Regardless of the protocol used by a mobile phone, whether GSM or TDMA, the RF transmitter switching creates severe noise for the power supply, as the RF power amplifier (PA) switches on and off at 217 Hz. At each of these events, a high current (typically, up to 1.7 A) is drawn from the power supply, creating a sudden voltage drop through the battery's equivalent series resistance (ESR). reaching up to 500 mV, Figure 1
Figure 1: TDMA noise propagation in a typical Li-ion battery-powered device
(Click on image to enlarge)
For a system-on-chip (SoC) designs which embed high-resolution audio converters with audio amplifiers, or for a high-sensitivity MEMS, such an amplitude jeopardizes the overall performance of the SoC(s). Specifically, the audio quality may be significantly altered by audible buzz sounds.
Such a noise is particularly audible since 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 approach for preventing GSM noise from degrading audio quality is to use a linear regulator (LR) with very low drop out (an LDO), as well as low output noise, to directly supply the audio amplifier from the lithium-ion battery. Such an LDO linear regulator is 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 regulator to reject noise from the input voltage, expressed as power supply rejection ratio (PSRR). But PSRR figures should also be considered along with the LR transient responses and drop-out characteristics.
Power-supply rejection ratio
PSRR is the ability of the regulator to maintain its output voltage as its input voltage varies. It must be specified over some frequency range, certainly including the critical 217 Hz value, 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 meeting maximum load demand (drop-out is maximal).
Usually, PSRR performance is specified at 10 kHz, which may not be appropriate to reflect the expected rejection of noise at 217 Hz.
Conclusion number 1: 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
PSRR is not the only specifications ensuring that no noise will be heard on the audio output. The transient response characteristic of the LR, and 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.
Assuming Vin is the input voltage, Vout is the output voltage, and Voutreg is the regulated output voltage, these are important points to consider:
Let 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 input voltage
- 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
Therefore, a LR exhibits a high PSSR only when the condition Vin > Voutreg + Vdrop is satisfied.
Consider the case shown in Figure 2, where the LR is providing the interface between a lithium-ion battery delivering 3.6 V and an analog block requiring a regulated 2.8 V supply.
Figure 2 Relationship between dropout and effective noise rejection of a linear regulator
(Click on image to enlarge)
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. The LR of line 1 (plain blue line) has a drop-out of less than 100 mV, and therefore keeps on regulating. In contrast, the LR of line 2 (red dotted line) cannot stand such a voltage drop, as it requires a minimum of 400 mV of difference (drop-out) between the input and the output voltage.
Conclusion number 2: the demand on a linear regulator with very low drop-out is even more critical as the battery voltage decreases over time, for instance, down to 2.9 V. It is important to maintain regulation over the full range of the battery voltage, which leads to the conclusion that the better the drop-out is, the better the battery runtime will be.
Misleading PSRR specification
It is not unusual 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
(for the case of regulator number in Figure 2)
and it will then demonstrate a PSRR of 0 dB in this case!
This is shown in Figure 2, where the residual TDMA noise present at the output of regulator number 2 is several hundreds of mV, and cannot be predicted using the PSRR figure.
Conclusion number 3: 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 to properly reject the noise from the input voltage under all operating conditions.
About the author
Marie Maurel has been Product Manager for mixed-signal silicon IP at Dolphin Integration (Meylan, France, www.dolphin-ip.com/jazz) for three years, in charge of audio ADCs, DACs, CODECs, and power management solutions. You can reach Marie at jazz@dolphin-ip.com.
