LM4835 Datasheet, PDF (17/34 Pages) National Semiconductor (TI) – Stereo 2W Audio Power Amplifiers with DC Volume Control and Selectable Gain

LM4835
www.ti.com
SNAS019F – MARCH 1999 – REVISED MAY 2013
Figure 4 shows that Amp1A's output serves as Amp2A'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.
P_DMAX = (V_DD)² / (2π²R_L) Single-Ended
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is higher
internal power dissipation for the same conditions.
The LM4835 has two operational amplifiers per channel. The maximum internal power dissipation per channel
operating in the bridge mode is four times that of a single-ended amplifier. From Equation 3, assuming a 5V
power supply and a 4Ω load, the maximum single channel power dissipation is 1.27W or 2.54W for stereo
operation.
P_DMAX = 4 * (V_DD)² / (2π²R_L) Bridge Mode
(3)
The LM4835's power dissipation is twice that given by Equation 2 or Equation 3 when operating in the single-
ended mode or bridge mode, respectively. Twice the maximum power dissipation point given by Equation 3 must
not exceed the power dissipation given by Equation 4:
P_DMAX′ = (T_JMAX − T_A) / θ_JA
(4)
The LM4835's T_JMAX = 150°C. In the LQ package soldered to a DAP pad that expands to a copper area of 5in²
on a PCB, the LM4835's θ_JA is 20°C/W. In the MTE package soldered to a DAP pad that expands to a copper
area of 2in² on a PCB, the LM4835's θ_JA is 41°C/W. At any given ambient temperature T_A, use Equation 4 to find
the maximum internal power dissipation supported by the IC packaging. Rearranging Equation 4 and substituting
P_DMAX for P_DMAX′ results in Equation 5. This equation gives the maximum ambient temperature that still allows
maximum stereo power dissipation without violating the LM4835's maximum junction temperature.
T_A = T_JMAX – 2*P_DMAX θ_JA
(5)
For a typical application with a 5V power supply and an 4Ω load, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the maximum junction temperature is approximately 99°C
for the LQ package and 45°C for the MTE package.
T_JMAX = P_DMAX θ_JA + T_A
(6)
Equation 6 gives the maximum junction temperature T_JMAX. If the result violates the LM4835's 150°C, reduce the
maximum junction temperature by reducing the power supply voltage or increasing the load resistance. Further
allowance should be made for increased ambient temperatures.
The above examples assume that a device is a surface mount part operating around the maximum power
dissipation point. Since internal power dissipation is a function of output power, higher ambient temperatures are
allowed as output power or duty cycle decreases.
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