MCP65R46 datasheet(24/34 Pages) MICROCHIP

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MCP65R41/6

DS22269A-page 24

 2010 Microchip Technology Inc.

4.6

PCB Surface Leakage

In applications where the low input bias current is

critical, the Printed Circuit Board (PCB) surface

leakage effects need to be considered. Surface

leakage is caused by humidity, dust or other type of

contamination on the board. Under low humidity

conditions, a typical resistance between nearby traces

is 1012

. A 5V difference would cause 5 pA of current

to flow. This is greater than the MCP65R41/6 family’s

bias current at +25°C (1 pA, typical).

The easiest way to reduce the surface leakage is to use

a guard ring around the 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

Figure 4-9.

FIGURE 4-9:

Example Guard Ring Layout

for Inverting Circuit.

Inverting Configuration (Figures 4-6 and 4-9):

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.

Non-inverting Configuration (Figure 4-4):

Connect the non-inverting pin (VIN+) to the

input pad without touching the guard ring.

Connect the guard ring to the inverting input

pin (VIN–).

4.7

Typical Applications

4.7.1

PRECISE COMPARATOR

Some applications require a higher DC precision. An

easy way to solve this problem is to use an amplifier

(such as the MCP6041, a 600 nA low power and

14 kHz bandwidth op amp) to gain-up the input signal

before it reaches the comparator. Figure 4-10 shows

an example of this approach, which also level shifts to

VPU using the Open-Drain option, MCP65R46.

FIGURE 4-10:

Precise Inverting

Comparator.

4.7.2

BISTABLE MULTI-VIBRATOR

A simple bistable multi-vibrator design is shown in

Figure 4-11. VREF needs to be between ground and the

maximum comparator internal VREF of 2.4V to achieve

oscillation. The output duty cycle changes with VREF.

FIGURE 4-11:

Bistable Multi-Vibrator.

4.7.3

OVER TEMPERATURE

PROTECTION CIRCUIT

The MCP65R41 device can be used as an over

temperature protection circuit using a thermistor. The

2.4V VREF can be used as stable reference to the

thermistor, the alert threshold and hysteresis threshold.

This is ideal for battery powered applications, where

the

change

temperature

and

output

toggle

thresholds would remain fixed as battery voltage

decays over time.

FIGURE 4-12:

Over Temperature Alert

Circuit.

Guard Ring

VSS

IN-

IN+

VREF

VDD

VOUT

VIN

VREF

VPU

RPU

MCP65R46

MCP6041

VREF

VDD

VREF

VOUT

MCP65R41

VOUT

VDD

VREF

VPU

RPU*

* Pull-up resistor required for the

MCP65R46 only.

Thermistor

VREF

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