MAX4465–MAX4469
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 pream-
plifiers. 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 20µA of
supply current. The MAX4466/MAX4468 have a mini-
mum stable gain of +5V/V while providing a 500kHz
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 10kΩload and
still typically swing within 20mV of the supply rails.
Figure 1 shows the output voltage swing of the
MAX4465 configured with AV= +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 micro-
amps of supply current, which can be significant in low-
power applications. The MIC_BIAS pin provides a
switched version of VCC 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 10nA, 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 shutdown waveform.
Common-Mode Rejection Ratio
A microphone preamplifier ideally only amplifies the
signal 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
common mode. The ratio of these voltages is called the
common-mode gain. The common-mode rejection ratio
is the logarithm of differential-mode gain to common-
mode gain. The high CMRR properties of the
MAX4465–MAX4469 provide outstanding performances
when configured as a noninverting microphone pream-
plifier.
Power-Up
The MAX4465–MAX4469 outputs typically settle within
1µs after power-up. Figure 6 shows the output voltage
on power-up.
Power Supplies and Layout
The MAX4465–MAX4469 operate from a single +2.4V
to +5.5V power supply. Bypass the power supply with a
1µF capacitor to ground. Good layout techniques are
necessary for the MAX4465–MAX4469 family. To
decrease stray capacitance, minimize trace lengths by
placing external components close to the op amp’s
pins. Surface-mount components are recommended. In
systems where analog and digital grounds are avail-
able, the MAX4465–MAX4469 should be connected to
the analog ground.