Datasheet

© 2009 Microchip Technology Inc. DS22233A-page 63
MCP434X/436X
8.4 Design Considerations
In the design of a system with the MCP43XX devices,
the following considerations should be taken into
account:
• Power Supply Considerations
• Layout Considerations
8.4.1 POWER SUPPLY
CONSIDERATIONS
The typical application will require a bypass capacitor
in order to filter high-frequency noise, which can be
induced onto the power supply's traces. The bypass
capacitor helps to minimize the effect of these noise
sources on signal integrity. Figure 8-6 illustrates an
appropriate bypass strategy.
In this example, the recommended bypass capacitor
value is 0.1 µF. This capacitor should be placed as
close (within 4 mm) to the device power pin (V
DD
) as
possible.
The power source supplying these devices should be
as clean as possible. If the application circuit has
separate digital and analog power supplies, V
DD
and
V
SS
should reside on the analog plane.
FIGURE 8-6: Typical Microcontroller
Connections.
8.4.2 LAYOUT CONSIDERATIONS
Several layout considerations may be applicable to
your application. These may include:
• Noise
• Footprint Compatibility
• PCB Area Requirements
8.4.2.1 Noise
Inductively-coupled AC transients and digital switching
noise can degrade the input and output signal integrity,
potentially masking the MCP43XX’s performance.
Careful board layout minimizes these effects and
increases the Signal-to-Noise Ratio (SNR). Multi-layer
boards utilizing a low-inductance ground plane,
isolated inputs, isolated outputs and proper decoupling
are critical to achieving the performance that the silicon
is capable of providing. Particularly harsh
environments may require shielding of critical signals.
If low noise is desired, breadboards and wire-wrapped
boards are not recommended.
8.4.2.2 Footprint Compatibility
The specification of the MCP43XX pinouts was done to
allow systems to be designed to easily support the use
of either the dual (MCP42XX) or quad (MCP43XX)
device.
Figure 8-7 shows how the dual pinout devices fit on the
quad device footprint. For the Rheostat devices, the
dual device is in the MSOP package, so the footprints
would need to be offset from each other.
FIGURE 8-7: Quad Pinout (TSSOP
Package) vs. Dual Pinout.
V
DD
V
DD
V
SS
V
SS
MCP434X/436X
0.1 µF
PIC
TM
Microcontroller
0.1 µF
U/D
CS
W
B
A
1
2
3
4
17
18
19
20
RESET
SDO
WP
V
DD
MCP43X1 Quad Potentiometers
TSSOP
5
6
7
14
15
16
P0W
P0B
P0A
P1A
P1W
P1B
V
SS
CS
SDI
SCK
MCP43X2 Quad Rheostat
1
2
3
4
11
12
13
14
P0B
SDO
P0W
V
DD
TSSOP
5
6
7
8
9
10
P2W
P1W
P2B
P3B
P3W
P1B
V
SS
CS
SDI
SCK
8
9
10
P3B
P3W
P3A
12
12
P2W
P2A
P2B
11
MCP42X1 Pinout
(1)
MCP42X2 Pinout
Note 1: Pin 15 (RESET) is the Shutdown (SHDN)
pin on the MCP42x1 device.