Datasheet

© 2002-2011 Microchip Technology Inc. DS21696H-page 19
MCP6541/1R/1U/2/3/4
4.4.2 INVERTING CIRCUIT
Figure 4-6 shows an inverting circuit for single-supply
using three resistors. The resulting hysteresis diagram
is shown in Figure 4-7.
FIGURE 4-6: Inverting Circuit With
Hysteresis.
FIGURE 4-7: Hysteresis Diagram for the
Inverting Circuit.
In order to determine the trip voltages (V
THL
and V
TLH
)
for the circuit shown in Figure 4-6, R
2
and R
3
can be
simplified to the Thevenin equivalent circuit with
respect to V
DD
, as shown in Figure 4-8.
FIGURE 4-8: Thevenin Equivalent Circuit.
Where:
Using this simplified circuit, the trip voltage can be
calculated using the following equation:
EQUATION 4-2:
Figure 2-20 and Figure 2-23 can be used to determine
typical values for V
OH
and V
OL
.
4.5 Bypass Capacitors
With this family of comparators, the power supply pin
(V
DD
for single supply) should have a local bypass
capacitor (i.e., 0.01 µF to 0.1 µF) within 2 mm for good
edge rate performance.
4.6 Capacitive Loads
Reasonable capacitive loads (e.g., logic gates) have
little impact on propagation delay (see Figure 2-31).
The supply current increases with increasing toggle
frequency (Figure 2-19), especially with higher
capacitive loads.
4.7 Battery Life
In order to maximize battery life in portable
applications, use large resistors and small capacitive
loads. Avoid toggling the output more than necessary.
Do not use Chip Select (CS
) frequently to conserve
start-up power. Capacitive loads will draw additional
power at start-up.
4.8 PCB Surface Leakage
In applications where low input bias current is critical,
PCB (Printed Circuit Board) surface leakage effects
need to be considered. Surface leakage is caused by
humidity, dust or other contamination on the board.
Under low humidity conditions, a typical resistance
between nearby traces is 10
12
Ω. A 5V difference would
cause 5 pA of current to flow. This is greater than the
MCP6541/1R/1U/2/3/4 family’s bias current at 25°C
(1 pA, typ.).
V
IN
V
OUT
MCP654X
V
DD
R
2
R
F
R
3
V
DD
V
OUT
High-to-LowLow-to-High
V
DD
V
OH
V
OL
V
SS
V
SS
V
DD
V
TLH
V
THL
V
IN
V
23
V
OUT
MCP654X
V
DD
R
23
R
F
+
-
V
SS
R
23
R
2
R
3
R
2
R
3
+
-------------------=
V
23
R
3
R
2
R
3
+
------------------- V
DD
× =
V
THL
V
OH
R
23
R
23
R
F
+
-----------------------
⎝⎠
⎜⎟
⎛⎞
V
23
R
F
R
23
R
F
+
----------------------
⎝⎠
⎛⎞
+=
V
TLH
V
OL
R
23
R
23
R
F
+
-----------------------
⎝⎠
⎜⎟
⎛⎞
V
23
R
F
R
23
R
F
+
----------------------
⎝⎠
⎛⎞
+=
V
TLH
= trip voltage from low-to-high
V
THL
= trip voltage from high-to-low