LM4865M National Semiconductor, LM4865M Datasheet - Page 8

• Manufacturer Part Number: LM4865M
• Description: IC, AUDIO PWR AMP, CLASS AB 750mW, SOIC8
• Manufacturer: National Semiconductor

Specifications of LM4865M

• Audio Control Type: Volume
• Output Power: 1W
• Control / Process Application: GSM Phones And Accessories, DECT, Office Phones, Hand Held Radio
• Supply Voltage Range: 2.7V To 5.5V
• No. Of Channels: 1
• Thd + N: 0.6% @ 8ohm, VDD=5V
• Load Impedance: 8ohm
• Operating Temperature Range: -40°C To +85°C
• Amplifier Case Style: MSOP
• Rohs Compliant: Yes
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

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Typical Performance Characteristics
Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4865 consists of two opera-
tional amplifiers internally. An external DC voltage sets the
closed-loop gain of the first amplifier, whereas two internal
20kΩ resistors set the second amplifier’s gain at -1. The
LM4865 can be used to drive a speaker connected between
the two amplifier outputs or a monaural headphone con-
nected between V
Figure 1 shows that the output of Amp1 serves as the input
to Amp2. This results in both amplifiers producing signals
that are identical in magnitude, but 180˚ out of phase.
Taking advantage of this phase difference, a load placed
between V
referred to as “bridge mode“ ). This mode is different from
single-ended driven loads that are connected between a
single amplifier’s output and ground.
Bridge mode has a distinct advantage over the single-ended
configuration: its differential drive to the load doubles the
output swing for a specified supply voltage. This results in
four times the output power when compared to a
single-ended amplifier under the same conditions. This in-
crease in attainable output assumes that the amplifier is not
current limited or the output signal is not clipped. To ensure
O
Supply Current vs Supply Voltage
1 and V
Output Power vs Supply Voltage
O
1 and GND.
O
2 is driven differentially (commonly
10102533
10102531
8
(Continued)
minimum output signal clipping when choosing an amplifier’s
closed-loop gain, refer to the Audio Power Amplifier De-
sign section.
Another advantage of the differential bridge output is no net
DC voltage across load. This results from biasing V
V
that single supply, single-ended amplifiers require. Eliminat-
ing an output coupling capacitor in a single-ended configu-
ration forces a single supply amplifier’s half-supply bias volt-
age across the load. The current flow created by the
half-supply bias voltage increases internal IC power dissipa-
tion and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful bridged or single-ended amplifier. 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.
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is an increase in
internal power dissipation point for a bridge amplifier oper-
ating at the same given conditions.
The LM4865 has two operational amplifiers in one package
and the maximum internal power dissipation is 4 times that
O
2 at half-supply. This eliminates the coupling capacitor
P
P
DMAX
Output Power vs Supply Voltage
DMAX
= 4*(V
= (V
DD
DD
)
2
)
/(2π
2
/(2π
2
R
2
R
L
) Single-Ended
L
) Bridge Mode
10102532
O
1 and
(1)
(2)