MAX44268 View Datasheet(PDF) - Maxim Integrated
Part Name
Description
Manufacturer
MAX44268 Datasheet PDF : 15 Pages
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MAX44268
1.3mm x 1.3mm, Low-Power
Dual Comparator with Reference
Use the following procedure to calculate resistor values.
1) Select R3. Input bias current at IN_+ is less than
15nA. To minimize errors caused by the input bias
current, the current through R3 should be at least
1.5µA. Current through R3 at the trip point is (VREF -
VOUT)/R3. Considering the two possible output states
in solving for R3 yields two formulas:
R3 = VREF/IR3 and R3 = [(VCC - VREF)/IR3] - R1
Use the smaller of the two resulting resistor values.
For example, for VCC = 5V, IR3 = -1.5µA, R1 =
200kI, and a VREF = 1.236V, the two resistor values
are 827kI and 1.5MI. Therefore, for R3 choose the
standard value of 825kI.
2) Choose the hysteresis band required (VHB). In this
example, the VHB = 50mV.
3) Calculate R2 according to the following equation:
R=2
(R1+
R3) VCC
+
VHB
(VREF
x R1)
R3
For this example, insert the value:
= R2 (200kΩ + 0.825MΩ) 505m.3= V 9.67kΩ
For this example, choose standard value R2 = 9.76kI.
4) Choose the trip point for VIN rising (VTHR) in such a
way that:
VTHR
>
VREF
1+
VHB
VCC
VTHR is the threshold voltage at which the com-
parator switches its output from low to high, as VIN
rises above the trip point. For this example, choose
VTHR = 3V.
5) Calculate R4 as follows:
= R4
R4 =
1
VTHR
VREFx R2
-
1
R2
-
1
R3
1
= 6.93kΩ
3
1.236V x
9.76
-
1
9.76
-
1
825
For this example, choose a standard value of 6.98kI.
6) Verify the trip voltages and hysteresis as follows:
= VTHR
VREF
x
R2
1
R2
+
1
R3
+
1
R4
V= THF
VREF
x
R2
1
R2
+
1
R1+ R3
+
1
R4
-
R2
R1+ R3
x
VCC
The hysteresis network in Figure 2 can be simplified if the
reference voltage is chosen to be at midrail and the trip
points of the comparator are chosen to be symmetrical
about the reference voltage. Use the circuit in Figure 3
if the reference voltage can be designed to be at the
center of the hysteresis band. For the symmetrical case,
follow the same steps outlined in the paragraph above
to calculate the resistor values except that in this case,
resistor R4 approaches infinity (open). So in the previ-
ous example, using comparator B with VREF = 2.5V, if
VTHR = 2.525V and VTHF = 2.475V then using the above
formulas, results in R1 = 200kI, R2 = 9.09kI, and R3 =
825kI, R4 = not installed.
Logic-Level Translator
Due to the open-drain output of the IC, the device can
translate between two different logic levels (Figure 4). If
the internal 4mV hysteresis is not sufficient, then external
resistors can be added to increase the hysteresis as
shown in Figure 2 and Figure 3.
VCC
R3
R2
VIN
MAX44268
R1
OUT
VREF
GND
Figure 3. Simplified External Hysteresis Network if VREF is at
the Center of the Hysteresis Band
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