LM4985TM/NOPB National Semiconductor, LM4985TM/NOPB Datasheet - Page 24

• Manufacturer Part Number: LM4985TM/NOPB
• Description: IC AMP AUDIO PWR .135W AB 12USMD
• Manufacturer: National Semiconductor
• Series: Boomer®r
• Type: Class ABr

Specifications of LM4985TM/NOPB

• Output Type: Headphones, 2-Channel (Stereo)
• Max Output Power X Channels @ Load: 135mW x 2 @ 16 Ohm
• Voltage - Supply: 2.3 V ~ 5.5 V
• Features: I²C, Shutdown, Thermal Protection, Volume Control
• Mounting Type: Surface Mount
• Package / Case: 12-MicroSMD
• Operational Class: Class-AB
• Audio Amplifier Output Configuration: 2-Channel Stereo
• Output Power (typ): 135x2@16OhmW
• Audio Amplifier Function: Headphone
• Single Supply Voltage (typ): 3/5V
• Dual Supply Voltage (typ): Not RequiredV
• Power Supply Requirement: Single
• Rail/rail I/o Type: No
• Power Supply Rejection Ratio: 77dB
• Single Supply Voltage (min): 2.3V
• Single Supply Voltage (max): 5.5V
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Operating Temp Range: -40C to 85C
• Operating Temperature Classification: Industrial
• Mounting: Surface Mount
• Pin Count: 12
• For Use With: LM4985TMEVAL - BOARD EVALUATION LM4985TM
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: LM4985TMTR

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Application Information
AMPLIFIER CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4985 has three internal power
amplifiers. Two of the amplifiers which amplify signals ap-
plied to their inputs, have internally configurable gain. The
remaining third amplifier provides both half-supply output
bias and AC ground return.
Loads, such as a headphone speaker, are connected be-
tween OUT1 and CNTGND or OUT2 and CNTGND. This
configuration does not require an output coupling capacitor.
The classical single-ended amplifier configuration, where
one side of the load is connected to ground, requires large,
expensive output coupling capacitors.
A configuration such as the one used in the LM4985 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs OUT1, OUT2, and CNTGND are all biased
at 1/2 V
eliminates the need for output coupling capacitors which are
required in a single-supply, single-ended amplifier configura-
tion. Without output coupling capacitors in a typical single-
supply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
The LM4985 eliminates these output coupling capacitors
when operating in Output Capacitor-less (OCL) mode. Un-
less shorted to ground, VoC is internally configured to apply
a 1/2 V
This voltage matches the bias voltage present on VoA and
VoB outputs that drive the headphones. The headphones
operate in a manner similar to a bridge-tied load (BTL).
Because the same DC voltage is applied to both headphone
speaker terminals this results in no net DC current flow
through the speaker. AC current flows through a headphone
speaker as an audio signal’s output amplitude increases on
the speaker’s terminal.
The headphone jack’s sleeve is not connected to circuit
ground when used in OCL mode. Using the headphone
output jack as a line-level output will place the LM4985’s 1/2
V
sents no difficulty when the external equipment uses capaci-
tively coupled inputs. For the very small minority of equip-
ment that is DC coupled, the LM4985 monitors the current
supplied by the amplifier that drives the headphone jack’s
sleeve. If this current exceeds 500mA
shutdown, protecting the LM4985 and the external equip-
ment.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier. When operating in capacitor-coupled mode (C-
CUPL), Equation 1 states the maximum power dissipation
point for a single-ended amplifier operating at a given supply
voltage and driving a specified output load.
When operating in the OCL mode, the LM4985’s three op-
erational amplifiers produce a maximum power dissipation
given in Equation 2:
DD
bias voltage on a plug’s sleeve connection. This pre-
P
DD
DD
DMAX
, no net DC voltage exists across each load. This
bias voltage to a stereo headphone jack’s sleeve.
= [2(V
P
DMAX
DD
)
= 2(V
2
/ (2π
DD
2
R
)
2
L
)] + [V
/ (2π
PEAK
2
R
DD
L
)
2
, the amplifier is
/ (4πR
L
)]
(1)
(2)
24
The maximum power dissipation point obtained from Equa-
tion 1 or Equation 2 must not be greater than the power
dissipation that results from Equation 3:
For package TMD12AAA, θ
the LM4985. Depending on the ambient temperature, T
the system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 2 is greater than that of
Equation 3, then either the supply voltage must be de-
creased, the load impedance increased or T
For a typical application using a 3.6V power supply, with a
32Ω load, the maximum ambient temperature possible with-
out violating the maximum junction temperature is approxi-
mately 144˚C provided that device operation is around the
maximum power dissipation point. Thus, for typical applica-
tions, power dissipation is not an issue. Power dissipation is
a function of output power and thus, if typical operation is not
around the maximum power dissipation point, the ambient
temperature may be increased accordingly. Refer to the
Typical Performance Characteristics curves for power dissi-
pation information for lower output powers.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be
as close to the device as possible.
Typical applications employ a regulator with 10µF tantalum
or electrolytic capacitor and a ceramic bypass capacitor
which aid in supply stability. This does not eliminate the need
for bypassing the supply nodes of the LM4985. A bypass
capacitor value in the range of 0.1µF to 1µF is recommended
for C
MICRO POWER SHUTDOWN
The LM4985’s micropower shutdown is activated or deacti-
vated through its I
1 for the I
registers. Each amplifier within the LM4985 can be shut-
down individually.
Please observe the following protocol when placing an indi-
vidual amplifier channel in shutdown while the other channel
remains active. The protocol requires activating both chan-
nels’ shutdown simultaneously, then deactivating the shut-
down of the channel whose output is desired (or leaving the
desire channel in shutdown mode). Also, when operating in
the C-CUPL mode, a short delay time is required between
activating one channel after placing both channels in shut-
down. If the user finds that both channels activate when only
one was chosen, increase the delay.
SELECTION OF INPUT CAPACITOR SIZE
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor, C
be expensive and may compromise space efficiency in por-
table designs. In many cases, however, the headphones
used in portable systems have little ability to reproduce
signals below 60Hz. Applications using headphones with this
limited frequency response reap little improvement by using
a high value input capacitor.
In addition to system cost and size, turn on time is affected
by the size of the input coupling capacitor C
S
.
2
C Address, Register Select, and Mode Control
P
DMAX
2
C digital interface . Please refer to Table
= (T
JA
JMAX
= 190˚C/W. T
i
. A high value capacitor can
- T
A
) / θ
JA
JMAX
A
i
. A larger input
reduced.
= 150˚C for
A
, of
(3)