LM4730TA datasheet(13/17 Pages) NSC

13 / 17 page

Application Information (Continued)

To achieve a transient free power-up and power-down, the

voltage seen at the input terminals should be ideally the

same. Such a signal will be common-mode in nature, and

will be rejected by the LM4730. In Figure 3, the resistor R

INP

serves to keep the inputs at the same potential by limiting the

voltage difference possible between the two nodes. This

should significantly reduce any type of turn-on pop, due to an

uneven charging of the amplifier inputs. This charging is

based on a specific application loading and thus, the system

designer may need to adjust these values for optimal perfor-

mance.

As shown in Figure 3, the resistors labeled R

BI help bias up

the LM4730 off the half-supply node at the emitter of the

2N3904. But due to the input and output coupling capacitors

in the circuit, along with the negative feedback, there are two

different values of R

BI, namely 10 k

Ω and 200 kΩ. These

resistors bring up the inputs at the same rate resulting in a

popless turn-on. Adjusting these resistors values slightly

may reduce pops resulting from power supplies that ramp

extremely quick or exhibit overshoot during system turn-on.

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components is required to meet

the design targets of an application. The choice of external

component values that will affect gain and low frequency

response are discussed below.

The gain of each amplifier is set by resistors R

f and Ri for the

non-inverting configuration shown in Figure 1. The gain is

found by Equation (4) below:

V =1+Rf /Ri (V/V)

(4)

For best noise performance, lower values of resistors are

used. A value of 1k

Ω is commonly used for R

i and then

setting the value of R

f for the desired gain. For the LM4730

the gain should be set no lower than 10V/V and no higher

than 50V/V. Gain settings below 10V/V may experience

instability and using the LM4730 for gains higher than 50V/V

will see an increase in noise and THD.

The combination of R

i with Ci (see Figure 1) creates a high

pass filter. The low frequency response is determined by

these two components. The -3dB point can be found from

Equation (5) shown below:

i =1/(2

πR

iCi) (Hz)

(5)

If an input coupling capacitor is used to block DC from the

inputs as shown in Figure 4, there will be another high pass

filter created with the combination of C

IN and RIN. When

using a input coupling capacitor R

IN is needed to set the DC

bias point on the amplifier’s input terminal. The resulting

-3dB frequency response due to the combination of C

IN and

IN can be found from Equation (6) shown below:

IN =1/(2

πR

INCIN) (Hz)

(6)

PHYSICAL IC MOUNTING CONSIDERATIONS

Mounting of the TO-220 package to a heat sink must be

done such that there is sufficient pressure from the mounting

screw to insure good contact with the heat sink for efficient

heat flow. Over tightening the mounting screw will cause the

TO-220 package to warp reducing contact area with the heat

sink. Less contact with the heat sink will increase the thermal

resistance from the TO-220 package case to the heat sink

(

CS) resulting in higher operating die temperatures and

possible unwanted thermal shut down activation. Extreme

over tightening of the mounting screw will cause severe

physical stress resulting in cracked die and catastrophic IC

failure. The recommended maximum mounting screw torque

is 40 inch-lbs or 3.3 foot-lbs (4.5 newton-meter).

Additionally, if the mounting screw is used to force the TO-

220 package into correct alignment with the heat sink, pack-

age stress will be increased. This increase in package stress

will result in reduced contact area with the heat sink increas-

ing die operating temperature and possible catastrophic IC

failure.

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