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LME49610 Datasheet, PDF (15/21 Pages) National Semiconductor (TI) – High Performance, High Fidelity, High Current Audio Buffer
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LME49610
www.ti.com
SNAS435B – APRIL 2008 – REVISED APRIL 2013
Procedure
1. First determine the maximum power dissipated by the LME49610, PD(MAX). For the simple case of the buffer
driving a resistive load, and assuming equal supplies, PD(MAX) is given by:
PDMAX(AC) = (IS x VS) + (VS)2 / (2π2RL) (Watts)
(3)
PDMAX(DC) = (IS x VS) + (VS)2 / RL (Watts)
where
• VS = |VEE| + VCC (V)
• IS = quiescent supply current (A)
Equation 3 is for sinusoidal output voltages and Equation 4 is for DC output voltages
(4)
2. Determine the maximum allowable die temperature rise,
TRISE(MAX) = TJ(MAX) - TA(MAX) °C
(5)
3. Using the calculated value of TRISE(MAX) and PD(MAX), find the required value of junction to ambient thermal
resistance combining Equation 1 and Equation 4 to derive Equation 6:
θJA = TRISE(MAX) / PD(MAX) (°C/W)
(6)
4. Finally, choose the minimum value of copper area from Figure 31 based on the value for θJA.
Example
Assume the following conditions: VS = |VEE| + VCC = 30V, RL = 32Ω, IS = 15mA, sinusoidal output voltage, TJ(MAX)
= 125°C, TA(MAX) = 85°C
Applying Equation 3:
PDMAX = (IS x VS) + (VS)2 / 2π2RL
(7)
= (15mA)(30V) + 900V2 / 632Ω
= 1.87W
Applying Equation 4:
TRISE(MAX) = 125°C – 85°C
(8)
= 40°C
Applying Equation 6:
θJA = 40°C/1.87W
(9)
= 21.4°C/W
Examining the Copper Area vs. θJA plot (see Figure 31) indicates 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 copper. Higher dissipation may require forced air flow. As a safety margin, an extra 15% heat
sinking capability is recommended.
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 reactive loads. It is particularly important to
determine dissipation when driving large load capacitance.
The LME49610’s dissipation in DC circuit applications is easily 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 = IPK / CL
(10)
Copyright © 2008–2013, Texas Instruments Incorporated
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