TS12011/TS12012
Adding Hysteresis to the TS12012 Open-Drain
Option
R3 = 1/[VTHR/(VREFOUT x R1) - (1/R1) - (1/R2)]
The TS12012 has open-drain output and requires an
external pull-up resistor to VDD as shown in Figure 3.
6) As before, the last step is to verify the trip
voltages and hysteresis band with the
standard resistor values used in the circuit:
For VCOMPIN+ rising:
VTHR = VREFOUT x R1 x (1/R1+1/R2+1/R3)
For VCOMPIN+ falling:
VTHF = VREFOUT x R1 x(1/R1+1/R3+1/(R2+R4))
-(R1/(R2+R4)) x VDD
and Hysteresis Band is given by VTHR – VTHF
Figure 3. Using Four Resistors Introduces
Additional Hysteresis in the TS12012
Pilot Light Flame Detector with Low-Battery
Lockout Circuit
Additional hysteresis can be generated using positive
feedback; however, the formulae differ slightly from
those of the push-pull option TS12011. The
procedure to calculate the resistor values for the
TS12012 is as follows:
The TS12011 can be used to create a pilot flame
detector with low-battery lockout circuit as shown in
Figure 4. The circuit is able to detect when the
thermocouple does not detect the pilot flame and
when the battery in the circuit drops to 1.39V. This
circuit makes use of the op-amp, comparator, and
0.58V reference in the TS12011. In this example, a
type R thermocouple is used. It generates a voltage
range from 9mV to 17mV that corresponds to a
temperature range of 900ºC to 1500ºC, which is
typical of a methane pilot flame. If the pilot flame is
removed, the temperature drops; hence, the output
voltage generated by the thermocouple is drops to a
minimum voltage of 0.1mV that is applied to the non-
inverting input of the op-amp. This switches the
output voltage of the op-amp to a LOW state and in
turn, switches Q1 off. If, however, the battery voltage
drops from 1.5V to 1.39V, the comparator output will
switch from an output HIGH to a LOW. This will turn
off Q2 and the output of the op-amp will turn Q1 off.
The complete circuit consumes approximately 95µA
of supply current at VDD = 1.5V.
1) As in the previous section, resistor R2 is
chosen according to the formulae:
R2 = VREFOUT/150nA
or
R2 = (VDD- VREFOUT)/150nA - R4
where the smaller of the two resulting resistor
values is the best starting value.
2) As before, the desired hysteresis band
(VHYSB) is set to 100mV.
3) Next, resistor R1 is then computed according
to the following equation:
PC Board Layout and Power-Supply Bypassing
R1 = (R2 + R4) x (VHYSB/VDD)
While power-supply bypass capacitors are not
typically required, it is good engineering practice to
use 0.1uF 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
4) The trip point for VCOMPIN+ rising (VTHR) is
chosen (again, remember that VTHF is the trip
point for VCOMPIN+ falling). This is the
threshold voltage at which the comparator
switches its output from low to high as
VCOMPIN+ rises above the trip point.
5) With the VTHR from Step 4 above, resistor R3
is computed as follows:
TS12011_12DS r1p0
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RTFDS
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