MAX9724A(2007) 查看數據表（PDF） - Maxim Integrated

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MAX9724A
(Rev.:2007)

60mW, DirectDrive, Stereo Headphone Amplifier with Low RF Susceptibility and Shutdown

Maxim Integrated 

60mW, DirectDrive, Stereo Headphone Amplifier

with Low RF Susceptibility and Shutdown

Flying Capacitor (C1)

The value of the flying capacitor (see the Functional

Diagram/Typical Operating Circuits) affects the charge

pump’s load regulation and output resistance. A C1

value that is too small degrades the device’s ability to

provide sufficient current drive, which leads to a loss of

output voltage. Increasing the value of C1 improves load

regulation and reduces the charge-pump output resis-

tance to an extent. See the Output Power vs. Load

Resistance and Charge-Pump Capacitor Size graph in

the Typical Operating Characteristics. Above 1µF, the

on-resistance of the switches and the ESR of C1 and C2

dominate.

Hold Capacitor (C2)

The hold capacitor value (see the Functional

Diagram/Typical Operating Circuits) and ESR directly

affect the ripple at PVSS. Increasing the value of C2

reduces output ripple. Likewise, decreasing the ESR of

C2 reduces both ripple and output resistance. Lower

capacitance values can be used in systems with low

maximum output power levels. See the Output Power

vs. Load Resistance and Charge-Pump Capacitor Size

graph in the Typical Operating Characteristics.

Power-Supply Bypass Capacitor (C3)

The power-supply bypass capacitor (see the Functional

Diagram/Typical Operating Circuits) lowers the output

impedance of the power supply, and reduces the

impact of the MAX9724A/MAX9724B’s charge-pump

switching transients. Bypass VDD with C3, the same

value as C1, and place it physically close to the VDD

and PGND pins.

Amplifier Gain

The gain of the MAX9724B amplifier is internally set to

-1.5V/V. All gain-setting resistors are integrated into the

device, reducing external component count. The inter-

nally set gain, in combination with DirectDrive, results in

a headphone amplifier that requires only five small

capacitors to complete the amplifier circuit: two for the

charge pump, two for audio input coupling, and one for

power-supply bypassing (see the Functional

Diagram/Typical Operating Circuits).

The gain of the MAX9724A amplifier is set externally as

shown in Figure 3, the gain is:

AV = -RF/RIN (V/V)

Choose feedback resistor values in the tens of kΩ

range. Lower values may cause excessive power dissi-

pation and require impractically small values of RIN for

large gain settings. The high-impedance state of the

outputs can also be degraded during shutdown mode

if an inadequate feedback resistor is used since the

equivalent output impedance during shutdown is

14kΩ||Rf (RF is equal to 30kΩ for the MAX9724B). The

source resistance of the input device may also need to

be taken into consideration. Since the effective value of

RIN is equal to the sum of the source resistance of the

input device and the value of the input resistor connect-

ed to the inverting terminal of the headphone amplifier

(20kΩ for the MAX9724B), the overall closed-loop gain

of the headphone amplifier can be reduced if the input

resistor is not significantly larger than the source resis-

tance of the input device.

RF

LEFT RIN

AUDIO

INPUT

MAX9724A

INL

OUTL

RIGHT RIN

INR

AUDIO

INPUT

OUTR

RF

Figure 3. Gain Setting for the MAX9724A

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