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%