IS31AP4088D
Integrated Silicon Solution, Inc. – www.issi.com
Rev. B, 01/03/2014
11
APPLICATION INFORMATION
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The IS31AP4088D’s QFN (die attach paddle) package
provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered.
This allows rapid heat transfer from the die to the
surrounding PCB copper traces, ground plane and,
finally, surrounding air.
The QFN package must have it’s DAP soldered to a
copper pad on the PCB. The DAP’s PCB copper pad is
connected to a large plane of continuous unbroken
copper. This plane forms a thermal mass and heat sink
and radiation area. Place the heat sink area on either
outside plane in the case of a two-sided PCB, or on an
inner layer of a board with more than two layers.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the IS31AP4088D consists of
two pairs of operational amplifiers, forming a
two-channel (Channel A and Channel B) stereo
amplifier. External feedback resistors R
F
and input
resistors R
IN
set the closed-loop gain of Amp A (OUT-)
and Amp B (OUT-) whereas two internal 20kΩ
resistors set Amp A’s (OUT+) and Amp B’s (OUT+)
gain at 1. The IS31AP4088D drives a load, such
speaker, connected between the two amplifier outputs,
OUTA− and OUTA+.
Figure 1 shows that Amp A’s (OUT-) output serves as
Amp A’s (OUT+) input. This results in both amplifiers
producing signals identical in magnitude, but 180° out
of phase. Taking advantage of this phase difference, a
load is placed between OUTA− and OUTA+ and driven
differentially (commonly referred to as “bridge mode”).
This results in a differential gain of
A
V
= 2×(R
F
/R
IN
) (1)
Bridge mode amplifiers are different from single-ended
amplifiers that drive loads connected between a single
amplifier’s output and ground. For a given supply
voltage, bridge mode has a distinct advantage over the
single-ended configuration: its differential output
doubles the voltage swing across the load. This
produces four times the output power when compared
to a single-ended amplifier under the same conditions.
This increase in attainable output power assumes that
the amplifier is not current limited
Another advantage of the differential bridge output is
no net DC voltage across the load. This is
accomplished by biasing Channel A’s and Channel B’s
outputs at half-supply. This eliminates the coupling
capacitor that single supply, single ended amplifiers
require. Eliminating an output coupling capacitor in a
single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load.
This increases internal IC power dissipation and may
permanently damage loads such as speakers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing
is critical for low noise performance and high power
supply rejection. Applications that employ a 5V
regulator typically use a 10μF in parallel with a 0.1μF
filter capacitor to stabilize the regulator’s output,
reduce noise on the supply line, and improve the
supply’s transient response. However, their presence
does not eliminate the need for a local 1.0μF tantalum
bypass capacitance connected between the
IS31AP4088D’s supply pins and ground. Keep the
length of leads and traces that connect capacitors
between the IS31AP4088D’s power supply pin and
ground as short as possible.
MICRO-POWER SHUTDOWN
The voltage applied to the SDB pin controls the
IS31AP4088D’s shutdown function. Activate
micro-power shutdown by applying GND to the SDB
pin. When active, the IS31AP4088D’s micro-power
shutdown feature turns off the amplifier’s bias circuitry,
reducing the supply current. The low 0.1μA typical
shutdown current is achieved by applying a voltage
that is as near as GND as possible to the SDB pin.
There are a few ways to control the micro-power
shutdown. These include using a single-pole,
single-throw switch, a microprocessor, or a
microcontroller. When use a switch, connect an
external 100kΩ resistor between the SDB pin and
GND. Select normal amplifier operation by closing the
switch. Opening the switch sets the SDB pin to ground
through the 100kΩ resistor, which activates the micro
power shutdown. The switch and resistor guarantee
that the SDB pin will not float. This prevents unwanted
state changes. In a system with a microprocessor or a
microcontroller, use a digital output to apply the control
voltage to the SDB pin. Driving the SDB pin with active
circuitry eliminates the pull up resistor.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the IS31AP4088D’s performance requires
properly selecting external components. Though the
IS31AP4088D operates well when using external
components with wide tolerances, best performance is
achieved by optimizing component values.
The IS31AP4088D is unity-gain stable, giving a
designer maximum design flexibility. The gain should
be set to no more than a given application requires.
This allows the amplifier to achieve minimum THD+N
and maximum signal-to-noise ratio. These parameters
are compromised as the closed-loop gain increases.
However, low gain demands input signals with greater
voltage swings to achieve maximum output power.
Fortunately, many signal sources such as audio
CODECs have outputs of 1VRMS (2.83V
P-P
). Please
refer to the Audio Power Amplifier Design section for
