LM48860

SNAS398D – JANUARY 2008 – REVISED MAY 2013

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APPLICATION INFORMATION

SUPPLY VOLTAGE SEQUENCING

It is a good general practice to first apply the supply voltage to a CMOS device before any other signal or supply

on other pins. This is also true for the LM48860 audio amplifier which is a CMOS device.

Before applying any signal to the inputs or shutdown pins of the LM48860, it is important to apply a supply

voltage to the VDD pins. After the device has been powered, signals may be applied to the shutdown pins (see

MICRO POWER SHUTDOWN) and input pins.

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM48860 features a low noise inverting charge pump that generates an internal negative supply voltage.

This allows the outputs of the LM48860 to be biased about GND instead of a nominal DC voltage, like traditional

headphone amplifiers. Because there is no DC component, the large DC blocking capacitors (typically 220µF)

are not necessary. The coupling capacitors are replaced by two, small ceramic charge pump capacitors, saving

board space and cost.

Eliminating the output coupling capacitors also improves low frequency response. In traditional headphone

amplifiers, the headphone impedance and the output capacitor form a high pass filter that not only blocks the DC

component of the output, but also attenuates low frequencies, impacting the bass response. Because the

LM48860 does not require the output coupling capacitors, the low frequency response of the device is not

degraded by external components.

In addition to eliminating the output coupling capacitors, the ground referenced output nearly doubles the

available dynamic range of the LM48860 when compared to a traditional headphone amplifier operating from the

same supply voltage.

OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED

The LM48860 contains advanced circuitry that virtually eliminates output transients ('clicks and pops'). This

circuitry prevents all traces of transients when the supply voltage is first applied or when the part resumes

operation after coming out of shutdown mode.

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 1, the LM48860 has two internal operational amplifiers. The two amplifiers have internally

configured gain.

Since this is an output ground-referenced amplifier, the LM48860 does not require output coupling capacitors.

POWER DISSIPATION

From the graph (THD+N vs Output Power , VDD = 3V, RL = 16Ω, f = 1kHz, 22kH BW, two channels in phase,

page 6) assuming a 3V power supply and a 16

Ω load, the maximum power dissipation point and thus the

maximum package dissipation point is 281mW. The maximum power dissipation point obtained must not be

greater than the power dissipation that results from Equation 1.

PDMAX = (TJMAX - TA) / (θJA)

(1)

For the DSBGA package

θ JA = 59.3°C/W. TJMAX = 150°C for the LM48860. Depending on the ambient

temperature, TA, of the system surroundings, Equation 1 can be used to find the maximum internal power

dissipation supported by the IC packaging. If the maximum power dissipation from the graph is greater than that

of Equation 1, then either the supply voltage must be decreased, the load impedance increased or TA reduced

(see power derating curves). For the application of a 5V power supply, with a 16

Ω load, the maximum ambient

temperature possible without violating the maximum junction temperature is approximately 110°C provided that

device operation is around the maximum power dissipation point. Power dissipation is a function of output power

and thus, if typical operation is not around the maximum power dissipation point, the ambient temperature may

be increased accordingly.

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