LME49610 Datasheet, PDF (14/18 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer

Applying Equation (5):
θ_JA = 40°C/1.87W
= 21.4°C/W
Examining the Copper Area vs. θ_JA plot (see Figure 6) indi-
cates that a thermal resistance of 21.4°C/W is possible with
a 8–10in² plane of one layer of 1oz copper. Other solutions
include using two layers of 1oz copper or the use of 2oz cop-
per. Higher dissipation may require forced air flow. As a safety
margin, an extra 15% heat sinking capability is recommend-
ed.
When amplifying AC signals, wave shapes and the nature of
the load (reactive, non-reactive) also influence dissipation.
Peak dissipation can be several times the average with reac-
tive loads. It is particularly important to determine dissipation
when driving large load capacitance.
The LME49610’s dissipation in DC circuit applications is eas-
ily computed using Equation (3). After the value of dissipation
is determined, the heat sink copper area calculation is the
same as for AC signals.
SLEW RATE
A buffer’s voltage slew rate is its output signal’s rate of change
with respect to an input signal’s step changes. For resistive
loads, slew rate is limited by internal circuit capacitance and
operating current (in general, the higher the operating current
for a given internal capacitance, the higher the slew rate).
However, when driving capacitive loads, the slew rate may be
limited by the available peak output current according to the
following expression.
dv/dt = I_PK / C_L
(6)
Output voltages with high slew rates will require large output
load currents. For example if the part is required to slew at
1000V/μs with a load capacitance of 1nF, the current de-
manded from the LME49610 is 1A. Therefore, fast slew rate
is incompatible with a capacitive load of this value. Also, if
C_L is in parallel with the load, the peak current available to the
load decreases as C_L increases.
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