Maxim MAX9777, MAX9778 manual Detailed Description, Input Multiplexer, Headphone Sense Enable

Stereo 3W Audio Power Amplifiers with Headphone Drive and Input Mux

MAX9777/MAX9778

Detailed Description

The MAX9777/MAX9778 feature 3W BTL speaker amplifiers, 200mW headphone amplifiers, input multi- plexers, headphone sensing, and comprehensive click- and-pop suppression. The MAX9777/MAX9778 are stereo BTL/headphone amplifiers. The MAX9777 is controlled through an I2C-compatible, 2-wire serial

interface. The MAX9778 is controlled through five logic inputs: MUTE, SHDN, HPS_EN, GAINA/B, and IN1/2

(see the Selector Guide). The MAX9777/MAX97778 fea- ture exceptional PSRR (100dB at 1kHz), allowing these devices to operate from noisy digital supplies without the need for a linear regulator.

The speaker amplifiers use a BTL configuration. The signal path is composed of an input amplifier and an output amplifier. Resistor RIN sets the input amplifier’s gain, and resistor RF sets the output amplifier’s gain. The output of these two amplifiers serves as the input to

aslave amplifier configured as an inverting unity-gain follower. This results in two outputs, identical in magni- tude, but 180° out of phase. The overall gain of the speaker amplifiers is twice the product of the two amplifier gains (see the Gain-Setting Resistors section). A feature of this architecture is that there is no phase inversion from input to output.

When configured as a headphone (single-ended) ampli- fier, the slave amplifier is disabled, muting the speaker and the main amplifier drives the headphone. The MAX9777/MAX9778 can deliver 3W of continuous power into a 3Ω load with less than 1% THD+N in speaker mode, and 200mW of continuous average power into a 16Ω load with less than 1% THD+N in headphone mode. These devices also feature thermal-overload protection.

Input Multiplexer

Each amplifier features a 2:1 input multiplexer, allowing input selection between two stereo sources. Both multi- plexers are controlled by bit 1 in the control register (MAX9777) or by the IN1/2 pin (MAX9778). A logic-low selects input IN_1 and a logic-high selects input IN_2.

The input multiplexer can also be used to further expand the number of gain options available from the MAX9777/MAX9778 family. Connecting the audio source to the device through two different input resis- tors (Figure 1) increases the number of gain options from two to four. Additionally, the input multiplexer allows a speaker equalization network to be switched into the speaker signal path. This is typically useful in optimizing acoustic response from speakers with small physical dimensions.

Headphone Sense Enable

The HPS input is enabled by HPS_EN (MAX9778) or the HPS_D bit (MAX9777). HPS_D or HPS_EN determines whether the device is in automatic detection mode or fixed-mode operation (see Tables 1a and 1b).

MAX9777

15kΩ MAX9778

IN_1

AUDIO
INPUT	30kΩ

IN_2

Figure 1. Using the Input Multiplexer for Gain Setting

BIAS

These devices operate from a single 5V supply, and fea- ture an internally generated, power-supply independent, common-mode bias voltage of 2.5V referenced to GND. BIAS provides both click-and-pop suppression and sets the DC bias level for the audio outputs. BIAS is internally connected to the noninverting input of each speaker amplifier (see the Typical Application Circuits and Functional Diagrams). Choose the value of the bypass capacitor as described in the BIAS Capacitor section. No external load should be applied to BIAS. Any load lowers the BIAS voltage, affecting the overall perfor- mance of the device.

Table 1a. MAX9777 HPS Setting

	INPUTS			GAIN
			MODE
HPS_D		SPKR/HP
	HPS			PATH*
BIT		BIT
0	0	X	BTL	A
0	1	X	SE	B
1	X	0	BTL	A or B
1	X	1	SE	A or B

*Note:

A—GAINA path selected B—GAINB path selected

A or B—Gain path selected by GAINAB control bit in register 02h

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