MAX4465–MAX4469
Low-Cost, Micropower, SC70/SOT23-8, Microphone
Preamplifiers with Complete Shutdown
7
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 10kΩ load and still
typically swing within 16mV of the supply rails. Figure 1
shows the output voltage swing of the MAX4465 config-
ured with A
V
= +10.
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
version of V
CC
to the bias components. Figure 3 shows
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 unconnected. Figure 5 shows the
shutdown 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)
GND.
