TSM921 TOUCHSTONE [Touchstone Semiconductor Inc], TSM921 Datasheet - Page 14

• Manufacturer Part Number: TSM921
• Description: Ultra-Low-Power, Single/Dual-Supply Comparators
• Manufacturer: TOUCHSTONE [Touchstone Semiconductor Inc]

TSM921-TSM924
high impedance on the feedback. The following
procedure explains the steps to design the circuit for
a desired hysteresis:
Page 14
Figure 3. External Hysteresis
1. Choose R3. As the leakage current at the
2. Next, the desired hysteresis band (V
3. Calculate R1.
4. Choose the trip point for V
5. Calculate R2.
IN+ pin is less than 1nA, the current through
R3 should be at least 100nA to minimize
offset voltage errors caused by the input
leakage current. For R3
current through R3 is V
point. In this case, a 10MΩ resistor is a good
standard value for R3.
set. In this example, V
R1
In this example, a 100kΩ, 1% standard
value resistor is selected for R1.
which is the threshold voltage at which the
comparator switches its output from low to
high as V
example, choose V
10MΩ x
R3 x
IN
50mV
V
rises above the trip point. In this
V+
5V
H
THR
100kΩ
HB
= 3V.
is set to 50mV.
REF
IN
/R3 at the trip
11.8MΩ, the
rising (V
HB
THR
) is
),
Board Layout and Bypassing
While power-supply bypass capacitors are not
typically required, it is good engineering practice to
use 0.1μF bypass capacitors close to the device’s
power
impedance is high, the power supply leads are long,
or there is excessive noise on the power supply
traces. To reduce stray capacitance, it is also good
engineering practice to make signal trace lengths as
short as possible. Also recommended are a ground
plane and surface mount resistors and capacitors.
TYPICAL APPLICATION CIRCUITS
Auto-Off Power Source
A timed auto power-off circuit can be designed as
shown in Figure 4 where the output of the TSM921
is the switched power-supply output. With an internal
reference, hysteresis, high current output, and a
2.5 μA supply current, the TSM921 provides a
wealth of features that make it perfect for this
application.
quiescent current with a 10mA load, the TSM921 is
able to generate a voltage of VBATT – 0.12V. As
shown in Figure 4, three resistors are used to
generate a hysteresis band of 100mV and sets the
IN+ trip point to 50mV when IN+ is going low. The
maximum power-on period of the OUT pin before
power-down occurs can be determined by the RC
time constant as follows:
6. The last step is to verify the trip voltages and
R2
In this example, a
value resistor is selected for R2.
hysteresis
resistance values:
V
V
supply
THR
THF
5.44kΩ
1.182V x 100kΩ
While
V
V
V
THR
REF
REF
pins
V
3
THR
x R1 x
–
x R1
band
consuming
R1 x V+
when
1
R3
1
R1
R1
using
1
1
100kΩ
4.9kΩ, 1% standard
+
the
TSM921_24DS r1p0
R3
R2
1
1
1
only
+
power
the
R3
10MΩ
1
1
3.5μA
standard
supply
RTFDS
of