TSM921 TOUCHSTONE [Touchstone Semiconductor Inc], TSM921 Datasheet - Page 13

• Manufacturer Part Number: TSM921
• Description: Ultra-Low-Power, Single/Dual-Supply Comparators
• 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 2
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 (TSM921 and TSM923)
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 the HYST pin to the
REF pin. The hysteresis band, V
the REF and HYST pin multiplied by a factor of 2.
The HYST pin can accept a voltage between REF
TSM921_24DS r1p0
Figure 1. Threshold Hysteresis Band
THR
is
, and decrease the trip point for the
a
well-established
THF
, by the same amount. If no
HB
, is voltage across
THF
technique
(for the falling
HB
) is defined
THR
to
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 TSM923, the hysteresis is the same for both
comparators.
Hysteresis (TSM922 and TSM924)
Relative to adding hysteresis with the HYST pin as
was done for the TSM921 and the TSM923, the
circuit in Figure 3 uses positive feedback along with
two external resistors to set the desired hysteresis
for the TSM924. The circuit consumes more current
and it slows down the hysteresis effect due to the
Figure 2. Programming the HYST Pin
1.182
2 x I
REF
V
V
I
H
REF
REF
is the primary source of current out of the
HB
(mv)
TSM921-TSM924
V
2
H
Page 13
RTFDS