MCP6476 datasheet(16/46 Pages) MICROCHIP

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MCP6476/6R/6U/7/9

DS20006419B-page 16

 2020-2021 Microchip Technology Inc.

4.1.4

NORMAL OPERATION

The input stage of the MCP6476/6R/6U/7/9 op amp

uses two differential input stages in parallel. One oper-

ates at a low Common-Mode Input Voltage (VCM), while

the other operates at a high VCM. With this topology,

the device operates with a VCM up to 300 mV above

VDD and 300 mV below VSS. The input offset voltage is

measured at VCM = VSS – 0.3V and VDD + 0.3V to

ensure proper operation.

The transition between the input stages occurs when

VCM is near VDD – 0.9V (see Figures 2-3 and 2-4). For

the best distortion performance and gain linearity, with

noninverting gains, avoid this region of operation.

4.2

Rail-to-Rail Output

The output voltage range of the MCP6476/6R/6U/7/9

op amp is 0.006V (typical) and 5.494V (typical) when

RL = 10 k is connected to VDD/2 and VDD = 5.5V.

Refer to Figures 2-16 and 2-17 for more information.

4.3

Start-up

The MCP6476/6R/6U/7/9 family of parts quickly con-

trols the output when power (VDD) is initially applied to

the device (start-up). Bypass capacitors are removed

during the start-up testing to minimize inrush currents

(see Figure 4-3). When the op amp is controlled and is

off, the output impedance is high and VOUT will be VL or

1V. When the op amp turns on, the output becomes

low-impedance and VOUT will follow the input sine

wave; this is used as the start-up time.

FIGURE 4-3:

Start-up Test Circuit.

Figure 4-4 shows the input voltage (blue line) for the

MCP6477 and the output voltage (black line). When

power is first applied to the MCP6477, the output is

turned off (Point A) and driven by the load. After 6 µs,

the output is turned on (Point B) and VOUT follows the

input sine wave.

FIGURE 4-4:

Start-up Time Voltages.

4.4

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. While a unity gain buffer (G = +1 V/V) is the

most sensitive to the capacitive loads, all gains show

the same general behavior.

When driving large capacitive loads with the

MCP6476/6R/6U/7/9 op amp, a small series resistor at

the output (RISO in Figure 4-5) improves the feedback

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

load resistive at higher frequencies. The bandwidth is

generally lower than the bandwidth with no capacitance

load.

FIGURE 4-5:

Output Resistor, RISO,

Stabilizes Large Capacitive Loads.

VDD

VSS

VL = 1V

VDD

5.5V

VOUT

VIN

RISO

VOUT

–

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