參數(shù)資料
型號(hào)： LM4855LQ/NOPB
廠商： National Semiconductor
文件頁(yè)數(shù)： 9/27頁(yè)
文件大?。?/td> 0K
描述： IC AMP AUDIO PWR 1.5W MONO 24LLP
標(biāo)準(zhǔn)包裝： 1
系列： Boomer®
類(lèi)型： AB 類(lèi)
輸出類(lèi)型： 1-通道（單聲道），帶立體聲耳機(jī)
在某負(fù)載時(shí)最大輸出功率 x 通道數(shù)量： 1.5W x 1 @ 4 歐姆; 115mW x 2 @ 32 歐姆
電源電壓： 2.6 V ~ 5 V
特點(diǎn)： 消除爆音，靜音，關(guān)機(jī)，SPI，熱保護(hù)，音量控制
安裝類(lèi)型： 表面貼裝
供應(yīng)商設(shè)備封裝： 24-LLP-EP（4x5）
封裝/外殼： 24-WFQFN 裸露焊盤(pán)
包裝： 標(biāo)準(zhǔn)包裝
配用： LM4855LQBD-ND - BOARD EVALUATION LM4855LQ
其它名稱(chēng)： LM4855LQDKR
SNAS164C – JUNE 2002 – REVISED MAY 2013
EXPOSED-DAP MOUNTING CONSIDERATIONS
The LM4855's exposed-DAP (die attach paddle) package (NHW) provides a low thermal resistance between the
die and the PCB to which the part is mounted and soldered. This allows rapid heat transfer from the die to the
surrounding PCB copper area heatsink, copper traces, ground plane, and finally, surrounding air. The result is a
low voltage audio power amplifier that produces 1.1W dissipation in a 8
load at ≤ 1% THD+N. This high power
is achieved through careful consideration of necessary thermal design. Failing to optimize thermal design may
compromise the LM4855's high power performance and activate unwanted, though necessary, thermal shutdown
protection.
The NHW package must have its DAP soldered to a copper pad on the PCB. The DAP's PCB copper pad is
then, ideally, connected to a large plane of continuous unbroken copper. This plane forms a thermal mass, heat
sink, and radiation area. Place the heat sink area on either outside plane in the case of a two-sided or multi-layer
PCB. (The heat sink area can also be placed on an inner layer of a multi-layer board. The thermal resistance,
however, will be higher.) Connect the DAP copper pad to the inner layer or backside copper heat sink area with
6 (3 X 2) (NHW) vias. The via diameter should be 0.012in - 0.013in with a 1.27mm pitch. Ensure efficient thermal
conductivity by plugging and tenting the vias with plating and solder mask, respectively.
Best thermal performance is achieved with the largest practical copper heat sink area. If the heatsink and
amplifier share the same PCB layer, a nominal 2.5in2 (min) area is necessary for 5V operation with a 4
load.
Heatsink areas not placed on the same PCB layer as the LM4855 should be 5in2 (min) for the same supply
voltage and load resistance. The last two area recommendations apply for 25°C ambient temperature. Increase
the area to compensate for ambient temperatures above 25°C. In all circumstances and under all conditions, the
junction temperature must be held below 150°C to prevent activating the LM4855's thermal shutdown protection.
Further detailed and specific information concerning PCB layout and fabrication and mounting an NHW (WQFN)
is found in TI's AN1187.
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3
AND 4
LOADS
Power dissipated by a load is a function of the voltage swing across the load and the load's impedance. As load
impedance decreases, load dissipation becomes increasingly dependent on the interconnect (PCB trace and
wire) resistance between the amplifier output pins and the load's connections. Residual trace resistance causes
a voltage drop, which results in power dissipated in the trace and not in the load as desired. For example, 0.1
Ω
trace resistance reduces the output power dissipated by a 4
Ω load from 1.7W to 1.6W. The problem of
decreased load dissipation is exacerbated as load impedance decreases. Therefore, to maintain the highest load
dissipation and widest output voltage swing, PCB traces that connect the output pins to a load must be as wide
as possible.
Poor power supply regulation adversely affects maximum output power. A poorly regulated supply's output
voltage decreases with increasing load current. Reduced supply voltage causes decreased headroom, output
signal clipping, and reduced output power. Even with tightly regulated supplies, trace resistance creates the
same effects as poor supply regulation. Therefore, making the power supply traces as wide as possible helps
maintain full output voltage swing.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4855 consists of three pairs of output amplifier blocks (A4-A6). A4, A5, and A6
consist of bridged-tied amplifier pairs that drive LOUT, ROUT, and SPKROUT respectively. The LM4855 drives a
load, such as a speaker, connected between outputs, SPKROUT+ and SPKROUT-. In the amplifier block A6, the
output of the amplifier that drives SPKROUT- serves as the input to the unity gain inverting amplifier that drives
SPKROUT+.
This results in both amplifiers producing signals identical in magnitude, but 180° out of phase. Taking advantage
of this phase difference, a load is placed between SPKROUT- and SPKROUT+ and driven differentially
(commonly referred to as 'bridge mode'). This results in a differential or BTL gain of:
AVD = 2(Rf/Ri) = 2
(1)
Copyright 2002–2013, Texas Instruments Incorporated
17
Product Folder Links: LM4855
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