A value of 50k to 100k is suggested for R

selected using the above equation.

V

The Vdd supply must be between 3 to 5 volts for the

LM4804. This voltage can be bootstrapped from a much

lower input voltage by simply connecting the V

V

ripple voltage source (less than 200 millivolts), it may be

advisable to use an RC filter to clean it up. Excessive ripple

on V

SETTING THE SWITCHING FREQUENCY

The switching frequency of the oscillator is selected by

choosing an external resistor (R3) connected between V

IN

and the FREQ pin. See the graph titled "Frequency vs V

the Typical Performance Characteristics section of the data

sheet for choosing the R3 value to achieve the desired

switching frequency. A high switching frequency allows the

use of very small surface mount inductors and capacitors

and results in a very small solution size. A switching fre-

quency between 300kHz and 2MHz is recommended.

OUTPUT DIODE SELECTION

A Schottky diode should be used for the output diode. The

forward current rating of the diode should be higher than the

peak input current, and the reverse voltage rating must be

higher than the output voltage. Do not use ordinary rectifier

diodes, since slow switching speeds and long recovery times

cause the efficiency and the load regulation to suffer. Table 1

shows a list of the diode manufacturers.

LLP PACKAGE DEVICES

The LM4804 is offered in the 14 lead LLP surface mount

package to allow for increased power dissipation compared

to the MSOP-8. For details of the thermal performance as

well as mounting and soldering specifications, refer to Ap-

plication Note AN-1187.

RECOMMENDED PRINTED CIRCUIT BOARD LAYOUT

Figure 5 through Figure 10 show the recommended four-

layer PC board layout that is optimized for the LQ-packaged,

3V 1.7W LM4804 mono-BTL audio amplifier and its associ-

ated external components. This circuit is designed for use

with an external 3V to 4.2V supply and speakers with 4

Ω or

higher impedance (8

Ω nominal).

The LM4804 circuit board is easy to use. Apply between 3V

and 4.2V (equivalent to, respectfully, a discharged or a fully

charged Li-ion or NMH battery) and ground to JP2’s V

GND pins, respectively. Connect a speaker with an imped-

ance of 4

Ω or greater (8Ω nominal) between the board’s

VO1 (-) and VO2 (+) pins. An audio signal is applied to JP1

between the V

The circuit board is configured for a gain of 20 or 26dB (VO2

-VO1 with respect to VIN). An inverting gain of -10 at VO1 is

set by Rf (200k

Ω) versus Ri (20kΩ). The extra gain of 2 is a

product of the BTL output (VO2 - VO1). Gain can be modi-

fied by changing Rf’s value with respect to Ri.

TABLE 1. Suggested Manufacturers List

Inductors

Capacitors

Diodes

Coilcraft

Tel: (800) 322-2645

Fax: (708) 639-1469

Sprague/ Vishay

Tel: (207) 324-4140

Fax: (207) 324-7223

Motorola

Tel: (800) 521-6274

Fax: (602) 244-6609

Coiltronics

Tel: (407) 241-7876

Fax: (407) 241-9339

Kemet

Tel: (864) 963-6300

Fax: (864) 963-6521

International Rectifier (IR)

Tel: (310) 322-3331

Fax: (310) 322-3332

Pulse Engineering

Tel: (619) 674-8100

Fax: (619) 674-8262

Nichicon

Tel: (847) 843-7500

Fax: (847) 843-2798

General Semiconductor

Tel: (516) 847-3222

Fax: (516) 847-3150

www.national.com

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