IS31AP4088D
Integrated Silicon Solution, Inc. – www.issi.com
Rev. B, 01/03/2014
13
result is in Equation (4).
OLOUTPECK P2RV (3)
V
CC
≥ V
OUTPEAK
+ (V
ODTOP
+ V
ODBOT
) (4)
The Output Power vs. Supply Voltage graph for an 8Ω
load indicates a minimum supply voltage of 4.35V for a
1W output at 1% THD+N. This is easily met by the
commonly used 5V supply voltage. The additional
voltage creates the benefit of headroom, allowing the
IS31AP4088D to produce peak output power in excess
of 1.2W at 5V of V
CC
and 1% THD+N without clipping
or other audible distortion. The choice of supply
voltage must also not create a situation that violates
maximum power dissipation.
After satisfying the IS31AP4088D’s power dissipation
requirements, the minimum differential gain needed to
achieve 1W dissipation in an 8Ω load is found using
Equation (5).
inrmsormsINLOV /VVV/RPA (5)
Thus, a minimum gain of 2.83 allows the
IS31AP4088D’s to reach full output swing and
maintain low noise and THD+N performance. For this
example, let A
V
= 3.
The amplifier’s overall gain is set using the input, R
IN
,
and feedback resistors, R
F
. With the desired input
impedance set at 20kΩ, the feedback resistor is found
using Equation (6).
R
F
/R
IN
= A
V
/2 (6)
The value of R
F
is 30kΩ.
The last step in this design example is setting the
amplifier’s −3dB frequency bandwidth. To achieve the
desired ±0.25dB pass band magnitude variation limit,
the low frequency response must extend to at least
one-fifth the lower bandwidth limit and the high
frequency response must extend to at least five times
the upper bandwidth limit. The gain variation for both
response limits is 0.17dB, well within the ±0.25dB
desired limit. The results are an
f
L
= 100Hz/5 = 20Hz
and an
f
H
= 20kHz×5 = 100kHz.
As mentioned in the External Components section, R
IN
and C
IN
create a high pass filter that sets the amplifier’s
lower band pass frequency limit. Find the coupling
capacitor’s value using Equation (7).
C
IN
≥ 1/(2Πr
In
f
L
) (7)
The result is
1/(2π×20kΩ×20Hz) = 0.398μF
Use a 0.39μF capacitor, the closest standard value.
The product of the desired high frequency cut off
(100kHz in this example) and the differential gain, A
V
,
determines the upper pass band response limit. With
A
V
= 3 and f
H
= 100kHz, the closed-loop gain
bandwidth product (GBWP) is 300kHz. With this
margin, the amplifier can be used in designs that
require more differential gain while avoiding
performance-restricting bandwidth limitations.
