tor, C

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

V

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 BYPASS CAPACITOR FOR AUDIO

AMPLIFIER

Besides minimizing the input capacitor value, careful consid-

eration should be paid to the bypass capacitor value. Bypass

capacitor, C

turn-on pops since it determines how fast the amplifier turns

on. The slower the amplifier’s outputs ramp to their quies-

cent DC voltage (nominally 1/2 V

pop. Choosing C

C

virtually clickless and popless shutdown function. Although

the device will function properly, (no oscillations or motor-

boating), with C

more susceptible to turn-on clicks and pops. Thus, a value of

C

sensitive designs.

SELECTING FEEDBACK CAPACITOR FOR AUDIO

AMPLIFIER

The LM4805 is unity-gain stable which gives the designer

maximum system flexability. However, a typical application

requires a closed-loop differential gain of 10. In this case a

feedback capacitor (C

to bandwidth limit the amplifier.

This feedback capacitor creates a low pass filter that elimi-

nates possible high frequency oscillations. Care should be

taken when calculating the -3dB frequency because an in-

correct combination of R

desired frequency

SELECTING OUTPUT CAPACITOR (C

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

CONVERTER

The output voltage is set using the external resistors R1 and

R2 (see Figure 1). A value of approximately 15k is recom-

mended for R2 to establish a divider current of approxi-

mately 92µA. R1 is calculated using the formula:

R1=R2X(V

(5)

FEED-FORWARD COMPENSATION FOR BOOST

CONVERTER

Although the LM4805’s internal Boost converter is internally

compensated, the external feed-forward capacitor C

required for stability (see Figure 1). Adding this capacitor

puts a zero in the loop response of the converter. The

recommended frequency for the zero fz should be approxi-

mately 6kHz. C

C

(6)

SELECTING DIODES

The external diode used in Figure 1 should be a Schottky

diode. A 20V diode such as the MBR0520 is recommended.

The MBR05XX series of diodes are designed to handle a

maximum average current of 0.5A. For applications exceed-

ing 0.5A average but less than 1A, a Microsemi UPS5817

can be used.

DUTY CYCLE

The maximum duty cycle of the boost converter determines

the maximum boost ratio of output-to-input voltage that the

converter can attain in continuous mode of operation. The

duty cycle for a given boost application is defined as:

Duty Cycle = V

This applies for continuous mode operation.

INDUCTANCE VALUE

The first question we are usually asked is: “How small can I

make the inductor.” (because they are the largest sized

component and usually the most costly). The answer is not

simple and involves trade-offs in performance. Larger induc-

tors mean less inductor ripple current, which typically means

less output voltage ripple (for a given size of output capaci-

tor). Larger inductors also mean more load power can be

delivered because the energy stored during each switching

cycle is:

E = L/2 X (lp)2

Where “lp” is the peak inductor current. An important point to

observe is that the LM4805 will limit its switch current based

on peak current. This means that since lp(max) is fixed,

increasing L will increase the maximum amount of power

available to the load. Conversely, using too little inductance

may limit the amount of load current which can be drawn

from the output.

Best performance is usually obtained when the converter is

operated in “continuous” mode at the load current range of

interest, typically giving better load regulation and less out-

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