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> Automotive
> APP 4607

Sep 28, 2009

Keywords: automotive headphone amp, auto HPA,
HPA, head phone amp, auto head phone amp, auto headphone amp,
headphone amp, headphone amplifier
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parts

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APPLICATION NOTE 4607

Add a Discrete Jack-Sensing Circuit to the
MAX13330/MAX13331 Automotive Headphone Amplifier

By: Jon
Wallace

Abstract: This
application note describes how to add a discrete jack-sensing
(JACKSENSE) circuit to the MAX13330/MAX13331 DirectDrive®
headphone amplifier.

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this brief webcast by Maxim on
TechOnline

Introduction
The MAX13330/MAX13331
automotive headphone amplifier
meets the requirements of most automotive applications, but
there are instances where other features are needed. Sensing
when a headphone is inserted into the audio jack is sometimes
an application requirement. This capability can easily be
added to any DirectDrive headphone amplifier, as long as there
is a microcontroller available to decode the additional
signals generated.

Jack-Sensing Circuit
Adding a discrete jack-sensing
circuit to a device typically requires just a few low-cost
passive components. The exact values of each component depend
on the system requirements, but the basic circuit should
remain the same. Typically a four-contact audio jack is used
where the extra contact is a normally-closed switch connected
to the left or right audio contact. When a plug is inserted
the switch opens. The circuit in Figure 1 shows the
basic jack-sensing circuit. Figure 2 shows a schematic
diagram of the MAX13330/MAX13331 with the jack-sensing
circuit.

The jack-sensing circuit can be used to
automatically enable the MAX13330/MAX13331 when a plug is
inserted into the jack. Connect the output of the jack-sensing
circuit directly to the active-low SHDN
pin to implement this function.

Figure
1. Jack-sensing circuit.

Figure
2. Schematic diagram of MAX13331 with discrete jack-sensing
circuit.

Filter Selection
The RC filter consists of R2 and C1,
and allows the jack-sensing detection times to be adjusted and
the audio signals to be filtered. R2 and C1 can be selected to
meet the requirements of each application. Table 1
gives a typical range of C1 capacitor values along with
associated jack detection times and RC filter
parameters.

If audio will be present when no plug is
inserted into the jack and if the rising threshold of the
digital input is < 2.2V, then select a capacitor value that
will filter out the audio signal. The MAX13330/MAX13331 has a
maximum audio signal of ~2.2V. Therefore, a capacitor value of
0.47µF is adequate to keep the jack-sensing output below 0.6V,
which will be a "0" for most digital inputs. To minimize the
jack detection times, adjust the capacitor value based on the
specific digital input threshold.

A Schmitt-trigger inverter
or buffer can be added to eliminate the need to filter the
audio signal, as the rising-edge threshold will be above the
maximum audio signal. With the small cost of the
inverter/buffer, this modification provides the added
flexibility of adjusting the jack detection
times.

Table 1. Typical C1 Filter Values

C1 (µF)

Jack-Present Detection Time (ms)*

Plug-Removal Detection Time
(ms)**

Cutoff Freq. (Hz)

-12dB (Hz)

0.01

1

0.4

320

1234

0.047

5

1.6

68

264

0.1

10

3.5

32

123

0.22

23

8

15

44

0.47

49

16

7

27

1.0

104

35

3

12

*MAX13330/MAX13331 disabled
(EN low)
**MAX13330/MAX13331 enabled (EN high) with no
audio present.

Conclusion
There are varying requirements which impact
the design of a jack-sensing circuit. but adding a discrete
jack-sensing output to the MAX13330/MAX13331 headphone
amplifier typically requires only a few low-cost components.
The jack-sensing circuit can control the enable state of the
MAX13330/MAX13331 directly and/or be sent into the digital
input of an available microcontroller, depending on the
application requirements.

DirectDrive
is a registered trademark and registered service mark of Maxim
Integrated Products, Inc.

Related parts

MAX13330

Automotive DirectDrive®
Headphone Amplifiers with Output Protection and
Diagnostics

Free
samples

MAX13331

Automotive DirectDrive®
Headphone Amplifiers with Output Protection and
Diagnostics

Free
samples

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Download,
PDF format (40kB)
APP 4607: Sep 28, 2009

AN4607, AN 4607, APP4607,
Appnote4607, Appnote 4607

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