SSM2250RM 查看數據表（PDF） - Analog Devices
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SSM2250RM Datasheet PDF : 10 Pages
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SSM2250
The output coupling capacitor creates a high-pass filter with a
cutoff frequency of:
f
−3dB
=
1
2πRL CC
(2)
Where, RL is the resistance of the headphone, and
CC is the output coupling capacitor.
Although a majority of headphones have around 80 Ω of resistance,
this resistance can vary between models and manufacturers. Head-
phone resistances are commonly between 32 Ω to 600 Ω. Using a
220 µF capacitor as shown in Figure 12, the worst-case –3 dB corner
frequency would be 22 Hz, with a 32 Ω headphone load. Smaller
output capacitors could be used at the expense of low frequency
response to the headphones.
An input coupling capacitor should be used to remove dc bias
from the inputs to the SSM2250. Again, the input coupling
capacitor in combination with the input resistor will create a
high-pass filter with a corner frequency of:
f−3dB
=
1
2π R1C1
(3)
Using the values shown in Figure 2, where R1␣ =␣ 20 kΩ and
C1␣ =␣ 1 µF, will create a corner frequency of 8 Hz. This is
acceptable, as the PC 99 audio requirement specifies the com-
puter audio system bandwidth to be 20 Hz to 20 kHz.
Pin 10 on the SSM2250 provides the proper bias voltage for the
amplifiers. A 0.1 µF capacitor should be connected here to
reduce sensitivity to noise on the power supply. A larger capaci-
tor can be used should more rejection from power supply noise
be required.
The SSM2250 has excellent phase margin and is stable even
under heavy loading. Therefore, a feedback capacitor in parallel
with R2 is not required, as it is in some competitors’ products.
Power Dissipation
An important advantage in using a bridged output configuration
is the fact that bridged output amplifiers are more efficient than
single-ended amplifiers in delivering power to a load.
1.5
VDD = 5V
1.25
1.0
RL = 4⍀
0.75
RL = 8⍀
0.5
RL = 16⍀
0.25
0
0
0.25
0.5
0.75
1.0
1.25
1.5
OUTPUT POWER – W
Figure 13. Power Dissipation vs. Output Power in BTL Mode
2
PDISS , MAX
= 2VDD
π 2RL
(4)
Using Equation 4 and the power derating curve in Figure 15,
the maximum ambient temperature can be easily found. This
ensures that the SSM2250 will not exceed its maximum junc-
tion temperature of 150°C.
The power dissipation for a single-ended output application where
an output coupling capacitor is used is shown in Figure 14.
0.35
VDD = 5V
0.3
RL = 4⍀
0.25
0.2
RL = 8⍀
0.15
0.1
RL = 16⍀
0.05
0
0
0.1
0.2
0.3
0.4
OUTPUT POWER – W
Figure 14. Power Dissipation vs. Single-Ended Output
Power (VDD␣ = 5␣ V)
The maximum power dissipation for a single-ended output is:
2
PDISS , MAX
= VDD
2π 2RL
(5)
Because the SSM2250 is designed to drive two single-ended
loads simultaneously, the worst-case maximum power dissipation
in SE Mode is twice the value of Equation␣ 5.
A thorough mathematical explanation behind Equation␣ 4 and
Equation␣ 5 is given in the SSM2211 data sheet, which can be
downloaded at http://www.analog.com.
Example: Given worst-case stereo headphone loads of 32 Ω,
the maximum power dissipation of the SSM2250 in SE Mode
with a 5 V supply would be:
( )2
5V
PDISS ,
MAX
= 2π2 32
= 79
Ω
mW
(6)
With an 8 Ω internal speaker attached, the maximum power
dissipation in BTL mode is (from Equation 4):
2 × (5 V )2
PDISS, MAX = π28 Ω = 633 mW
(7)
It can be easily seen that power dissipation from BTL Mode
operation is of greater concern than SE Mode.
Solving for Maximum Ambient Temperature
To protect the SSM2250 against thermal damage, the junction
temperature of the die should not exceed 150°C. The maximum
allowable ambient temperature of the application can be easily
found by solving for the expected maximum power dissipation in
Equation␣ 4 and Equation␣ 5, and using Equation␣ 8.
REV. 0
–7–
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