MCP6244 datasheet(12/38 Pages) MICROCHIP

12 / 38 page

MCP6241/1R/1U/2/4

DS21882D-page 12

A significant amount of current can flow out of the

inputs when the common mode voltage (VCM) is below

ground (VSS); see Figure 2-19. Applications that are

high impedance may need to limit the useable voltage

range.

4.1.3

NORMAL OPERATION

The input stage of the MCP6241/1R/1U/2/4 op amps

use two differential CMOS input stages in parallel. One

operates at low common mode input voltage (VCM),

while the other operates at high VCM. WIth this topol-

ogy, the device operates with VCM up to 0.3V above

VDD and 0.3V below VSS.

4.2

Rail-to-Rail Output

The output voltage range of the MCP6241/1R/1U/2/4

op amps is VDD –35mV (maximum) and VSS +35mV

(minimum) when RL =10kΩ is connected to VDD/2 and

VDD = 5.5V. Refer to Figure 2-14 for more information.

4.3

Capacitive Loads

Driving large capacitive loads can cause stability

problems for voltage-feedback op amps. As the load

capacitance increases, the feedback loop’s phase

margin decreases and the closed-loop bandwidth is

reduced. This produces gain peaking in the frequency

response, with overshoot and ringing in the step

response. A unity-gain buffer (G = +1) is the most

sensitive to capacitive loads, but all gains show the

same general behavior.

When driving large capacitive loads with these op

amps (e.g., > 70 pF when G = +1), a small series

resistor at the output (RISO in Figure 4-4) improves the

feedback loop’s phase margin (stability) by making the

output load resistive at higher frequencies. The

bandwidth will be generally lower than the bandwidth

with no capacitive load.

FIGURE 4-4:

Output Resistor, RISO

stabilizes large capacitive loads.

Figure 4-5 gives recommended RISO values for

different capacitive loads and gains. The x-axis is the

normalized load capacitance (CL/GN), where GN is the

circuit’s noise gain. For non-inverting gains, GN and the

signal gain are equal. For inverting gains, GN is

1 + |Signal Gain| (e.g., –1 V/V gives GN = +2 V/V).

FIGURE 4-5:

Recommended RISO Values

for Capacitive Loads.

After selecting RISO for your circuit, double-check the

resulting

frequency

response

peaking

and

step

response overshoot. Evaluation on the bench and

simulations with the MCP6241/1R/1U/2/4 SPICE

macro model are very helpful. Modify RISO’s value until

the response is reasonable.

4.4

Supply Bypass

With this op amp, the power supply pin (VDD for

single-supply) should have a local bypass capacitor

(i.e., 0.01 µF to 0.1 µF) within 2 mm for good high-

frequency performance. It can use a bulk capacitor

(i.e., 1 µF or larger) within 100 mm to provide large,

slow currents. This bulk capacitor can be shared with

other nearby analog parts.

4.5

Unused Op Amps

An unused op amp in a quad package (MCP6244)

should be configured as shown in Figure 4-6. Both

circuits prevent the output from toggling and causing

crosstalk. Circuit A can use any reference voltage

between the supplies, provides a buffered DC voltage,

and minimizes the supply current draw of the unused

amp.

Circuit

minimizes

the

number

components, but may draw a little more supply current

for the unused op amp.

FIGURE 4-6:

Unused Op Amps.

VIN

RISO

VOUT

MCP624X

–

1.E+02

1.E+03

1.E+04

1.E+01

1.E+02

1.E+03

1.E+04

Normalized Load Capacitance; CL/GN (F)

10p

100p

10n

10k

100

GN = +1 V/V

GN ≥ +2 V/V

VDD

¼ MCP6244 (A)

¼ MCP6244 (B)

VDD

VREF

REF

------------------

⋅

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