TSM971/72/73/82/84
hysteresis effectively forces one comparator input to
move quickly past the other input, moving the input
Figure 2. Programming the HYST Pin
typically in the range of 0.1μA and 4μA. It is also
important to ensure that the current from reference is
much larger than the HYST pin input current. Given
R2 = 2.4MΩ, the current sourced by the reference is
0.5μA. This allows the hysteresis band and R1 to be
approximated as follows:
Figure 1. Threshold Hysteresis Band
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
were reversed, the figure would be the same with an
inverted output.
R1(kΩ) = VHB(mv)
Hysteresis (TSM971/973 and TSM982)
For the TSM973 and TSM982, the hysteresis is the
same for both comparators.
Hysteresis can be generated with two external
resistors using positive feedback as shown in
Figure 2. Resistor R1 is connected between REF
and HYST and R2 is connected between HYST and
V-. This will increase the trip point for the rising input
voltage, VTHR, and decrease the trip point for the
falling input voltage, VTHF, by the same amount. If no
hysteresis is required, connect HYST to REF. The
hysteresis band, VHB, is voltage across the REF and
HYST pin multiplied by a factor of 2. The HYST pin
can accept a voltage between REF and REF-50mV,
where a voltage of REF-50mV generates the
maximum voltage across R1 and thus, the maximum
hysteresis and hysteresis band of 50mV and
100mV, respectively. To design the circuit for a
desired hysteresis band, consider the equations
below to acquire the values for resistors R1 and R2:
Hysteresis (TSM972 and TSM984)
Relative to adding hysteresis with the HYST pin as
was done for the TSM971, TSM973, and the
TSM982, the circuit in Figure 3 uses positive
feedback along with two external resistors to set the
desired hysteresis. The circuit consumes more
VHꢀ
R1 =
ꢀ
2 x ꢁREF
VHꢀ
2
1.182 ꢂ
ꢁREF
Figure 3. External Hysteresis
R2 =
current and it slows down the hysteresis effect due
to the high impedance on the feedback. Due to the
pull-up resistor on the output and its inability to
source current, upper threshold variations will
depend on the value of the pull-up resistor.
where IREF is the primary source of current out of the
reference pin and should be maintained within the
maximum current the reference can source. This is
TSM971_73/TSM982/TSM984DS r1p0
Page 13
RTFDS
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