TSM931 TOUCHSTONE [Touchstone Semiconductor Inc], TSM931 Datasheet - Page 13

• Manufacturer Part Number: TSM931
• Description: Ultra Low-Power Single/Dual-Supply Comparators with Reference
• Manufacturer: TOUCHSTONE [Touchstone Semiconductor Inc]

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
stabilizing analog comparator behavior and requires
external components. As shown in Figure 1, adding
comparator hysteresis creates two trip points: V
(for the rising input voltage) and V
input voltage). The hysteresis band (V
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.
Hysteresis (TSM931-TSM933)
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, V
falling input voltage, V
hysteresis is required, connect HYST to REF. The
hysteresis band, V
HYST pin multiplied by a factor of 2. The HYST pin
TSM931_34DS r1p0
Figure 1. Threshold Hysteresis Band
THR
is
, and decrease the trip point for the
a
HB
well-established
, is voltage across the REF and
THF
, by the same amount. If no
THF
technique
(for the falling
HB
) is defined
THR
to
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:
R1
R2
where I
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:
R1(kΩ)
For the TSM932-TSM933, the hysteresis is the
same for both comparators.
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
Figure 2. Programming the HYST Pin
1.182
2 x I
REF
V
I
V
H
REF
REF
is the primary source of current out of the
HB
(mv)
TSM931-TSM934
V
2
H
Page 13
RTFDS