MAX4298/MAX4299
Ultra-High PSRR Stereo Drivers +
Microphone Amp + 100mA Linear Regulator
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Regulator (MAX4299)
The MAX4299 also has an additional 100mA low-
dropout (LDO) regulator to provide clean analog power
for other sensitive analog circuitry on the PC board,
such as a typical PC99 audio codec or microphone
biasing. It is designed to provide good AC line regula-
tion. The nominal output voltage of REG is 3.3V, and is
adjustable between 1.2V and 4.5V by connecting a
resistor-divider from REG to GND. Connect FB to the
junction of the resistor-divider. The input impedance at
FB is typically 7kΩ, which should be considered in out-
put voltage calculations. REG should be bypassed to
GND with at least 10µF.
Applications Information
Capacitor Selection and
Regulator Stability
Normally, use a 10µF capacitor on REG (MAX4299 only)
and a 1µF capacitor on C
BYPASS
. Larger capacitor val-
ues and lower ESRs provide better supply-noise rejec-
tion and line-transient response. Reduce noise and
improve load-transient response, stability, and power-
supply rejection by using larger capacitors. For stable
operation over the full temperature range and load cur-
rents up to 100mA, a minimum of 10µF (REG) and 1µF
(C
BYPASS
) is recommended.
Use a 1µF bypass capacitor on BIAS to ensure a fully
clickless/popless power-up sequence. Smaller capacitor
values may be used here to decrease the power-up
time, but may cause the power-up transient to become
audible. Larger bypass capacitors are not necessary to
reduce noise and/or improve AC power-supply rejection.
SV
CC
is the standby power supply. If using an external
diode for charging, a 220µF reservoir capacitor on
SV
CC
provides standby power for the clickless power-
down sequence. Smaller capacitors here may cause an
audible output transient on power-down; 220µF or high-
er provides enough energy when C
BIAS
= 1µF. For larg-
er values of C
BIAS
, increase the reservoir capacitor
accordingly.
Mic Biasing (MAX4299)
Common microphone elements require resistive biasing
to power their internal circuitry. A 2kΩ resistor is typical-
ly used, and the microphone is AC-coupled to the
microphone amplifier. If the microphone element allows
low-voltage operation, biasing to the REG output pro-
vides excellent power-supply rejection.
Power Supply and Bypassing
The excellent PSRR of the MAX4298/MAX4299 allows
them to operate from noisy power supplies. In most
applications, a 0.1µF capacitor from V
CC
to GND is suf-
ficient. This bypass capacitor should be placed close
to the V
CC
pin.
Layout
Good layout improves performance by decreasing the
amount of stray capacitance and noise at the power
amplifier’s inputs and output. To decrease stray capaci-
tance, minimize PC board trace lengths and resistor
leads, and place external components as close to the
pins as possible.
Power Dissipation
The first equation below indicates the maximum power
dissipation point for a package that has two power
amplifiers operating at identical known supply voltages
and loads with sine wave inputs:
P
IC(DISS)
= (V
CC
)
2
/ (π
2
R
L
) [W]
For example, with a 5V power supply and a load of 16Ω,
the maximum power dissipation of the amplifiers alone
is 317mW.
The additional power dissipation due to the 100mA reg-
ulator operating at maximum current is nominally
170mW, but will increase if the output is reduced exter-
nally from its nominal 3.3V. The regulator power con-
sumption is given by:
P
REG
= (V
CC
- V
REG
) x 100mA [W]
To avoid thermal shutdown the sum of the regulator and
amplifier power dissipation must not exceed the absolute
maximum power-dissipation rating of the package.
Short-Circuit Protection and
Thermal Shutdown
The MAX4298/MAX4299 have short-circuit current pro-
tection on all outputs. They also have a thermal shut-
down function designed to protect the chip from junction
temperatures in excess of +150°C that may arise from
temporary short circuits or operation beyond the power
dissipation limit of the package. The driver amplifier out-
puts limit at around ±220mA, the regulator at 150mA,
and the microphone amplifier at +1.5mA/-12mA.
USB Applications
Universal serial bus (USB) interfaces are an increasingly
popular method of interfacing medium-speed (up to
12Mbps) PC peripherals. One of the great benefits of
the USB interface is the inclusion of a +5V supply. While
this supply works well for a mouse or keyboard, its sus-
ceptibility for noise pickup can be unsuitable for high-
fidelity audio applications. The MAX4298/MAX4299s’
excellent PSRR make them ideal candidates for USB
applications due to their insensitivity to the supply noise.
Of particular interest is an Internet-Protocol (IP) phone.
This PC peripheral uses the local internet service
provider as a free long-distance phone. The MAX4299,
with its integral microphone amp, headphone driver,
and linear regulator, can be a key element in the imple-
mentation of an IP phone that interfaces to the PC
through the USB.
