Low-Cost, Micropower, SC70/SOT23-8, Microphone

Preamplifiers with Complete Shutdown

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Detailed Description

The MAX4465–MAX4469 are low-power, micropower op

amps designed to be used as microphone preampli-

fiers. These preamplifiers are an excellent choice for

noisy environments because of their high common-

mode rejection and excellent power-supply rejection

ratios. They operate from a single +2.4V to +5.5V sup-

ply.

The MAX4465/MAX4467/MAX4469 are unity-gain stable

and deliver a 200kHz gain bandwidth from only 24µA of

supply current. The MAX4466/MAX4468 have a mini-

mum stable gain of +5V/V while providing a 600kHz

gain bandwidth product.

The MAX4467/MAX4468 feature a complete shutdown,

which is active-high, and a shutdown-controlled output

providing bias to the microphone. The MAX4465/

MAX4467/MAX4469 feature a slew rate suited to voice

channel applications. The MAX4466/MAX4468 can be

used for full-range audio, e.g., PC99 inputs.

Rail-to-Rail Output Stage

The MAX4465–MAX4469 can drive a 10k

Ω load and still

typically swing within 16mV of the supply rails. Figure 1

shows the output voltage swing of the MAX4465 config-

Switched Bias Supply

When used as a microphone amplifier for an electret

microphone, some form of DC bias for the microphone

is necessary. The MAX4467/MAX4468 have the ability to

turn off the bias to the microphone when the device is in

shutdown. This can save several hundred microamps of

supply current, which can be significant in low power

applications. The MIC_BIAS pin provides a switched

some typical values.

Driving Capacitive Loads

Driving a capacitive load can cause instability in many

op amps, especially those with low quiescent current.

The MAX4465/MAX4467/MAX4469 are unity-gain stable

for a range of capacitive loads up to 100pF. Figure 4

shows the response of the MAX4465 with an excessive

capacitive load.

Applications Information

Shutdown Mode

The MAX4467 and MAX4468 feature a low-power, com-

plete shutdown mode. When SHDN goes high, the sup-

ply current drops to 5nA, the output enters a high

impedance state and the bias current to the microphone

is switched off. Pull SHDN low to enable the amplifier.

Do not leave SHDN floating. Figure 5 shows the shut-

down waveform.

Common-Mode Rejection Ratio

A microphone preamplifier ideally only amplifies the sig-

nal present on its input and converts it to a voltage

appearing at the output. When used in noninverting

mode, there is a small output voltage fluctuation when

both inputs experience the same voltage change in the

( ) denotes SOT23 package of the MAX4467/MAX4468.

Pin Description (continued)

PIN

MAX4465

MAX4466

MAX4467

MAX4468

MAX4469

NAME

FUNCTION

5

7 (7)

8

Positive Supply. Bypass with a 0.1µF capacitor to

GND.

——

2

INA-

Inverting Amplifier Input A

——

3

INA+

Noninverting Amplifier Input A

——

6

INB-

Inverting Amplifier Input B

——

5

INB+

Noninverting Amplifier Input B

——

7

OUTB

Amplifier Output B

—

8 (6)

—

SHDN

Active-High Shutdown Input. Connect to GND

shutdown. Do not leave floating.

—

5 (5)

—

N.C.

No Connection. Not internally connected.