Datasheet

LM4936
www.ti.com SNAS269A –APRIL 2005–REVISED APRIL 2013
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 2.5in
2
(min) area is necessary for 5V operation with a 4Ω load.
Heatsink areas not placed on the same PCB layer as the LM4936 should be 5in
2
(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 systems using cooling fans, the LM4936MH
can take advantage of forced air cooling. With an air flow rate of 450 linear-feet per minute and a 2.5in
2
exposed
copper or 5.0in
2
inner layer copper plane heatsink, the LM4936MH can continuously drive a 3Ω load to full
power. In all circumstances and conditions, the junction temperature must be held below 150°C to prevent
activating the LM4936's thermal shutdown protection. The LM4936's power de-rating curve in the Typical
Performance Characteristics shows the maximum power dissipation versus temperature. Example PCB layouts
are shown in the LM4936 MH HTSSOP Board Artwork section. Further detailed and specific information
concerning PCB layout, fabrication, and mounting is available in Texas Instruments' AN1187.
PCB LAYOUT AND SUPPLY REGULATION CONSIDERATIONS FOR DRIVING 3Ω AND 4Ω
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, 0.1Ω
trace resistance reduces the output power dissipated by a 4Ω load from 2.1W to 2.0W. This 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 3, the LM4936 output stage consists of two pairs of operational amplifiers, forming a two-
channel (channel A and channel B) stereo amplifier. (Though the following discusses channel A, it applies
equally to channel B.)
Figure 3 shows that the first amplifier's negative (-) output serves as the second amplifier's input. 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 −OUTA and +OUTA and driven differentially (commonly referred to as
“bridge mode”). This results in a differential gain of
A
VD
= 2 * (R
f
/R
i
) (1)
Bridge mode amplifiers are different from single-ended amplifiers that drive loads connected between a single
amplifier's output and ground. For a given supply voltage, bridge mode has a distinct advantage over the single-
ended configuration: its differential output doubles the voltage swing across the load. This produces four
times the output power when 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 that the output signal is not clipped.
To ensure minimum output signal clipping when choosing an amplifier's closed-loop gain, refer to the AUDIO
POWER AMPLIFIER DESIGN section.
Another advantage of the differential bridge output is no net DC voltage across the load. This is accomplished by
biasing channel A's and channel B's outputs at half-supply. This eliminates the coupling capacitor that single
supply, single-ended amplifiers require. Eliminating an output coupling capacitor in a single-ended configuration
forces a single-supply amplifier's half-supply bias voltage across the load. This increases internal IC power
dissipation and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful single-ended or bridged amplifier. Equation 2
states the maximum power dissipation point for a single-ended amplifier operating at a given supply voltage and
driving a specified output load.
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