MAX9768 datasheet(21/23 Pages) AD

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Power Supplies

The MAX9768 has different supplies for each portion of

the device, allowing for the optimum combination of head-

room power dissipation and noise immunity. The speaker

amplifiers are powered from PVDD and can range from

4.5V to 14V. The remainder of the device is powered by

VDD. Power supplies are independent of each other so

sequencing is not necessary. Power may be supplied by

separate sources or derived from a single higher source

using a linear regulator to reduce the voltage, as shown

in Figure 10.

Component Selection

Input Filter

An input capacitor, CIN, in conjunction with the input resis-

tor of the MAX9768 forms a highpass filter that removes

the DC bias from an incoming signal. The AC-coupling

capacitor allows the amplifier to automatically bias the

signal to an optimum DC level. Assuming zero source

impedance, the -3dB point of the highpass filter is given by:

f 3dB

IN IN

2R C

−

Choose CIN so f-3dB is well below the lowest frequency of

interest. Use capacitors whose dielectrics have lowvoltage

coefficients, such as tantalum or aluminum electrolytic.

Capacitors with high-voltage coefficients, such as ceram-

ics, may result in increased distortion at low frequencies.

Other considerations when designing the input filter

include the constraints of the overall system and the

actual frequency band of interest. Although high-fidelity

audio calls for a flat-gain response between 20Hz and

20kHz, portable voice-reproduction devices such as cellu-

lar phones and two-way radios need only concentrate on

the frequency range of the spoken human voice (typically

300Hz to 3.5kHz). In addition, speakers used in portable

devices typically have a poor response below 300Hz.

Taking these two factors into consideration, the input

filter may not need to be designed for a 20Hz to 20kHz

response, saving both board space and cost due to the

use of smaller capacitors.

BIAS Capacitor

BIAS is the output of the internally generated DC bias

voltage. The BIAS bypass capacitor, CBIAS, improves

PSRR and THD+N by reducing power supply and other

noise sources at the common-mode bias node. Bypass

BIAS with a 2.2μF capacitor to GND.

Table 8. Suggested Values for LC filter

Figure 8. Output Filter for PWM Mode

RL (Ω)

L3, L4 (µH)

C15 (µF)

C11, C12 (µF)

R6, R7 (Ω)

C13, C14 (µF)

0.33

0.01

7.5

0.68

0.22

0.01

0.47

0.1

0.01

0.33

BOOT_+

0.1µF

OUT_+

1, 2

C11

C15

MAX9768

C13

14, 18

BOOT_-

0.1µF

OUT_-

C12

C14

MAX9768

10W Mono Class D Speaker

Amplifier with Volume Control

www.maximintegrated.com

Maxim Integrated │ 21

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