MAX44269_1112 Datasheet, PDF (9/14 Pages) Maxim Integrated Products – 1.3mm x 1.3mm, Low-Power Dual Comparator

MAX44269
1.3mm x 1.3mm, Low-Power
Dual Comparator
Use the following procedure to calculate resistor values.
1) Select R3. Input bias current at IN_+ is less than15nA.
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 (V_REF - V_OUT)/
R3. Considering the two possible output states in solv-
ing for R3 yields two formulas:
R3 = V_REF/IR3 and R3 = [(V_CC - V_REF)/IR3] - R1
Use the smaller of the two resulting resistor values.
For example, for V_CC = 5V, IR3 = -1.5µA, R1 = 200kI,
and a V_REF = 1.24V, the two resistor values are 827kI
and 1.5MI. Therefore, for R3 choose the standard
value of 825kI.
2) Choose the hysteresis band required (V_HB). In this
example, the V_HB = 50mV.
3) Calculate R2 according to the following equation:
R=2
(R1
+

R3)
V_CC
+
V_HB
(V_REF 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 V_IN rising (V_THR) in such a
way that:
V_THR
>
V_REF

¹⁺

V_HB
V_CC



V_THR is the threshold voltage at which the com-
parator switches its output from low to high, as V_IN
rises above the trip point. For this example, choose
V_THR = 3V.
5) Calculate R4 as follows:
= R4
R4 =
1



V_THR
V_REF^{x R2}



−


1
R2


−


1
R3


1
= 6.93kΩ

_1.24
3
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
V_REF
x
R2


1
R2


+


1
R3


+
1
 _R4


V⁼ _THF
V_REF
x
R2


1
R2


+


1
R1+ R3


+


1
R4


−
R2
R1+ R3
x
V_CC
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 previous
example with V_REF = 2.5V, if V_THR = 2.525V and V_THF
= 2.475V then using the above formulas, we get R1 =
200kI, R2 = 9.09kI and R3 = 825kI, R4 = not installed.
Jack Detect
The IC can be used to detect peripheral devices
connected to a circuit. This includes a simple jack-
detect scheme for cell phone applications. The Typical
Application Circuit shows how the device can be used in
conjunction with an external reference to detect a remote
key connection and an accessory ID input. The open-
drain output of the devices allows the output logic level
to be controlled independent of the peripheral device’s
load, making interfacing and controlling external devices
as simple as monitoring a few digital inputs on a micro-
controller or codec.
VCC
R3
R2
V_IN
MAX44269
R1
OUT
VREF
GND
Figure 3. Simplified External Hysteresis Network if V_REF is at
the Center of the Hysteresis Band
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