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| 型號： | LM4891MBD |
| 廠商： | National Semiconductor |
| 文件頁數(shù)： | 4/21頁 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION LM4891M |
| 標(biāo)準(zhǔn)包裝： | 1 |
| 系列： | Boomer® |
| 放大器類型： | AB 類 |
| 輸出類型： | 1-通道（單聲道） |
| 在某負(fù)載時(shí)最大輸出功率 x 通道數(shù)量： | 1W x 1 @ 8 歐姆 |
| 電源電壓： | 2.2 V ~ 5.5 V |
| 工作溫度： | -40°C ~ 85°C |
| 板類型： | 完全填充 |
| 已用 IC / 零件： | LM4891 |
| 已供物品： | 板 |
| 相關(guān)產(chǎn)品： | LM4891MX-ND - IC AMP AUDIO PWR 1W MONO 8SOIC LM4891MMTR-ND - IC AMP AUDIO PWR 1W MONO 8MSOP LM4891M-ND - IC AMP AUDIO PWR 1W MONO 8SOIC |
OBSOLETE
SNAS122E – MAY 2001 – REVISED APRIL 2013
APPLICATION INFORMATION
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4891 has two operational amplifiers internally, allowing for a few different amplifier
configurations. The first amplifier's gain is externally configurable, while the second amplifier is internally fixed in
a unity-gain, inverting configuration. The closed-loop gain of the first amplifier is set by selecting the ratio of Rf to
Ri while the second amplifier's gain is fixed by the two internal 20 k resistors. Figure 1 shows that the output of
amplifier one serves as the input to amplifier two which results in both amplifiers producing signals identical in
magnitude, but out of phase by 180°. Consequently, the differential gain for the IC is
AVD= 2 *(Rf/Ri)
By driving the load differentially through outputs Vo1 and Vo2, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is different from the classical single-ended amplifier
configuration where one side of the load is connected to ground.
A bridge amplifier design has a few distinct advantages over the single-ended configuration, as it provides
differential drive to the load, thus doubling output swing for a specified supply voltage. Four times the output
power is possible as compared to a single-ended amplifier under the same conditions. This increase in attainable
output power assumes that the amplifier is not current limited or clipped. In order to choose an amplifier's closed-
loop gain without causing excessive clipping, please refer to the AUDIO POWER AMPLIFIER DESIGN section.
A bridge configuration, such as the one used in LM4891, also creates a second advantage over single-ended
amplifiers. Since the differential outputs, Vo1 and Vo2, are biased at half-supply, no net DC voltage exists across
the load. This eliminates the need for an output coupling capacitor which is required in a single supply, single-
ended amplifier configuration. Without an output coupling capacitor, the half-supply bias across the load would
result in both increased internal IC power dissipation and also possible loudspeaker damage.
EXPOSED-DAP PACKAGE PCB MOUNTING CONSIDERATIONS FOR THE LM4891LD
The LM4891LD's exposed-DAP (die attach paddle) package (NGZ) provides a low thermal resistance between
the die and the PCB to which the part is mounted and soldered. The LM4891LD package should have its DAP
soldered to the grounded copper pad (heatsink) under the NGZ (the NC pins, no connect, and ground pins
should also be directly connected to this copper pad-heatsink area). The area of the copper pad (heatsink) can
be determined from the NGZ Power Derating graph. If the multiple layer copper heatsink areas are used, then
these inner layer or backside copper heatsink areas should be connected to each other with 4 (2 x 2) vias. The
diameter for these vias should be between 0.013 inches and 0.02 inches with a 0.050 inch pitch-spacing. Ensure
efficient thermal conductivity by plating through and solder-filling the vias. Further detailed information concerning
PCB layout, fabrication, and mounting an WSON package is available from Texas Instrument's Package
Engineering Group under application note AN1187.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or
single-ended. A direct consequence of the increased power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. Since the LM4891 has two operational amplifiers in one package, the
maximum internal power dissipation is 4 times that of a single-ended amplifier. The maximum power dissipation
for a given application can be derived from the power dissipation graphs or from Equation 1.
PDMAX = 4*(VDD)
2/(2π2R
L)
(1)
It is critical that the maximum junction temperature (TJMAX) of 150°C is not exceeded. TJMAX can be determined
from the power derating curves by using PDMAX and the PC board foil area. By adding additional copper foil, the
thermal resistance of the application can be reduced from a free air value of 150°C/W, resulting in higher PDMAX.
Additional copper foil can be added to any of the leads connected to the LM4891. It is especially effective when
connected to VDD, GND, and the output pins. Refer to the application information on the LM4891 reference
design board for an example of good heat sinking. If TJMAX still exceeds 150°C, then additional changes must be
made. These changes can include reduced supply voltage, higher load impedance, or reduced ambient
temperature. Internal power dissipation is a function of output power. Refer to the TYPICAL PERFORMANCE
CHARACTERISTICS curves for power dissipation information for different output powers and output loading.
12
Copyright 2001–2013, Texas Instruments Incorporated
Product Folder Links: LM4891
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