TSM931-TSM934
APPLICATIONS INFORMATION
Hysteresis
As a result of circuit noise or unintended parasitic
feedback, many analog comparators often break into
oscillation within their linear region of operation
especially when the applied differential input voltage
approaches 0V (zero volt). Externally-introduced
hysteresis is
a
well-established technique to
stabilizing analog comparator behavior and requires
external components. As shown in Figure 1, adding
comparator hysteresis creates two trip points: VTHR
(for the rising input voltage) and VTHF (for the falling
input voltage). The hysteresis band (VHB) is defined
as the voltage difference between the two trip points.
When a comparator’s input voltages are equal,
hysteresis effectively forces one comparator input to
move quickly past the other input, moving the input
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.
Figure 2. Programming 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:
VHꢁ
R1 ꢀ
ꢀ
2 x IREF
VHꢁ
2
1.182 ꢂ
IREF
R2 ꢀ
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
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
R1(kΩ) ꢀ VHB(mv)
Hysteresis (TSM931-TSM933)
For the TSM932-TSM933, 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
Hysteresis (TSM934)
Relative to adding hysteresis with the HYST pin as
was done for the TSM931-TSM933, the circuit in
Figure 3 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 due to the high impedance on the
TSM931_34DS r1p0
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RTFDS