G1428 Datasheet, PDF (13/16 Pages) Global Mixed-mode Technology Inc – 2W Stereo Audio Amplifier

Global Mixed-mode Technology Inc.
G1428
Application Information
Gain setting via GAIN0 and GAIN1 inputs
The internal gain setting is determined by two input
terminals, GAIN0 and GAIN1. The gains listed in Table
1 are realized by changing the taps on the input resis-
tors inside the amplifier. This will cause the internal
input impedance, Z_I, to be dependent on the gain set-
ting. Although the real input impedance will shift by
30% due to process variation from part-to-part, the
actual gain settings are controlled by the ratios of the
resistors and the actual gain distribution from part-to-
part is quite good.
Table 1
GAIN0
0
0
1
1
X
GAIN1
0
1
0
1
X
SE/ BTL
0
0
0
0
1
A_V (V/V)
-2
-6
-12
-24
-1
Input Resistance
The typical input impedance at each gain setting is
given in the Table 2. Each gain setting is achieved by
varying the input resistance of the amplifier, which can
be over 6 times from its minimum value to the maxi-
mum value. As a result, if a single capacitor is used in
the input high pass filter, the -3dB or cut-off frequency
will be also change over 6 times. To reduce the varia-
tion of the cut-off frequency, an additional resistor can
be connected from the input pin of the amplifier to the
ground, as shown in Figure 1. With the extra resistor,
the cut-off frequency can be re-calculated using equa-
tion : f_-3dB= 1/ 2πC(R||R_I). Using small external R
can reduce the variation of the cut-off frequency. But
the side effect is small external R will also let (R||R_I)
become small, the cut-off frequency will be larger and
degraded the bass-band performance. The other side
effect is with extra power dissipation through the ex-
ternal resistor R to the ground. So using the external
resistor R to flatting the variation of the cut-off fre-
quency, the user must also consider the bass-band
performance and the extra power dissipation to
choose the accepted external resistor R value.
C
Zi
Zf
Input Signal
IN
R
Table 2
Zi (Kohm)
15
30
45
90
A_V (V/V)
-24
-12
-6
-2
Input Capacitor
In the typical application, an input capacitor C_i is re-
quired to allow the amplifier to bias the input signal to
the proper dc level for optimum operation. In this
case ,C_i and the input impedance of the amplifier, Z_i,
form a high-pass filter with the -3dB determined by the
equation: f_-3dB= 1/ (2πR_I C_i)
The value of C_i is important to consider as it directly
affects the bass performance of the application circuit.
For example, if the input resistor is 15kΩ, the input
capacitor is 1µF, the flat bass response will be down to
10.6Hz.
Because the small leakage current of the input ca-
pacitors will cause the dc offset voltage at the input to
the amplifier that reduces the operation headroom,
especially at the high gain applications. The low-
leakage tantalum or ceramic capacitors are suggested
to be used as the input coupling capacitors. When
using the polarized capacitors, it is important to let the
positive side connecting to the higher dc level of the
application.
Power Supply Decoupling
The G1428 is a high-performance CMOS audio ampli-
fier that requires adequate power supply decoupling to
make sure the output total harmonic distortion (THD)
as low as possible. The optimum decoupling is using
two capacitors with different types that target different
types of noise on the power supply leads. For high
frequency transients, spikes, a good low ESR ceramic
capacitor works best, typically 0.1µF/1µF used and
placed as close as possible to the G1428 VDD lead. A
larger aluminum electrolytic capacitor of 10uF or
greater placed near the device power is recommended
for filtering low-frequency noise.
Optimizing DEPOP Operation
Circuitry has been implemented in G1428 to mini-
mize the amount of popping heard at power-up and
when coming out of shutdown mode. Popping oc-
curs whenever a voltage step is applied to the
speaker and making the differential voltage gener-
ated at the two ends of the speaker. To avoid the
popping heard, the bypass capacitor should be
chosen promptly, 1/(C_Bx170kΩ) ≦ 1/(C_I*(R_I+R_F)).
Figure 1
Ver: 1.2
Mar 31, 2005
TEL: [phone]
http://www.gmt.com.tw
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