Rainbow Electronics MAX9758 User Manual

Page 23

Flying Capacitor (C1)

The value of the flying capacitor (C1) affects the load
regulation and output resistance of the charge pump. A
C1 value that is too small degrades the device’s ability to
provide sufficient current drive, which leads to a loss of
output voltage. Increasing the value of C1 improves load
regulation and reduces the charge-pump output resis-
tance to an extent. See the Output Power vs. Charge-
Pump Capacitance 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 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 MAX9756/MAX9757/MAX9758’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 MAX9756/MAX9757/MAX9758
Evaluation Kit for a suggested layout).

Powering Other Circuits

from a Negative Supply

An additional benefit of the MAX9756/MAX9757/
MAX9758 is the internally generated negative supply
voltage (CPV

). CPV

is used by the MAX9756/

MAX9757/MAX9758 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 operation 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.

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 heat 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

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 0

Ω trace, a 100mΩ

trace reduces the power delivered to a 4

Ω load from

2.1W to 2W. Large output, supply, and GND traces also
improve the power dissipation of the device. The
MAX9756/MAX9757/MAX9758 thin QFN package fea-
tures an exposed thermal pad on its underside. This pad
lowers the package’s thermal resistance by providing a
direct-heat conduction path from the die to the PC
board. Connect the exposed thermal pad to GND by
using a large pad and multiple vias to the GND plane.

MAX9756/MAX9757/MAX9758

2.3W Stereo Speaker Amplifiers and DirectDrive

Headphone Amplifiers with Automatic Level Control

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