MCP6549 datasheet(14/28 Pages) MICROCHIP

14 / 28 page

MCP6546/7/8/9

DS21714C-page 14

 2003 Microchip Technology Inc.

3.7

Battery Life

In order to maximize battery life in portable

applications, use large resistors and small capacitive

loads. Also, avoid toggling the output more than

necessary and do not use chip select (CS) to conserve

power for short periods of time. Capacitive loads will

draw additional power at start-up.

3.8

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 1012

Ω. A 5V difference

would cause 5 pA. If current-to-flow, this is greater

than the MCP6546/7/8/9 family’s bias current at 25°C

(1 pA, typ).

The easiest way to reduce surface leakage is to use a

guard ring around sensitive pins (or traces). The guard

ring is biased at the same voltage as the sensitive pin.

An example of this type of layout is shown in

Figure 3-6.

FIGURE 3-6:

Example Guard Ring Layout

for Inverting Circuit.

Inverting Configuration (Figures 3-3 and 3-6):

Connect the guard ring to the non-inverting

input pin (VIN+). This biases the guard ring

to the same reference voltage as the

comparator (e.g., VDD/2 or ground).

Connect the inverting pin (VIN–) to the input

pad without touching the guard ring.

3.9

Typical Applications

3.9.1

PRECISE COMPARATOR

Some applications require higher DC precision. An

easy way to solve this problem is to use an amplifier

(such as the MCP6041) to gain-up the input signal

before it reaches the comparator. Figure 3-7 shows an

example of this approach.

FIGURE 3-7:

Precise Inverting

Comparator.

3.9.2

WINDOWED COMPARATOR

Figure 3-8 shows one approach to designing a

windowed comparator. The wired-OR connection

produces a high output (logic 1) when the input voltage

is between VRB and VRT (where VRT > VRB).

FIGURE 3-8:

Windowed comparator.

Guard Ring

VIN-VIN+

VREF

VDD

VOUT

VIN

VREF

VPU

RPU

MCP6546

MCP6041

VRT

1/2

VRB

VIN

VPU

RPU

VOUT

MCP6547

1/2

MCP6547

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