TSM931-TSM934
1
R2 ꢀ
VTHR
1
1
ꢂꢃ
ꢄ ꢂ
ꢂ
ꢅ
VREF x R1 R1 R3
1
ꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁꢁ ꢀ
3
1
1
ꢂꢃ
ꢄ ꢂ
ꢂ
ꢅ
1.182V x 100kΩ 100kΩ 10MΩ
ꢀꢁ65.44kΩ
In this example, a 64.9kΩ, 1% standard
value resistor is selected for R2.
6. The last step is to verify the trip voltages and
hysteresis band using the standard
resistance values:
Figure 3. External Hysteresis
feedback. The following procedure explains the
steps to design the circuit for a desired hysteresis:
1
1
1
VTHR ꢀ VREFx R1 x ꢃ
+
+
ꢄ
R1 R2 R3
1. Choosing R3. As the leakage current at the
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 ꢀ 11.8MΩ, the
current through R3 is VREF/R3 at the trip
point. In this case, a 10MΩ resistor is a good
standard value for R3.
ꢀ
R1 x V+
VTHF ꢀ VTHR
–
R3
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 supply pins when the power supply
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.
2. Next, the desired hysteresis band ( VHB) is
set. In this example, VHB is set to 50mV.
3. Calculating R1.
VHꢁ
R1 ꢀ R3 x
V+
50mV
ꢀ 10MΩ x
5V
TYPICAL APPLICATION CIRCUITS
Auto-Off Power Source
ꢀꢁ100kΩ
A timed auto power-off circuit can be designed as
shown in Figure 4 where the output of the TSM931
is the switched power-supply output. With an internal
reference, hysteresis, high current output, and a 2.5
μA supply current, the TSM931 provides a wealth of
features that make it perfect for this application.
While consuming only 3.5μA of quiescent current
with a 10mA load, the TSM931 is able to generate a
voltage of VBATT – 0.12V. As shown in the figure,
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
In this example, a 100kΩ, 1% standard
value resistor is selected for R1.
4. Choose the trip point for VIN rising (VTHR),
which is the threshold voltage at which the
comparator switches its output from low to
high as VIN rises above the trip point. In this
example, choose VTHR = 3V.
5. Calculating R2.
Page 14
TSM931_34DS r1p0
RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
