MAX921–MAX924
comparator is used with the reference, the combined
peak-to-peak noise is about 1mV. This, of course, is
much higher than the RMS noise of the individual
components. Care should be taken in the layout to
avoid capacitive coupling from any output to the
reference pin. Crosstalk can significantly increase the
actual noise of the reference.
__________Applications Information
Hysteresis
Hysteresis increases the comparators’ noise margin by
increasing the upper threshold and decreasing the
lower threshold (see Figure 2).
Hysteresis (MAX921/MAX923)
To add hysteresis to the MAX921 or MAX923, connect
resistor R1 between REF and HYST, and connect
resistor R2 between HYST and V- (Figure 3). If no
hysteresis is required, connect HYST to REF. When
hysteresis is added, the upper threshold increases by
the same amount that the lower threshold decreases.
The hysteresis band (the difference between the upper
and lower thresholds, V
HB
) is approximately equal to
twice the voltage between REF and HYST. The HYST
input can be adjusted to a maximum voltage of REF
and to a minimum voltage of (REF – 50mV). The
maximum difference between REF and HYST (50mV)
will therefore produce a 100mV max hysteresis band.
Use the following equations to determine R1 and R2:
Where I
REF
(the current sourced by the reference)
should not exceed the REF source capability, and
should be significantly larger than the HYST input
current. I
REF
values between 0.1µA and 4µA are
usually appropriate. If 2.4MΩ is chosen for R2 (I
REF
=
0.5µA), the equation for R1 and V
HB
can be
approximated as:
When hysteresis is obtained in this manner for the
MAX923, the same hysteresis applies to both comparators.
Hysteresis (MAX922/MAX924)
Hysteresis can be set with two resistors using positive
feedback, as shown in Figure 4. This circuit generally
draws more current than the circuits using the HYST
pin on the MAX921 and MAX923, and the high
feedback impedance slows hysteresis. The design
procedure is as follows:
1. Choose R3. The leakage current of IN+ is under
1nA (up to +85°C), so the current through R3 can be
around 100nA and still maintain good accuracy.
The current through R3 at the trip point is V
REF
/R3,
or 100nA for R3 = 11.8MΩ. 10MΩ is a good
practical value.
2. Choose the hysteresis voltage (V
HB
), the voltage
between the upper and lower thresholds. In this
example, choose V
HB
= 50mV.
3. Calculate R1.
4. Choose the threshold voltage for V
IN
rising (V
THR
).
In this example, choose V
THR
= 3V.
5. Calculate R2.
A 1% preferred value is 64.9kΩ.
