LM4962 datasheet(14/22 Pages) NSC

14 / 22 page

Application Information (Continued)

PROPER SELECTION OF EXTERNAL COMPONENTS

Proper selection of external components in applications us-

ing integrated power amplifiers, and switching DC-DC con-

verters, is critical for optimizing device and system perfor-

mance. Consideration to component values must be used to

maximize overall system quality.

The best capacitors for use with the switching converter

portion of the LM4962 are multi-layer ceramic capacitors.

They have the lowest ESR (equivalent series resistance)

and highest resonance frequency, which makes them opti-

mum for high frequency switching converters.

When selecting a ceramic capacitor, only X5R and X7R

dielectric types should be used. Other types such as Z5U

and Y5F have such severe loss of capacitance due to effects

of temperature variation and applied voltage, they may pro-

vide as little as 20% of rated capacitance in many typical

applications. Always consult capacitor manufacturer’s data

curves before selecting a capacitor. High-quality ceramic

capacitors can be obtained from Taiyo-Yuden.

POWER SUPPLY BYPASSING

As with any amplifier, proper supply bypassing is critical for

low noise performance and high power supply rejection. The

capacitor location on both V1 and V

DD pins should be as

close to the device as possible.

SELECTING INPUT CAPACITOR FOR AUDIO

AMPLIFIER

One of the major considerations is the closedloop bandwidth

of the amplifier. To a large extent, the bandwidth is dictated

by the choice of external components shown in Figure 1. The

input coupling capacitor, C

i, forms a first order high pass filter

which limits low frequency response. This value should be

chosen based on needed frequency response for a few

distinct reasons.

High value input capacitors are both expensive and space

hungry in portable designs. Clearly, a certain value capacitor

is needed to couple in low frequencies without severe at-

tenuation. But ceramic speakers used in portable systems,

whether internal or external, have little ability to reproduce

signals below 100Hz to 150Hz. Thus, using a high value

input capacitor may not increase actual system perfor-

mance.

In addition to system cost and size, click and pop perfor-

mance is affected by the value of the input coupling capaci-

tor, C

i. A high value input coupling capacitor requires more

charge to reach its quiescent DC voltage (nominally 1/2

DD). This charge comes from the output via the feedback

and is apt to create pops upon device enable. Thus, by

minimizing the capacitor value based on desired low fre-

quency response, turn-on pops can be minimized.

SELECTING FEEDBACK CAPACITOR FOR AUDIO

AMPLIFIER

The LM4962 is unity-gain stable which gives the designer

maximum system flexability. However, to drive ceramic

speakers, a typical application requires a closed-loop differ-

ential gain of 10. In this case a feedback capacitor (C

f2) will

be needed as shown in Figure 1 to bandwidth limit the

amplifier.

This feedback capacitor creates a low pass filter that elimi-

nates possible high frequency noise. Care should be taken

when calculating the -3dB frequency because an incorrect

combination of R

f and Cf2 will cause rolloff before the de-

sired frequency

SELECTING OUTPUT CAPACITOR (C

O) FOR BOOST

CONVERTER

A single 4.7µF to 10µF ceramic capacitor will provide suffi-

cient output capacitance for most applications. If larger

amounts of capacitance are desired for improved line sup-

port and transient response, tantalum capacitors can be

used. Aluminum electrolytics with ultra low ESR such as

Sanyo Oscon can be used, but are usually prohibitively

expensive. Typical AI electrolytic capacitors are not suitable

for switching frequencies above 500 kHz because of signifi-

cant ringing and temperature rise due to self-heating from

ripple current. An output capacitor with excessive ESR can

also reduce phase margin and cause instability.

In general, if electrolytics are used, we recommended that

they be paralleled with ceramic capacitors to reduce ringing,

switching losses, and output voltage ripple.

SELECTING INPUT CAPACITOR (Cs1) FOR BOOST

CONVERTER

An input capacitor is required to serve as an energy reservoir

for the current which must flow into the coil each time the

switch turns ON. This capacitor must have extremely low

ESR, so ceramic is the best choice. We recommend a

nominal value of 4.7µF, but larger values can be used. Since

this capacitor reduces the amount of voltage ripple seen at

the input pin, it also reduces the amount of EMI passed back

along that line to other circuitry.

SETTING THE OUTPUT VOLTAGE (V

1) OF BOOST

CONVERTER

The output voltage is set using the external resistors R

2 and

5 (see Figure 1). A value of approximately 25k

Ω is recom-

mended for R2 to establish the open loop gain of the boost

converter.

1 =VFB [1+(R2 /R5)]

(5)

FEED-FORWARD COMPENSATION FOR BOOST

CONVERTER

Although the LM4962’s internal Boost converter is internally

compensated, the external feed-forward capacitor C

f is re-

quired for stability (see Figure 1). Adding this capacitor puts

a zero in the loop response of the converter. The recom-

mended frequency for the zero fz should be approximately

60kHz. C3 can be calculated using the formula:

3 =1/(2

π xR

2 x fz)

(6)

SELECTING A SOFT-START CAPACITOR (Css)

The soft-start function charges the boost converter reference

voltage slowly, which allows the output of the boost con-

verter to ramp up slowly thus limiting the transient current at

startup.

Selecting a soft-start capacitor (Css) value presents a trade

off between the wake-up time of the boost converter (T

WUBC)

and the startup transient current. Using a larger capacitor

value will increase wake-up time and decrease startup tran-

sient current; on the flip side, using a smaller capacitor value

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