LM4917SD datasheet(11/18 Pages) NSC

11 / 18 page

Application Information

ELIMINATING THE OUTPUT COUPLING CAPACITOR

The LM4917 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows

the outputs of the LM4917 to be biased about GND instead

of a nominal DC voltage, like traditional headphone amplifi-

ers. 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. 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

LM4917 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 dy-

namic range of the LM4917 when compared to a traditional

headphone amplifier operating from the same supply volt-

age.

OUTPUT TRANSIENT (’CLICK AND POPS’)

ELIMINATED

The LM4917 contains advanced circuitry that virtually elimi-

nates 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.

To ensure optimal click and pop performance under low gain

configurations (less than 0dB), it is critical to minimize the

RC combination of the feedback resistor R

F and stray input

capacitance at the amplifier inputs. A more reliable way to

lower gain or reduce power delivered to the load is to place

a current limiting resistor in series with the load as explained

in the Minimizing Output Noise / Reducing Output Power

section.

AMPLIFIER CONFIGURATION EXPLANATION

As shown in Figure 2, the LM4917 has two operational

amplifiers internally. The two amplifiers have externally con-

figurable gain, and the closed loop gain is set by selecting

the ratio of R

f to Ri. Consequently, the gain for each channel

of the IC is

V = -(Rf /Ri)

Since this an output ground-referenced amplifier, by driving

the headphone through R

OUT (Pin 11) and LOUT (Pin 8), the

LM4917 does not require output coupling capacitors. The

typical single-ended amplifier configuration where one side

of the load is connected to ground requires large, expensive

output capacitors.

POWER DISSIPATION

Power dissipation is a major concern when using any power

amplifier and must be thoroughly understood to ensure a

successful design. Equation 1 states the maximum power

dissipation point for a single-ended amplifier operating at a

given supply voltage and driving a specified output load.

DMAX =(VDD)

2 /(2

π2R

(1)

Since the LM4917 has two operational amplifiers in one

package, the maximum internal power dissipation point is

twice that of the number which results from Equation 1. Even

with the large internal power dissipation, the LM4917 does

not require heat sinking over a large range of ambient tem-

perature. From Equation 1, assuming a 3V power supply and

a16

Ω load, the maximum power dissipation point is 28mW

per amplifier. Thus the maximum package dissipation point

is 56mW. The maximum power dissipation point obtained

must not be greater than the power dissipation that results

from Equation 2:

DMAX =(TJMAX -TA)/(

JA)

(2)

For package TSSOP,

JA = 109˚C/W. TJMAX = 150˚C for the

LM4917. Depending on the ambient temperature, T

A,ofthe

system surroundings, Equation 2 can be used to find the

maximum internal power dissipation supported by the IC

packaging. If the result of Equation 1 is greater than that of

Equation 2, then either the supply voltage must be de-

creased, the load impedance increased or T

A reduced. For

the typical application of a 3V power supply, with a 16

Ω load,

the maximum ambient temperature possible without violating

the maximum junction temperature is approximately 119.9˚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. Refer to the Typical Perfor-

mance Characteristics curves for power dissipation infor-

mation for lower output powers.

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 3V power supply typi-

cally use a 4.7µF in parallel with a 0.1µF ceramic filter

capacitors to stabilize the power supply’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 0.1µF supply bypass capacitor, C

S, con-

nected between the LM4917’s supply pins and ground. Keep

the length of leads and traces that connect capacitors be-

tween the LM4917’s power supply pin and ground as short

as possible.

MICRO POWER SHUTDOWN

The voltage applied to the SD_LC (shutdown left channel)

pin and the SD_RC (shutdown right channel) pin controls the

LM4917’s shutdown function. When active, the LM4917’s

micropower shutdown feature turns off the amplifiers’ bias

circuitry, reducing the supply current. The trigger point is

0.3*CPV

DD for a logic-low level, and 0.7*CPVDD for logic-

high level. The low 0.01µA(typ) shutdown current is achieved

by appling a voltage that is as near as ground a possible to

the SD_LC/SD_RC pins. A voltage that is higher than ground

may increase the shutdown current.

There are a few ways to control the micro-power shutdown.

These include using a single-pole, single-throw switch, a

microprocessor, or a microcontroller. When using a switch,

connect an external 100k

Ω pull-up resistor between the

SD_LC/SD_RC pins and V

DD. Connect the switch between

the SD_LC/SD_RC pins and ground. Select normal amplifier

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