Component Selection
Gain-Setting Resistors External feedback components set the gain of the MAX9777/MAX9778. Resistor RIN sets the gain of the input amplifier (AVIN), and resistor RF sets the gain of the second stage amplifier (AVOUT):
| ⎛ 10kΩ ⎞ | ⎛ | R | ⎞ |
| AVIN = − ⎜ | | ⎟ | , AVOUT = − ⎜ | F | ⎟ |
| RIN | 10kΩ |
| ⎝ | ⎠ | ⎝ | ⎠ |
| | |
Combining AVIN and AVOUT, RIN and RF set the single- ended gain of the device as follows:
⎛ 10kΩ ⎞ | ⎛ | R | ⎞ | ⎛ | R | ⎞ |
AV = AVIN ⋅ AVOUT = − ⎜ | | ⎟ | ⋅ − ⎜ | F | ⎟ | = +⎜ | F | ⎟ |
| | |
⎝ | RIN | ⎠ | ⎝ | 10kΩ | ⎠ | ⎝ RIN ⎠ |
| |
As shown, the two-stage amplifier architecture results in a noninverting gain configuration, preserving absolute phase through the MAX9777/MAX9778. The gain of the device in BTL mode is twice that of the sin- gle-ended mode. Choose RIN between 10kΩ and 15kΩ and RF between 15kΩ and 100kΩ.
Input Filter The input capacitor (CIN), in conjunction with RIN, forms a highpass filter that removes the DC bias from an incoming signal. The AC-coupling capacitor allows the amplifier to bias the signal to an optimum DC level. Assuming zero-source impedance, the -3dB point of the highpass filter is given by:
1
f−3dB = 2πRINCIN
Choose RIN according to the Gain-Setting Resistors sec- tion. Choose the CIN such that f-3dBis well below the lowest frequency of interest. Setting f-3dBtoo high affects the amplifier’s low-frequency response. Use capacitors whose dielectrics have low-voltage coefficients, such as tantalum or aluminum electrolytic. Capacitors with high- voltage coefficients, such as ceramics, may result in an increased distortion at low frequencies.
Other considerations when designing the input filter include the constraints of the overall system, the actual frequency band of interest, and click-and- pop suppression.
Output-Coupling Capacitor The MAX9777/MAX9778 require output-coupling capacitors to operate in single-ended (headphone) mode. The output-coupling capacitor blocks the DC component of the amplifier output, preventing DC cur- rent from flowing to the load. The output capacitor and
the load impedance form a highpass filter with a -3dB point determined by:
1
f−3dB = 2πRLCOUT
As with the input capacitor, choose COUT such that
f is well below the lowest frequency of interest.
Setting f too high affects the amplifier‘s low-fre- quency response.
Load impedance is a concern when choosing COUT. Load impedance can vary, changing the -3dB point of the output filter. A lower impedance increases the cor- ner frequency, degrading low-frequency response. Select COUT such that the worst-case load/COUT com- bination yields an adequate response. Select capaci- tors with low ESR to minimize resistive losses and optimize power transfer to the load.
If layout constraints require a physically smaller output- coupling capacitor, decrease the value of COUT and add series resistance to the output of the MAX9777/MAX9778 (see Figure 9). With the added series resistance at the output, the cutoff frequency of the highpass filter is:
| f−3dB | = | 1 |
| 2π(RL + RSERIES )COUT |
| | |
Since the cutoff frequency of the output highpass filter is inversely proportional to the product of the total load resistance seen by the outputs (RL + RSERIES) and
COUT, increase the total resistance seen by the MAX9777/MAX9778 outputs by the same amount COUT is decreased to maintain low-frequency performance. Since the added series resistance forms a voltage- divider with the headphone speaker resistance for fre- quencies within the passband of the highpass filter, there is a loss in voltage gain. To compensate for this loss, increase the voltage gain setting by an amount equal to the attenuation due to the added series resis- tance. Use the following equation to approximate the required voltage gain compensation:
AV _ COMP = 20log⎛⎜ RL + RSERIES ⎞⎟
⎝ RL ⎠
COUTRSERIES
OUT_+
RL
Figure 9. Reducing COUT by Adding RSERIES
