Datasheet

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MAX98307/MAX98308
3.3W Mono Class DG Multilevel Audio Amplifier
Detailed Description
The MAX98307/MAX98308 fully differential mono Class
DG multilevel power amplifiers with integrated inverting
charge pumps offer highly efficient, high-power audio
solutions for portable applications.
The new Class DG multilevel modulation scheme extends
the dynamic range of the output signal by employing a
charge-pump-generated negative rail, which is used as
needed to extend the supply range. When the negative
rail is not needed, the output is drawn entirely from the
standard supply. This scheme results in high efficiency
over a wide output power range.
The power amplifier incorporates active emissions limit-
ing edge rate and overshoot control circuitry in combi-
nation with the multilevel output modulation scheme to
greatly reduce EMI. These features eliminate the need for
output filtering as compared to traditional Class D ampli-
fiers, which reduces an application’s component count.
The MAX98307 has an adjustable gain set by external
resistors. The MAX98308 has preset fixed gains of 8.5dB,
11.5dB, 14.5dB, 17.5dB, and 20.5dB set by a gain select
input (GAIN).
Class DG Multilevel Operation
The ICs’ filterless Class DG multilevel amplifiers feature
a proprietary Maxim output stage that offers higher effi-
ciency over a greater output power range than previous
amplifiers. The amplifier combines Class D switching
output efficiency and Class G supply level shifting with a
multilevel output modulation scheme that with a 5V supply
has efficiency better than 80% efficiency over the 0.35W
to 2.2W output range.
The Class DG multilevel output stage uses pulse-width
modulation (PWM), a rail-to-rail digital output signal with
variable duty cycle, to approximate an analog input signal
as in a Class D amplifier. Rail-to-rail operation ensures that
any dissipation at the output is due solely to the R
DS(ON)
of the power output MOSFETs. The Class DG multilevel
output stage also senses the magnitude of the output
signal and switches the supply rails as needed to more
efficiently supply the required signal power. For a low out-
put signal swing requirement (below the battery supply rail
V
PVDD
), the output range is between V
PVDD
and ground.
When output swing above V
PVDD
is required, V
PVSS
, an
internal inverting charge-pump-generated negative rail
replaces ground as the lower supply. The high output
swing range is then V
PVDD
to V
PVSS
, approximately dou-
ble the low swing range. This approach efficiently man-
ages power consumption by switching the operating rails
as needed according to the output swing requirements.
Additionally, multilevel output modulation is employed in to
draw the maximum possible power from the lower imped-
ance battery supply rail, V
PVDD
, rather than the higher
impedance charge-pump-generated rail V
PVSS
. This is
accomplished by generating PWM signals that swing
from ground to V
PVDD
or from ground to V
PVSS
at either
end of the bridge tied load (BTL) rather than continually
swinging from V
PVDD
to V
PVSS
.
The signals are modu-
lated in such a way that V
PVSS
is used only as necessary
to generate low-end signal swing.
These combined operations ensure that power dissipa-
tion due to R
DS(ON)
loss and charge-pump impedance
is minimized, and that efficiency and output power is
maximized across the audio range. Class DG multilevel
operation is shown as Figure 1.
Figure 1. Class DG Multilevel Operation
MAX98307 fig01
400µs/div
OUT+
5V/div
OUT-
5V/div
OUT+ - OUT-
10V/div