Datasheet

MCP6546/6R/6U/7/8/9
DS21714G-page 16 © 2002-2012 Microchip Technology Inc.
FIGURE 4-3: The MCP6546/6R/6U/7/8/9
Comparators’ Internal Hysteresis Eliminates
Output Chatter Caused By Input Noise Voltage.
4.2 Open-Drain Output
The open-drain output is designed to make level-
shifting and wired-OR logic easy to implement. The
output can go as high as 10V for 9V battery-powered
applications. The output stage minimizes switching cur-
rent (shoot-through current from supply-to-supply)
when the output changes state. See Figures 2-15, 2-18
and 2-37 through 2-41, for more information.
4.3 MCP6548 Chip Select (CS)
The MCP6548 is a single comparator with a Chip
Select (CS
) pin. When CS is pulled high, the total
current consumption drops to 20 pA (typical). 1 pA
(typical) flows through the CS pin, 1 pA (typical) flows
through the output pin and 18 pA (typical) flows through
the V
DD
pin, as shown in Figure 1-1. When this
happens, the comparator output is put into a high-
impedance state. By pulling CS low, the comparator is
enabled. If the CS
pin is left floating, the comparator will
not operate properly. Figure 1-1 shows the output
voltage and supply current response to a CS
pulse.
The internal CS
circuitry is designed to minimize
glitches when cycling the CS
pin. This helps conserve
power, which is especially important in battery-powered
applications.
4.4 Externally Set Hysteresis
Greater flexibility in selecting hysteresis, or input trip
points, is achieved by using external resistors.
Input offset voltage (V
OS
) is the center (average) of the
(input-referred) low-high and high-low trip points. Input
hysteresis voltage (V
HYST
) is the difference between
the same trip points. Hysteresis reduces output
chattering when one input is slowly moving past the
other, thus reducing dynamic supply current. It also
helps in systems where it is best not to cycle between
states too frequently (e.g., air conditioner thermostatic
control).
4.4.1 INVERTING CIRCUIT
Figure 4-4 shows an inverting circuit for a single-supply
application using three resistors, besides the pull-up
resistor. The resulting hysteresis diagram is shown in
Figure 4-5.
FIGURE 4-4: Inverting Circuit with
Hysteresis.
FIGURE 4-5: Hysteresis Diagram for the
Inverting Circuit.
In order to determine the trip voltages (V
THL
and V
TLH
)
for the circuit shown in Figure 4-4, R
2
and R
3
can be
simplified to the Thevenin equivalent circuit with
respect to V
DD
, as shown in Figure 4-6.
FIGURE 4-6: Thevenin Equivalent Circuit.
-3
-2
-1
0
1
2
3
4
5
6
7
8
Time (100 ms/div)
Output Voltage (V)
-30
-25
-20
-15
-10
-5
0
5
10
15
20
25
Input Voltage (10 mV/div)
V
OUT
V
IN
–
V
DD
= 5.0V
Hysteresis
V
IN
V
OUT
MCP654X
V
DD
R
2
R
F
R
3
V
PU
R
PU
V
DD
I
OL
I
RF
I
PU
V
OUT
High-to-LowLow-to-High
V
OH
V
OL
V
SS
V
SS
V
DD
V
TLH
V
THL
V
IN
V
PU
V
TLH
= trip voltage from low to high
V
THL
= trip voltage from high to low
V
23
V
OUT
MCP654X
V
PU
R
23
R
F
+
-
R
PU