SAE 800
Digital / Analog Converter, Loudness and Junction Control
The DAC converts the 4-bit words from the logic into the appropriate staircase currents with the
particular tone frequency. The sum current
I
I
drives the following current amplifier. The loudness
generator produces the DAC reference current
I
L
for all three tones. This requires connecting an
external resistor to ground. The chip temperature is monitored by the junction control. At
temperatures of more then approx. 170 ˚C the stop input will switch the output current
I
I
to zero. The
output current is enabled again once the chip has cooled down to approx. 150 ˚C.
Current Amplifier
The current amplifier with a gain of 1600 boosts the current
I
I
from approx. 470
µA
maximum to
approx. 750 mA maximum. The output stage consists of an NPN transistor with its emitter on power
GND and collector on pin Q.
The current control insures that the output stage only conducts defined currents. In conjunction with
the integrated thermal shutdown, this makes the configuration shortcircuit-protected within wide
limits. Because of the absence of feedback the circuit is also extremely stable and therefore
uncritical in applications. Resistor
R
L
on pin L sets the output voltage swing. This assumes that the
resistive component of the loudspeaker impedance
R
Q
responds similarly as the resistance
R
L
.
The output amplitude of the current
I
I
reaches the maximum
I
Imax
≅
3 x
V
L
/
R
L
at a time
t
of 2.33 s
(only 3 tone mode), so
R
L
has to be scaled for this point.
The following applies:
I
Q
=
I
Imax
x B = (
V
S
–
V
sat
) /
R
Q
≈
0.8
V
S
/
R
Q
3 x B x (
V
L
/
R
L
)
≈
0.8
V
S
/
R
Q
the result is:
R
L
=
R
Q
x 3 x B x (
V
L
/ 0.8
V
S
)
R
L
=
R
Q
x K x (
V
L
/ 0.8
V
S
)
with: B = 1600
with: K = 4800
Semiconductor Group
6
SIEMENS [ Siemens Semiconductor Group ]
