IS31AP4912
Integrated Silicon Solution, Inc. – www.issi.com 7
Rev. C, 12/23/2015
APPLICATION INFORMATION
CHARGE PUMP CONVERTER
IS31AP4912 integrate a charge pump converter to
change input supply voltage (V
CC
) into a negative
voltage providing a 0V reference voltage for output.
The charge pump converter only needs three external
components: supply decoupling capacitor, output
bypass capacitor and flying capacitor.
Choose low ESR capacitors to ensure the best
operating performance and place the capacitors as
close as possible to the IS31AP4912.
GAIN SETTING
The input resistors (R
IN
) and feedback resistors (R
F
)
set the gain of the amplifier according to Equation (1).
IN
F
R
R
ain G
V
V
(1)
For example, in Figure 1:
R
F
= 20k, R
IN
= 20k,
so,
1G
20
20
ain
V
V
Resistor matching is very important in the amplifiers.
The balance of the output on the reference voltage
depends on matched ratios of the resistors. CMRR,
PSRR, and cancellation of the second harmonic
distortion diminish if resistor mismatch occurs.
Therefore, it is recommended to use 1% tolerance
resistors or better to keep the performance optimized.
Matching is more important than overall tolerance.
Resistor arrays with 1% matching can be used with a
tolerance greater than 1%.
Place the input resistors very close to the IS31AP4912
to limit noise injection on the high-impedance nodes.
INPUT CAPACITOR (C
IN
)
The input capacitors and input resistors form a high
pass filter with the corner frequency, f
C
, determined in
Equation (2).
ININ
CR
c
f
2
1
(2)
For example, in Figure 1:
R
IN
= 20k, C
IN
= 0.47µF,
so,
Hz
c
f
Fk
17
47.0202
1
The value of the input capacitor is important to
consider as it directly affects the bass (low frequency)
performance of the circuit. The capacitors should have
a tolerance of ±10% or better, because any mismatch
in capacitance causes an impedance mismatch at the
corner frequency and below.
DESIGN NOTE
COMPONENT SELECTION
The value and ESR of the output capacitor for charge
pump will affect output ripple and transient
performance. A X7R or X5R ceramic capacitor in 2.2F
should be recommended. The flying capacitor should
use a 2.2µF X7R or X5R ceramic capacitor.
All the capacitors should support at least 10V.
PCB LAYOUT
The decoupling capacitors should be placed close to
the VCC pin and the output capacitors should be
placed close to the VSS pin. The flying capacitor
should be placed close to the CN and CP pins. The
input capacitors and input resistors should be placed
close to the INR and INL pins and the traces must be
parallel to prevent noise. The traces of OUTR and
OUTL pins connected to the headphone should be as
possible as short and wide. The recommended width is
0.5mm.
Trace width should be at least 0.75mm for the power
supply and the ground plane. The SGND and PGND
pins of the IS31AP4912 must be routed separately
back to the decoupling capacitor in order to provide
proper device operation. If the SGND and PGND pins
are connected directly to each other, the part functions
without risk of failure, but the noise and THD
performance do not meet the specifications.
