 turns on, allowing current to
charge output capacitor C1. Local feedback formed by R4,
Q1 and Q2 creates a constant-current source from the 5V
input to C1. R4, R1 and Q2’s V
BE
also provide current
limiting in the case of an output short-circuit to ground. C2
reduces output ripple, while the R2-R3 feedback voltage
divider establishes the output voltage.
Auto Power-Off Source
Figure 8 shows the circuit for a 30mA power supply that
has a timed auto power-off function. The comparator
output is the switched power supply output. With a 10mA
load, it typically provides a voltage of (V
BAT
– 0.17V). The
whole circuit draws a mere 0.8µA of quiescent current with
V
BAT
= 5V. The three resistor voltage divider programs
50mV of hysteresis for the comparator, and sets the IN
–
voltage at 200mV. This gives an IN
+
trip threshold of
approximately 150mV
The RC time constant determines the maximum power-on
time of the OUT pin before power down occurs. This period
can be approximated by:
t = 4.6RC (seconds)
The actual time will vary with both the leakage current of
the capacitor and the input current at the IN
+
pin.
Figure 7. 3.3V Output Low Dropout Linear Regulator
V
+
V
–
1
C3
2
7
8
R1
47k
GND
OUT
1540 F07
IN
+
IN
–
HYST
REF
3
4
5
6
–
+
R3
750k
1%
R2
430k
1%
R4
10Ω
Q1
TP0610L
V
IN
= 5V
Q2
2N3906
C1
10µF
V
OUT
3.3V
C2
2.2nF
LTC1540
V
+
IN
+
OUT
432k
V
BAT
V
–
21
7
8
3
GND
CR
(V
BAT
– 0.17V)
10mA
MOMENTARY
SWITCH
LTC1540
1540 F08
IN
–
HYST
REF
4
5
6
121k
2M
–
+
Figure 8. Auto Power-Off Switch Operates
on 0.8µA Quiescent Current
9
LTC1540
sn1540 1540fas
Low-Battery Detect
Figure 9 shows how to use the LTC1540 for a low-battery
detect, drawing only 1.4µA at V
BAT
= 2V. The circuit is
powered by a 2-cell NiCd battery. The V
BAT
pin could be as
low as 1.6V when the batteries are completely depleted.
The electrical specifications of the LTC1540 guarantee
operation down to a supply voltage of 2V, but it is still
functional with the supply as low as 1.6V. Some param-
eters, such as V
REF
and V
OS
, will be degraded on lower
supply voltages. The input voltage range extends from
0.9V below the positive supply to the negative supply.
TYPICAL APPLICATIONS
U
Figure 9. Low-Battery Detect Works Down to 1.6V
V
+
R1
3M
R2
1.1M
V
BAT
= ~1.6V TO 2.5V
V
–
21
7
8
OUT
GND
LBO
2-CELL
NiCd
LTC1540
1540 F09
IN
+
IN
–
REF
HYST
3
4
6
5
–
+
R3
40k
R4
1.2M
R5
1M
P1-P3P4-P6P7-P9P10-P12

LTC1540CMS8#PBF

Mfr. #:
Buy LTC1540CMS8#PBF
Manufacturer:
Analog Devices Inc.
Description:
Analog Comparators Nanopwr(300nA)COMPARATOR.w/2% Volt Ref
Lifecycle:
New from this manufacturer.
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