Datasheet

LM4936
SNAS269A –APRIL 2005–REVISED APRIL 2013 www.ti.com
P
DMAX
= (V
DD
)
2
/(2π
2
R
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 LM4936 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
/(2π
2
R
L
) Bridge Mode (3)
The LM4936's power dissipation is twice that given by Equation 2 or Equation 3 when operating in the single-
ended mode or bridge mode, respectively due to stereo operation. 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 LM4936's T
JMAX
= 150°C. In the MH package soldered to a DAP pad that expands to a copper area of 2in
2
on a PCB, the LM4936MH'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 LM4936'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 45°C
for the MH package.
T
JMAX
= P
DMAX
θ
JA
+ T
A
(6)
Equation 6 gives the maximum junction temperature T
JMAX
. If the result violates the LM4936's 150°C T
JMAX
,
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.
If the result of Equation 3 multiplied by 2 for stereo operation is greater than that of Equation 4, then decrease
the supply voltage, increase the load impedance, or reduce the ambient temperature. If these measures are
insufficient, a heat sink can be added to reduce θ
JA
. The heat sink can be created using additional copper area
around the package, with connections to the ground pin(s), supply pin and amplifier output pins. External, solder
attached SMT heatsinks such as the Thermalloy 7106D can also improve power dissipation. When adding a heat
sink, the θ
JA
is the sum of θ
JC
, θ
CS
, and θ
SA
. (θ
JC
is the junction-to-case thermal impedance, θ
CS
is the case-to-
sink thermal impedance, and θ
SA
is the sink-to-ambient thermal impedance.) Refer to the Typical Performance
Characteristics curves for power dissipation information at lower output power levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically use a 10µF in parallel with a 0.1µF filter capacitor to
stabilize the regulator's output, reduce noise on the supply line, and improve the supply's transient response.
However, their presence does not eliminate the need for a local 1µF tantalum bypass capacitance connected
between the LM4936's supply pins and ground. Do not substitute a ceramic capacitor for the tantalum. Doing so
may cause oscillation. Keep the length of leads and traces that connect capacitors between the LM4936's power
supply pin and ground as short as possible. Connecting a 1µF capacitor, C
B
, between the BYPASS pin and
ground improves the internal bias voltage's stability and the amplifier's PSRR. The PSRR improvements increase
as the BYPASS pin capacitor value increases. Too large a capacitor, however, increases turn-on time and can
compromise the amplifier's click and pop performance. The selection of bypass capacitor values, especially C
B
,
depends on desired PSRR requirements, click and pop performance (as explained in the following section,
SELECTING PROPER EXTERNAL COMPONENTS), system cost, and size constraints.
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