Keeping the best audio
quality in mobile phone by managing voltage drops created by 217 Hz
By Marie Maurel
Audio Product Line Manager
Source of major noise
protocol used by a mobile phone, GSM or TDMA, RF transmitter switching
creates the most notorious noise for the power supply. The RF power
amplifier switches on and off at a rate of 217 Hz. At each of these
events, a high current (up to 1.7A) is drawn from the power supply,
creating a sudden voltage drop on the battery equivalent serial resistance
(ESR) reaching up to 500 mV.
For a System-on-Chip
(SoC) embedding high-resolution audio converters with audio amplifiers,
or for a high sensitivity MEMS, such an amplitude jeopardizes the overall
performance of the SoC(s). More specifically, the audio quality may
be deeply altered by audible buzz sounds.
Such a noise
is particularly audible as it is not random. Indeed, noises with amplitudes
as low as 10 µV can be heard if they occur at a fixed rate of recurrence.
They could be even much more disturbing than a random noise of higher
amplitude, which will be considered as a background noise.
The best choice
for preventing GSM noise from degrading audio quality is to place a
Linear Regulator (LR) with very Low Drop-Out (LDO) and Output Noise
to supply directly the audio amplifier from the Lithium-Ion battery.
Such LDO linear regulator is then used as a clean-up or filtering module
for the amplifier power supply.
The usual key
criterion to select the best LR to prevent GSM noise is the capability
of the LR to reject noise from the input voltage, expressed as Power
Supply Rejection Ratio. But PSRR figures should also be pondered with
the LR transient responses and drop-out characteristics.
Power Supply Rejection Ratio
The Power Supply
Rejection Ratio is the ability of the regulator to maintain its output
voltage as its input voltage varies. PSRR must be specified over some
frequency range, certainly including the critical 217 Hz, and for the
maximum output current for which the regulator is designed. Indeed,
the capacity of rejection must be ensured even when the regulator is
the most solicited (drop-out is maximal).
performances are specified at 10 kHz and may not be appropriate to reflect
the expected rejection of noise at 217 Hz.
1 TDMA noise propagation in a typical Li-Ion battery powered device
The first conclusion
is that the PSRR specification must be analyzed over a complete frequency
range to compute the sensitivity of the regulator to any possible noise
source, including 217 Hz.
Transient response and drop-out
The PSRR is
not the only specifications ensuring that no noise will be heard on
the audio output. The transient response characteristic of the LR, thus
the capacity of the voltage regulator to maintain the output voltage
at the desired regulated value, under sudden supply or load change,
also is critical.
Let us convey
some important statements.
stands for input voltage, Vout stands for output voltage, Voutreg stands
for regulated output voltage :
Vdrop = minimum
drop-out of the regulator (minimum difference between Vin and Vout)
The LR is designed
to maintain a fixed output voltage, with a high PSRR, for a varying
When Vin > Voutreg
+ Vdrop, the LR performs normally and regulates the output voltage with
a 70 dB PSRR for example
When Vin < Voutreg
+ Vdrop, the LR does not regulate the output anymore and Vout = Vin-Vdrop,
the PSRR is 0 dB as the output voltage follows the input voltage
LR exhibits a high PSSR only when the condition Vin > Voutreg + Vdrop
Let us consider
a case depicted in Figure 2
where the LR is making the interface between a Lithium-Ion battery delivering
3.6 V and an analog block requiring a regulated 2.8 V supply. The figure
shows how a 500 mV transient on the battery voltage is transferred to
the LDO output depending on the effective drop-out of the linear regulator.
In Figure 2,
the LR n°1 (in blue plain line) has a drop-out of less than 100 mV
and therefore keeps on regulating, while the n°2 (in red doted line)
cannot stand such voltage drop as it requires a minimum of 400 mV of
difference (drop-out) between the input and the output voltage.
2 Relationship between dropout and effective noise rejection of a linear
conclusion is that the demand for a linear regulator with very low drop-out
is even more critical as the battery voltage decreases over time, down
to 2.9 V for instance. It is important to maintain regulation over the
full range of the battery voltage and this leads to the conclusion that
the better the drop-out is, the better the battery runtime will be.
Misleading PSRR specification
It is not rare
to see a LR specified with a 400 mV drop-out voltage. Even if such a
LR is specified for a PSRR of 85 dB, it will not be able to filter any
noise if the input voltage is lower than 3.0 + 0.4 = 3.4 V (case of
regulator n°2 on Figure
2) and it will then
demonstrate a PSRR of 0 dB in this case!
See the case
2, where the residual
TDMA noise present at the regulator n°2 output with several hundreds
of mV and cannot be predicted using the PSRR figure!
The third conclusion
is that the effective dropout must be considered as well to evaluate
the residual noise at the linear regulator output.
To be useful
for a SoC integrator willing to compare two linear regulators, the drop-out
must be given at the maximum current. This ensures that the LR is properly
calibrated to drive the given load and reject properly the noise from
the input voltage in all operating conditions.
LR with high performances are currently provided by Dolphin Integration
as an extension to its famous product line of embedded high resolution
audio CODECS. With such solutions, the Company's customers may benefit
from a higher audio quality over a longer time and a better immunity
to the noise on PCB power supplies.
For more information, visit
our website at www.dolphin-ip.com/jazz