TS9004
out of the region where oscillation occurs. Figure 1
illustrates the case in which an IN- input is a fixed
voltage and an IN+ is varied. If the input signals
3. Calculating R1.
Vꢀꢁ
R1 = R3 x
V+
50mV
= 10MΩ x
= 100ꢂΩ
5V
In this example, a 100ꢂΩ, 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.
Figure 1. Threshold Hysteresis Band
were reversed, the figure would be the same with an
inverted output. To add hysteresis to the TS9004,
the circuit in Figure 2 is implemented and uses
positive feedback along with two external resistors to
set the desired hysteresis. The circuit consumes
more current and it slows down the hysteresis effect
5. Calculating R2.
1
R2 =
VTꢀR
1
1
ꢀꢁ
ꢂ ꢃ
ꢃ
ꢃ
VREF x R1 R1 R3
1
=
3
1
1
ꢀꢁ
ꢂ ꢃ
ꢃ
ꢃ
1.182V x 100ꢂΩ 100ꢂΩ 10MΩ
= ꢄ5.44ꢂΩ
In this example, a ꢄ4.9ꢂΩ, 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:
1
1
1
VTꢀR = VREFx R1 x ꢁ
+
+
ꢂ
Figure 2. External Hysteresis
R1 R2 R3
due to the high impedance on the feedback. The
following procedure explains the steps to design the
circuit for a desired hysteresis:
ꢄ
ꢅ
R1 x V+
VTꢀF = VTꢀR
–
R3
Board Layout and Bypassing
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.
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.
Page 10
TS9004DS r1p0
RTFDS
TOUCHSTONE [ TOUCHSTONE SEMICONDUCTOR INC ]
