G1420
Global Mixed-mode Technology Inc.
Application Information
Input MUX Operation
There are two input signal paths – HP & Line. With the
prompt setting, G1420 allows the setting of different
gains for BTL and SE modes. Generally, speakers
typically require approximately a factor of 10 more
gain for similar volume listening levels as compared
with headphones.
-3 dB
SE Gain(HP)
BTL Gain(LINE)
To achieve headphones and speakers listening parity,
(RF(LINE/RI(LINE)) is suggested to be 5 times of (RF(HP)
=
-(RF(HP)/RI(HP)
)
fc
=
-2(RF(LINE)/RI(LINE)
)
Figure B
/
RI(HP)). The ratio of (RF(HP)/RI(HP)) can be determined by
the applications. When the optimum distortion per-
formance into the headphones (clear sound) is impor-
tant, gain of –1 ((RF(HP) / RI(HP)) = 1) is suggested.
Bridged-Tied Load Mode Operation
G1420 has two linear amplifiers to drive both ends of
the speaker load in Bridged-Tied Load (BTL) mode
operation. Figure C shows the BTL configuration. The
differential driving to the speaker load means that
when one side is slewing up, the other side is slewing
down, and vice versa. This configuration in effect will
double the voltage swing on the load as compared to a
ground reference load. In BTL mode, the peak-to-peak
voltage VO(PP) on the load will be two times than a
ground reference configuration. The voltage on the
load is doubled, this will also yield 4 times output
power on the load at the same power supply rail and
loading. Another benefit of using differential driving
configuration is that BTL operation cancels the dc off-
sets, which eliminates the dc coupling capacitor that is
needed to cancelled dc offsets in the ground reference
configuration. Low-frequency performance is then lim-
ited only by the input network and speaker responses.
Cost and PCB space can be minimized by eliminating
the dc coupling capacitors.
Single Ended Mode Operation
G1420 can drive clean, low distortion SE output power
into headphone loads (generally 16Ω or 32Ω) as in
Figure A. Please refer to Electrical Characteristics to
see the performances. A coupling capacitor is needed
to block the dc offset voltage, allowing pure ac signals
into headphone loads. Choosing the coupling capaci-
tor will also determine the 3 dB point of the high-pass
filter network, as Figure B.
fC=1/(2πRLCC)
For example, a 68uF capacitor with 32Ω headphone
load would attenuate low frequency performance be-
low 73Hz. So the coupling capacitor should be well
chosen to achieve the excellent bass performance
when in SE mode operation.
VDD
VDD
Vo(PP)
Vo(PP)
RL
2xVo(PP)
-Vo(PP)
CC
VDD
Vo(PP)
RL
Figure A
Figure C
TEL: 886-3-5788833
http://www.gmt.com.tw
Ver: 1.5
Aug 04, 2005
18
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
