Rainbow Electronics MAX9755 User Manual

Page 22

MAX9750/MAX9751/MAX9755

2.6W Stereo Audio Power Amplifiers and
DirectDrive Headphone Amplifiers

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resistance to an extent. See the Output Power vs.
Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristics. Above
2.2µF, the on-resistance of the switches and the ESR of
C1 and C2 dominate.

Output Capacitor (C2)

The output capacitor value and ESR directly affect the
ripple at CPV

. Increasing the value of C2 reduces

output ripple. Likewise, decreasing the ESR of C2
reduces both ripple and output resistance. Lower
capacitance values can be used in systems with low
maximum output power levels. See the Output Power
vs. Charge-Pump Capacitance and Load Resistance
graph in the Typical Operating Characteristics.

CPV

Bypass Capacitor

The CPV

bypass capacitor (C3) lowers the output

impedance of the power supply and reduces the
impact of the MAX9750/MAX9751/MAX9755’s charge-
pump switching transients. Bypass CPV

with C3, the

same value as C1, and place it physically close to
CPV

and PGND (refer to the MAX9750 Evaluation Kit

for a suggested layout).

Powering Other Circuits from a

Negative Supply

An additional benefit of the MAX9750/MAX9751/
MAX9755 is the internally generated negative supply volt-
age (CPV

). CPV

is used by the MAX9750/

MAX9751/MAX9755 to provide the negative supply for
the headphone amplifiers. It can also be used to power
other devices within a design. Current draw from CPV

should be limited to 5mA, exceeding this affects the oper-
ation of the headphone amplifier. A typical application is
a negative supply to adjust the contrast of LCD modules.

When considering the use of CPV

in this manner,

note that the charge-pump voltage of CPV

is roughly

proportional to CPV

and is not a regulated voltage.

The charge-pump output impedance plot appears in
the Typical Operating Characteristics.

Layout and Grounding

Proper layout and grounding are essential for optimum
performance. Use large traces for the power-supply
inputs and amplifier outputs to minimize losses due to
parasitic trace resistance, as well as route head away
from the device. Good grounding improves audio per-
formance, minimizes crosstalk between channels, and
prevents any switching noise from coupling into the
audio signal. Connect CPGND, PGND and GND
together at a single point on the PC board. Route
CPGND and all traces that carry switching transients
away from GND, PGND, and the traces and compo-
nents in the audio signal path.

Connect all components associated with the charge
pump (C2 and C3) to the CPGND plane. Connect V

and CPV

together at the device. Place the charge-

pump capacitors (C1, C2, and C3) as close to the
device as possible. Bypass HPV

and PV

with a

0.1µF capacitor to GND. Place the bypass capacitors
as close to the device as possible.

Use large, low-resistance output traces. As load imped-
ance decreases, the current drawn from the device out-
puts increase. At higher current, the resistance of the
output traces decrease the power delivered to the load.
For example, when compared to a 0Ω trace, a 100mΩ
trace reduces the power delivered to a 4Ω load from
2.1W to 2W. Large output, supply, and GND traces also
improve the power dissipation of the device.

The MAX9750/MAX9751/MAX9755 thin QFN package
features an exposed thermal pad on its underside. This
pad lowers the package’s thermal resistance by provid-
ing a direct heat conduction path from the die to the
printed circuit board. Connect the exposed thermal
pad to GND by using a large pad and multiple vias to
the GND plane.

MAX9750

INR

INL

1µF

22nF

OUTL+

OUTL-

OUTR+

OUTR-

OUT-

OUT+

22µF

10nF

20kΩ

10kΩ

MAX9711

Figure 11. Stereo Plus Subwoofer Application Circuit