Detailed Description
The MAX4364/MAX4365 bridged audio power ampli-
fiers can deliver 1.4W into 8Ω (MAX4364) or 1W into 8Ω
(MAX4365) while operating from a single 5V supply.
These devices consist of two high-output-current op
amps configured as a bridge-tied load (BTL) amplifier
(see
Typical Application Circuit/Functional Diagram
).
The gain of the device is set by the closed-loop gain of
the input op amp. The output of the first amplifier
serves as the input to the second amplifier, which is
configured as an inverting unity-gain follower in both
devices. This results in two outputs, identical in magni-
tude, but 180° out of phase.
BIAS
The MAX4364/MAX4365 feature an internally generated
common-mode bias voltage of V
CC
/2 referenced to
GND. BIAS provides both click-and-pop suppression
and the DC bias level for the audio signal. BIAS is inter-
nally connected to the noninverting input of one amplifi-
er, and should be connected to the noninverting input
of the other amplifier for proper signal biasing (see
Typical Application Circuit/Functional Diagram
).
Choose the value of the bypass capacitor as described
in the
BIAS Capacitor
section.
Shutdown
The MAX4364/MAX4365 feature a 10nA, low-power
shutdown mode that reduces quiescent current con-
sumption. Pulling SHDN high disables the device’s bias
circuitry, the amplifier outputs go high impedance, and
BIAS is driven to GND. Connect SHDN to GND for nor-
mal operation.
Current Limit
The MAX4364/MAX4365 feature a current limit that pro-
tects the device during output short circuit and over-
load conditions. When both amplifier outputs are
shorted to either V
CC
or GND, the short-circuit protec-
tion is enabled and the amplifier enters a pulsing mode,
reducing the average output current to a safe level. The
amplifier remains in this mode until the overload or
short-circuit condition is removed.
Applications Information
Bridge-Tied Load
The MAX4364/MAX4365 are designed to drive a load
differentially in a BTL configuration. The BTL configura-
tion (Figure 1) offers advantages over the single-ended
configuration, where one side of the load is connected
to ground. Driving the load differentially doubles the
output voltage compared to a single-ended amplifier
under similar conditions. Thus, the differential gain of
the device is twice the closed-loop gain of the input
amplifier. The effective gain is given by:
Substituting 2
V
OUT(P-P)
into the following equations
yields four times the output power due to doubling of
the output voltage.
Since the differential outputs are biased at midsupply,
there is no net DC voltage across the load. This elimi-
nates the need for DC-blocking capacitors required for
single-ended amplifiers. These capacitors can be
large, expensive, consume board space, and degrade
low-frequency performance.
Power Dissipation
Under normal operating conditions, the MAX4364/
MAX4365 can dissipate a significant amount of power.
The maximum power dissipation for each package is
given in the
Absolute Maximum Ratings
section under
Continuous Power Dissipation or can be calculated by
the following equation:
where T
J(MAX)
is +150°C, T
A
is the ambient temperature
and θ
JA
is the reciprocal of the derating factor in °C/W
as specified in the
Package Thermal Characteristics
section. For example, θ
JA
of the µMAX package is
206.3°C/W.
