G1421 Datasheet, PDF (23/24 Pages) Global Mixed-mode Technology Inc – 2W Stereo Audio Amplifier with No Headphone Coupling Capacitor Function

Global Mixed-mode Technology Inc.
G1421
Optimizing DEPOP Operation
Circuitry has been implemented in G1421 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_Bx100kΩ) ≦ 1/(C_I*(R_I+R_F)).
Where 100kΩ is the output impedance of the
mid-rail generator, C_B is the mid-rail bypass ca-
pacitor, C_I is the input coupling capacitor, R_I is the
input impedance, R_F is the gain setting impedance
which is on the feedback path. C_B is the most im-
portant capacitor. Besides it is used to reduce the
popping, C_B can also determine the rate at which
the amplifier starts up during startup or recovery
from shutdown mode.
De-popping circuitry of G1421 is shown on Figure E.
The PNP transistor limits the voltage drop across
the 50kΩ by slewing the internal node slowly when
power is applied. At start-up, the voltage at
BYPASS capacitor is 0. The PNP is ON to pull the
mid-point of the bias circuit down. So the capacitor
sees a lower effective voltage, and thus the charg-
ing is slower. This appears as a linear ramp (while
the PNP transistor is conducting), followed by the
expected exponential ramp of an R-C circuit.
Junction Temperature Measurement
Characterizing a PCB layout with respect to thermal
impedance is very difficult, as it is usually impossi-
ble to know the junction temperature of the IC.
G1421 TJ (pin 2) sources a current inversely pro-
portional to the junction temperature. Typically TJ
sources–120µA for a 5V supply at 25°C. And the
slope is approximately 0.22µA/°C. As the resistors
have a tolerance of ±20%, these values should be
calibrated on each device. When the temperature
sensing function is not used, TJ pin can be left
floating or tied to VDD to reduce the current con-
sumption.
Temperature sensing circuit is shown on Figure F.
VDD
R
R
5R
TJ
Bypass
50 kΩ
VDD
100 kΩ
Figure F
100 kΩ
Figure E
Ver: 1.6
Aug 04, 2005
TEL: [phone]
http://www.gmt.com.tw
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