Rainbow Electronics MAX9758 - Manual (Page 19)

Maxim’s patent-pending DirectDrive architecture uses a
charge pump to create an internal negative supply volt-
age. This allows the MAX9756/MAX9757/MAX9758 head-
phone amplifier output to be biased at GND, almost
doubling the dynamic range while operating from a single
supply. With no DC component, there is no need for the
large DC-blocking capacitors. Instead of two large
capacitors (220µF, typ), the MAX9756/MAX9757/
MAX9758 charge pump requires only two small ceramic
capacitors (1µF typ), conserving board space, reducing
cost, and improving the frequency response of the
headphone amplifier. See the Output Power vs. Charge-
Pump Capacitance graph in the Typical Operating
Characteristics for details of the possible capacitor
values.
Low-Frequency Response
In addition to the cost and size disadvantages, the DC-
blocking capacitors limit the low-frequency response of
the amplifier. The impedance of the headphone load to
the DC-blocking capacitor forms a highpass filter with
the -3dB point determined by:
where R
L
is the impedance of the headphone and
C OUT is the value of the DC-blocking capacitor.
The highpass filter is required by conventional single-
ended, single-supply headphone amplifiers to block the
midrail DC component of the audio signal from the
headphones. Depending on the -3dB point, the filter can
attenuate low-frequency signals within the audio band.
Larger values of C OUT reduce the attenuation but are
physically larger, more expensive capacitors. Figure 10
shows the relationship between the size of C
OUT and
the resulting low-frequency attenuation. Note that the
-3dB point for a 16
Ω headphone with a 100µF-blocking
capacitor is 100Hz, well within the audio band.
Charge Pump
The MAX9756/MAX9757/MAX9758 feature a low-noise
inverting charge pump to generate the negative rail
necessary for DirectDrive headphone operation. The
switching frequency is well beyond the audio range,
and does not interfere with the audio signals. The
switch drivers feature a controlled switching speed that
minimizes noise generated by turn-on and turn-off tran-
sients. Limiting the switching speed of the charge
pump minimizes the di/dt noise caused by the parasitic
bond wire and trace inductance.
Headphone Sense Input (HPS)
The headphone sense input (HPS) monitors the head-
phone jack and automatically configures the MAX9756/
MAX9757/MAX9758 based upon the voltage applied at
HPS. A voltage of less than 0.8V enables the speaker
amplifier. A voltage of greater than 2V disables the
speaker amplifiers and enables the headphone ampli-
fiers. For automatic headphone detection, connect HPS
to the control pin of a 3-wire headphone jack as shown
in Figure 11. With no headphone present, the output
impedance of the headphone amplifier pulls HPS low.
When a headphone plug is inserted into the jack, the
control pin is disconnected from the tip contact and
HPS is pulled to V
DD with 35µA.
f dB
R C
L OUT
− =
3
1
2
π
MAX9756/MAX9757/MAX9758
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
______________________________________________________________________________________ 19
0
-15.0
10 100 1k 10k 100k
LOW-FREQUENCY ROLLOFF
(R L = 16 Ω)
-12.0
-13.5
-6.0
-7.5
-9.0
-10.5
-3.0
-4.5
-1.5
FREQUENCY (Hz)
ATTENUATION (dB)
DirectDrive
330 μF
220 μF
100 μF
33 μF
Figure 10. Low-Frequency Attenuation of Common DC-
Blocking Capacitor Values
MAX9756/
MAX9757/
MAX9758
14k Ω
35 μA
14k Ω
V DD
HPS
HPL
HPR
SHUTDOWN
CONTROL
Figure 11. HPS Configuration