Rainbow Electronics MAX9758 - Manual (Page 18)

MAX9756/MAX9757/MAX9758
Since the volume control (VOL) ADC is ratiometric to
HPV
DD , any variations in HPV DD are negated. The gain
step sizes are not constant; the step sizes are
0.5dB/step at the upper extreme, 2dB/step in the
midrange, and 4dB/step at the lower extreme. Figure 8
shows the transfer function of the volume control for a
3.3V supply.
Low-Dropout Linear Regulator
The MAX9756/MAX9758s’ low-dropout linear regulator
(LDO) can be used to provide a clean power supply to
a CODEC or other circuitry. The LDO can be enabled
independently of the audio amplifiers. REGEN
enables/disables the LDO, set REGEN = V
DD to enable
the LDO or set REGEN = GND to disable. The LDO is
capable of providing up to 150mA continuous current
and features Maxim’s Dual Mode feedback. When SET
is connected to GND, the output is internally set to
approximately 4.65V. Adjust the output from 1.23V to
5V by connecting two external resistors, used as a volt-
age-divider, at SET (Figure 9).
The output voltage is set by the following equation:
where V SET = 1.23V.
To simplify resistor selection:
Since the input bias current at SET is nominally zero,
large resistance values can be used for R1 and R2 to
minimize power consumption without losing accuracy.
Up to 1.5M
Ω is acceptable for R2.
To minimize the current consumption, it is desirable to
use high-value resistors (> 10k
Ω for the external feed-
back divider (R1, R2). The input capacitance at SET
and the stray and wiring capacitance should be com-
pensated by placing a small capacitor (in the 10pF
range) across the upper feedback resistor R1 (see
Figure 9).
This capacitor creates a zero in the feedback loop to
reduce overshoot. Overcompensation can cause poor
stability in the high current range.
The regulator should be compensated with two 1µF
ceramic capacitors connected between IN and GND
and OUT and GND. X7R dielectric with 10% tolerance
is recommended.
The ESR of each capacitor should not exceed 40m
Ω
for good stability up to the full-rated current (150mA).
Place the capacitors as close as possible to the device
to limit the parasitic resistance and inductance. There
is no upper limit to the amount of additional bypass
capacitance.
DirectDrive Headphone Amplifier
Unlike the MAX9756/MAX9757/MAX9758, conventional
single-supply headphone amplifiers typically have their
outputs biased at half the supply voltage for maximum
dynamic range. Large coupling capacitors are needed
to block this DC bias from the headphones. Without
these capacitors, a significant amount of DC current
flows to the headphone, resulting in unnecessary
power dissipation and possible damage to both head-
phone and headphone amplifier.
R R
V
V
OUT
SET
1 2 1
=
⎛
⎝⎜ ⎞
⎠⎟
−
V V
R
R
OUT SET
= +
⎛
⎝⎜ ⎞
⎠⎟
1
1
2
2.3W Stereo Speaker Amplifiers and DirectDrive
Headphone Amplifiers with Automatic Level Control
18 ______________________________________________________________________________________
Figure 8. Volume Control Transfer Function
VOLUME CONTROL TRANSFER FUNCTION
V VOL (V)
GAIN (dB)
3.5 3.0 2.5 2.0 1.5 1.0 0.5
-60
-40
-20
0
20
40
-80
0 4.0
GAIN1 = GAIN2 = GAIN3 = 1
HPV
DD = 3.3V
SPEAKER MODE
HEADPHONE MODE
Figure 9. Adjustable Output Using External Feedback Resistors
MAX9756
MAX9758
OUT
SET
GND
1 μF 1 μF 10pF R1
R2