Diodes PAM8908 - Manual (Page 8)

PAM8901 / PAM8908
Document number: DSxxxxx Rev. 1 - 1
8 of 12
www.diodes.com
December 2012
© Diodes Incorporated
PAM8901 / PAM8908
A Product Line of
Diodes Incorporated
Application Information
The basic PAM8901 / PAM8908 application circuit is shown in page 1 and page 2.
Gain Control
The PAM8901 / PAM8908 has four gain settings which are controlled with pins G0 and G1. The following table gives an overview of the gain
function.
G0 Voltage G1 Voltage Amplifier Gain
≤0.6V ≤0.6V -6dB
≥1.4V ≤0.6V 0dB
≤0.6V ≥1.4V 3dB
≥1.4V ≥1.4V 6dB
Input Coupling Capacitors
Input coupling capacitors block any DC bias from the audio source and ensure maximum dynamic range. Input coupling capacitors also minimize
PAM8901 / PAM8908 turn-on pop to an inaudible level. The input capacitors are in series with PAM8901 / PAM8908 internal input resistors,
creating a high-pass filter. The following Equation calculates the highpass filter corner frequency.
C R 2
1
f
IN IN
TT
C
=
The input impedance, RIN, is dependent on device gain. Larger input capacitors decrease the corner frequency. See the following table for input
impedance values.
G0 Voltage G1 Voltage R
IN
≤0.6V ≤0.6V 26.4kΩ
≥1.4V ≤0.6V 19.8kΩ
≤0.6V ≥1.4V 16.5kΩ
≥1.4V ≥1.4V 13.2kΩ
For a given high-pass cutoff frequency, the minimum input coupling capacitor is found as:
R f 2
1
C
IN C
TT
IN
=
Example: Design for a 20Hz corner frequency with a PAM8901 / PAM8908 gain of +6dB. The input impedance table gives RIN as 13.2k Ω. The C
IN
Equation shows the input coupling capacitors must be at least 0.6µF to achieve a 20Hz highpass corner frequency. Choose a 0.68µF standard
value capacitor for each PAM8901 / PAM8908 input (X5R material or better is required for best performance).
Charge Pump Flying Capacitor, HPVDD Capacitor and HPVSS Capacitor
The PAM8901 / PAM8908 uses a built-in charge pump to generate a positive and negative voltage supply for the headphone amplifiers. The
charge pump flying capacitor connects between CAP+ and CAP-. It transfers charge to generate the positive and negative supply voltage. The
HPVDD capacitor or HPVSS capacitor must be at least equal in or larger than value to the flying capacitor to allow maximum charge transfer.
Use low equivalent-series-resistance (ESR) ceramic capacitors (X5R material or better is required for best performance) to maximize charge
pump efficiency. Typical values are 1µF for the HPVDD, HPVSS and flying capacitors.
Power Supply Decoupling Capacitors
The PAM8901 / PAM8908 TrueCapFree headphone amplifier requires adequate power supply decoupling to ensure that output noise and total
harmonic distortion (THD) remain low. Use good low equivalent-series-resistance (ESR) ceramic capacitors (X5R material or better is required
for best performance). Place a 2.2µF capacitor within 5mm of the PVDD pin. Reducing the distance between the decoupling capacitor and PVDD
minimizes parasitic inductance and resistance, improving PAM8901 / PAM8908 supply rejection performance. Use 0402 or smaller size
capacitors if possible.
Power Supply Sequencing
Use input coupl ing capaci tors to ensure inaudible turn-on pop. Activate the PAM8901 / PAM8908 after all audio sources have been activated
and their output voltages have settled. On powerd own, deactivate the PAM8901 / PAM8908 before deactivating the audio input source. The EN
pin controls device shutdown: Set to 0.6V or lower to deactivate the PAM8901 / PAM8908; set to 1.4V or higher to activate.