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december 2007 rev 1 1/18 AN2583 application note understanding and applying the m41t00aud real-time clock with audio introduction drawing on the fact that the squarewave generator built in to many st serial rtcs can be used as a tone generator, users have been employing st real-time clocks in printer-fax scanner units for several years. in these applications, the squarewave output is used to create the audible beeps these all-in-one (aio) machines make when a key is pressed or an error occurs. the beep signals drive an amplifier circuit which in turn drives a speaker. furthermore, other tone sources from within the aio also go to the speaker, so they are summed into the amplifier along with the squarewave signal. one of these sources is the phone line. it is audible while the fax is dialing and negotiating the connection. additionally, it is necessary to control the gain of the amplifier and to limit the bandwidth of the audio signal, so some form of volume control and filtering is required. as a result, st developed the m41t00aud which combines a real-time clock with an audio section. referring to figure 1 on page 2 , the clock is based on st's m41t00 and includes several enhancements such as a trickle charge circuit for charging backup capacitors and a voltage reference for precisely contro lling the backup switchover threshold. the audio section starts with an input amplifier for summing multiple signals of varying levels. next is an 8 khz low-pass filter, then a 16-step, digitally controlled gain stage with 3 db steps. finally, the output section is a bridged amplifier capable of driving 300 mw into an 8 load (with v cc at 3.3 v). thus, the m41t00aud is a highly integrated circuit perfect for all-in-one printer applications, as well as other consumer devices, where high functionality and low cost are key requirements. www.st.com
typical application AN2583 2/18 1 typical application figure 1. m41t00aud block diagram au t om a tic b a tte r y switchover & deselec t v cc 2 i 2 c (sda, scl) i 2 c v dd 32khz oscil l a t or 400khz i 2 c inter f ace osci osco uc m 4 1 t 0 0 a u d hours d a te d a y month year secs mins centu r y bits calibr a tion reference v pfd =2.80v oscil l a t or f ai l detec t ou t irq/ft/ou t trickle charge v in t v back v dd 256/512hz audio lpf adj gain ain aout+ aout ? v ss v bias write protec t fbk audio-in ai13927 v dd 2
AN2583 typical application 3/18 the typical aio application has audio sources in the system micro-controller and in the fax chipset originating with the phone line. these ar e summed into the inpu t amplifier which in turn drives the filter along side the internally generated 256/512 hz tones (see figure 2 ). figure 2. m41t00aud application example input resistors, r in , along with the feedback resistor, r f , control the gain into the part. each input can be added at a different level by changing its corresponding input resistor thus allowing signals of varying levels to be normalized. the input capacitor, c, blocks any dc into the part. since the m41t00aud uses a single supply, its internal mid-point is v dd /2 and not ground. thus, input signals would need to be centered at v dd /2 to connect directly. otherwise, the coupling capacitor is necessary. furthermore, it provides an input filter blocking low frequencies. when r in = 20 k and c = 0.1 uf, signals below approximately 80 hz will be attenuated. after the input amplifier is the low-pass filter with its knee at 8 khz. with the input capacitor, these form a band pass filter over the range of 80 hz to 8 khz. also input to the lpf is the 256/512 hz signal from the rtc timing chain. software controls whether it is passed into the lpf and which of the two frequencies is selected. software also controls the gain. a mute bit can be set to shut off all audio and a four-bit gain field controls the digital gain stage selecting between ?33 db and +12 db of gain in 3 db steps. the output is a bridged amplifier capable of sinking/sourcing over 300 ma. for v cc = 3.3 v, it can readily drive over 300 mw of power into an 8 speaker. uc pwm / dac tone generator fax chipset modem phone line lpf adj gain aout+ aout? m41t00aud audio section r i1 r i2 r f rtc timing chain 256/512hz c ai13928
audio characteristics AN2583 4/18 2 audio characteristics several parameters are used to characterize amplifier circuits. among them are gain, total harmonic distortion, power-supply rejection ratio and output power. the following is a brief primer on these topics. 2.1 harmonic distortion ideally, the output of an audio amplifier is a linear reproduction of its input; the output differs only in amplitude and is ident ical in all ot her respects. figure 3. ideal and non-ideal waveforms in reality, several things combine to distort the waveform resulting in the addition of harmonics in the output, as depicted in figure 4 . the amplifiers are not perfectly linear, so some reshaping of the waveform will occur. this happens in a symmetric fashion resulting in the addition of odd harmonics. the crossover point at which one drive transistor turns on and the other turns off is imperfect and results in crossover distortion as shown in figure 3 (exaggerated for effect). this too adds odd harmonics. the lack of perfect symmetry in the complementary output tr ansistors will cause the top and bottom halves of the waveforms to be asymmetric resulting in even harmonics. the result is that the amplifier outp ut will contain several artifacts not present in the input. ideal output is simply a linear scale of the input. asymmetric nature of waveforms due to imperfect symmetry of complementary p and n transistors reshaping of waveforms due to non-linear nature of transistors crossover distortion because switchover from p to n channel drivers does not occur perfectly 0 v dd / 2 v dd v in 0 v dd / 2 v dd v out 0 v dd / 2 v dd v out ideal with dist or tion t f 0 = 1 t ai13929
AN2583 audio characteristics 5/18 figure 4. harmonics 2.1.1 total harmonic distortion the total harmonic distortion, thd, is a measure of the linearity or purity of the amplification process and indicates to what extent the artifacts occur. a pure sine wave is input to the amplifier and the spectrum of the output is determined. the thd figure is a comparison of the output power or voltage levels of the harmonics to the fundamental. figure 5 illustrates an example output spectrum. p 1 is the fundamental frequency while p 2 to p 7 are the harmonics. the thd figure establishes the fraction of the output signal which is harmonics. figure 5. power output spectrum there are two methods used to calculate the thd. the first method compares the power of the harmonics to the fundamental. ai13930 0 f (v out ) v out , fundament al f 0 2f 0 3f 0 4f 0 5f 0 6f 0 7f 0 f v out odd harmonics v out even harmonics 0 f (p out ) f 0 2f 0 3f 0 4f 0 5f 0 6f 0 7f 0 f p 1 p 2 p 3 p 4 p 5 p 6 p 7 ai13931 1 7 6 5 4 3 2 p p p p p p p p frequency l fundamenta of power powers harmonic thd l + + + + + + = =
audio characteristics AN2583 6/18 figure 6. voltage output spectrum the other thd calculation method compares the voltages. since p is proportional to v 2 , this second method amounts to being the square root of the first. because thd is generally a number less than 1, thd v , being the square root of thd p , will be a larger number than thd p . 0 f (v out ) f 0 2f 0 3f 0 4f 0 5f 0 6f 0 7f 0 f v 1 v 2 v 3 v 4 v 5 v 6 v 7 ai13932 1 2 7 2 6 2 5 2 4 2 3 2 2 2 v v v v v v v v frequency l fundamenta of voltage voltages) (harmonic thd l + + + + + + = = 1 2 6 2 5 2 4 2 3 2 2 1 2 7 2 6 2 5 2 4 2 3 2 2 v v v v v v v r 1 r 1 v v v v v v v thd l + + + + + ? = + + + + + + = 1 7 6 5 4 3 2 1 2 7 2 6 2 5 2 4 2 3 2 2 p p p p p p p r v r v r v r v r v r v r v l l + + + + + + = + + + + + = p v thd thd =
AN2583 audio characteristics 7/18 the required thd specification for the m41t00 aud is less than 2% at 1 khz and 300 mw. the voltage formula was used in the measurement for this, and makes for the more difficult specification since it tends to yield the higher number. example: let v 1 = 1 v, v 2 = 0.1 v, v 3 = 0.08 v, v 4 = 0.06 v and v 5 = 0.04 v thus the voltage formula makes for a more challenging specification. 2.2 amplifier primer several types of amplifiers are possible for fa brication in semiconductor ics. the following is a brief overview of the types and their power capabilities. 2.2.1 simple amplifier figure 7 illustrates a simple amplifie r circuit. the input sect ion provides any signal conditioning - gain and offset - necessary for driving the output transistors shown here. the midpoint voltage, v x , is biased at v dd /2. at its maximum, v x will swing up to v dd ? v dsp-min when q2 is fully on, and down to v dsn-min when q1 is fully on. figure 7. basic single supply amplifier for the purposes of analysis, we can assume v dsp-min and v dsn-min are small compared to v dd and thus ignore them here. with that in mind, the point v x will swing between gnd (0 v) and v dd . for audio applications, it is necessary that the output signal be purely ac. there should be no dc component in it. the output signal must swing symmetrically about ground. % 7 . 14 147 . 0 1 0.04 0.06 0.08 0.1 thd 2 2 2 2 2 v = = + + + = 2.16% 0.0216 thd thd 2 v p = = = v dd v x v l v in + v dsp ? + v dsn ? c r l + ? q2 q1 gnd gnd ai13933
audio characteristics AN2583 8/18 therefore, a capacitor, c, is inserted in series between v x and the load, r l . this will have the effect of blocking any dc in the output. so, while the point v x will swing between v dd and ground, with an average (dc) level of v dd /2, the output voltage v l will swing between v dd /2 and ?v dd /2, with an average of 0v. this assumes no voltage drop across the capacitor. if the capacitance is sufficiently hi gh for the frequency range of interest, then the drop across the capacitor will be small and the assumption valid. if the signal at v x is a sine wave swinging between v dd and ground, we can write that as and where f is the frequency of the sine wave. the signal at the load then, with the dc component removed, is 2.2.2 output power a high priority for many audio enthusiasts is the power output of their amplifiers. they want to get as much power as they can without high costs. the two main factors affecting the amplifier power output are the available voltage supply and the circuit topology. for a generic sinusoidal signal, the average power to the load is calculated as a function of the voltage. either the peak-to-peak voltage, v pp , or the peak voltage, v pk , can be used. these are illustrated in figure 8 . figure 8. voltage terminology for the circuit in figure 7 , at maximum power, v x will swing from v dd to ground as shown in figure 9 . at the load, with the dc component removed, v l will swing between v dd /2 and ?v dd /2. t sin 2 v 2 v ) t sin 1 ( 2 v ) t ( v dd dd dd x ? + = + ? = f 2 = t sin 2 v ) t ( v dd l ? = v pp v pk ai13934
AN2583 audio characteristics 9/18 figure 9. simple amplifier outputs waveforms the average or continuous power to a resistive load, r, for a sinusoidal signal, is this shows the power calculation using both the peak voltage v pk and the peak-to-peak voltage v pp . for the circuit in figure 7 , the expression becomes at maximum swing, v l-pk equals v dd /2, so the maximum continuous power to the load is equation 1 thus, for the topology in figure 7 , the power to the load is a function v dd and r l . to get more power to the load, the user must increase v dd or decrease r l , or use a different circuit topology. 0 v dd / 2 v dd v x v xpp v dd / 2 0 v dd / 2 v l v x-pk v lpp v l-pk ai13935 r 8 v r 2 v p 2 pp 2 pk av = = l 2 pp - l l 2 pk - l av - l r 8 v r 2 v p = = () l 2 dd l 2 dd l 2 pk - l max - av - l r 8 v r 2 2 / v r 2 v p = = =
audio characteristics AN2583 10/18 2.2.3 getting more power one technique for increasing the deliverable power is to add a negative supply equal in magnitude to the positive supply. this is depicted in figure 10 . with this addition, the midpoint, v x , is at ground. one advantage of this is that no coupling capacitor is required thus removing any frequency roll off effects due to using an imperfect, real-world capacitor. figure 10. amplifier with symmetric supplies thus, without the capacitor, v l = v x . for this topology, ignoring the v ds drops in q1 and q2 as before, v x will swing between v dd and ?v dd as shown in figure 11 . figure 11. output of amplifier with symmetric supplies as before, the continuous power to the load is at maximum continuous output power, v l-pk equals v dd , so this becomes thus, compared to equation 1 , by adding the negative supply, the maximum power increased by a factor of 4. the same effect could also be achieved by doubling v dd in the circuit of figure 7 . but either way, adding a supply or doubling the voltage can be costly. it is preferred to get the increase in power without such additional costs. v dd v x v l v in + v dsp ? + v dsn ? r l + ? v ss = ?v dd q2 q1 gnd ai13936 v dd 0 v dd v l = v x v lpp v l-pk ai13937 l 2 pk - l av - l r 2 v p = l 2 dd l 2 pk - l max - av - l r 2 v r 2 v p = =
AN2583 audio characteristics 11/18 2.2.4 bridged amplifiers a bridged amplifier uses two identical amplifiers driven 180 out of phase. the circuit shown in figure 12 uses two single supply amplifiers, a x . these would each be comparable to the amplifier of figure 7 . the input signal is connected unchanged to the left amplifier and inverted prior to going to the right amplifier thus making the right one 180 out of phase with the left. the outputs of the two amps, v l+ and v l? , are each biased at v dd /2. thus, the quiescent difference between them is 0 v. therefore, no dc blocking capacitor is required at the load. figure 12. bridged amplifiers each output can swing between v dd and ground as shown in figure 13 . figure 13. outputs of bridged amplifier v in r l v dd v l + ? v l+ v l? a x v dd a x ai13938 0 v dd / 2 v l? v dd v l+ 0 v dd v l v l-pk v dd ai13939
audio characteristics AN2583 12/18 at maximum swing, for sinusoidal outputs, v l+ and v l- can be written as and the load voltage, v l , is the difference between the two outputs. equation 2 thus, bridging has the effect of doubling the load voltage without adding or changing the voltage supply. in this example, the two amplif iers bridged together were each single supply devices, but dual-supply amp lifiers such as that in figure 10 can also be bridged. the result will be the same - the deliverable load voltage will be double that of usi ng a single amplifier. 2.2.5 bridged power for the circuit in figure 12 , the peak value of v l is v dd as shown in figure 13 . the maximum continuous power is then this is the same power as for the case of the dual-supply amplifier in figure 10 , and four times the power of that in figure 7 . by bridging two single-supply amplifiers, four times the power is available without the added cost of a second power supply. hence, bridging is an economical and popular way of increasing amplifier power without expensive power supplies. t sin 2 v ) t ( v dd l ? = + + ? = ? ? = ? ? = l dd dd ? l v t sin 2 v ) 180 t sin( 2 v ) t ( v + + + + = ? ? = ? = l l l ? l l l v 2 ) v ( v v v v l 2 dd l 2 pk - l max - av - l r 2 v r 2 v p = =
AN2583 audio characteristics 13/18 2.2.6 automotive example an automobile is a great example of the benefits of bridging. the standard supply of 12 v limits the load voltage unless an expensive dc-dc power supply is added to the system. for the circuit of figure 7 , with v dd = 12 v and an 8 ohm load, we get thus, the maximum continuous power delivered to an 8 ohm speaker would be 2.25 w. taking into account the v ds drops in the drive transistors will further lower that. conversely, most cars will be operating at a little more than 12 v, usually somewhere in the range 13.6 to 14.4 v, so that tends to offset the v ds drops. for the circuit of figure 12 , again with 12 v and 8 ohms, the power calculation is bridging delivers 9 w, four times the previous case, as expected. and with a 4 ohm speaker, the load power goes up to 18 w. for maximum power in the 12 v automotive environment, bridging offers a low cost alternative to expensive power supplies by increasing the available audio power by a factor of 4. combined with 4-ohm speakers, users can achieve outstanding audio performance at moderate cost. similarly, in any environment where the voltage supply options are limited, bridging provides a significant power improvement fo r a nominal increase in silicon. w 25 . 2 64 144 r 8 v p l 2 dd max - av - l = = = w 9 16 144 r 2 v p l 2 dd max - av - l = = =
audio characteristics AN2583 14/18 2.3 power supply rejection ratio the psrr is a measure of how well the device keeps power supply noise out of its output. it is measured by setting the input to 0 and coupling a 200 mv pp sign wave into v dd . the output signal is filtered to reject frequencies other than the test signal, in this case 1khz. a fast fourier transform engine, or fft, is helpful in implementing the filter and measuring the output level. since the psrr is in the range of ?55 to ?65 db, the output signal will be on the order of 200 uv. without the fft and f iltering, the signal will be lost in the noise floor. figure 14. psrr test setup the psrr is calculated as follows using the standard formula for power gain in db. psrr = 20 log ( v out / v n ) example: if the output is measured as 350 uv, the psrr will be psrr = 20 log ( 350 u / 200 m ) = ?55.1 db ai13940 v n test signal scope and/or other test eqpt v dd + v n v in v out v dd
AN2583 audio characteristics 15/18 2.4 gain there are three stages of gain in the m41t00aud. the input amplifier's gain is controlled by the input and feedback resistors chosen by the user. the lpf is at unity gain and thus does not contribute. the programmable gain stage can be adjusted from a gain (attenuation) of 0.022x all the way up to a 4x multiplication. lastly, the bridge amplifier provides a fixed gain of 2x which is a consequence of two amplifiers in parallel, one at a gain of 1x and one at ?1x, resulting in a net gain of 2 as shown in equation 2 on page 12 . figure 15. end-to-end audio path the gain is usually measured in decibels or db . this is defined as the ten times the log of the ratio of the output power to the input power normalizing the resistances, this reduces to example: in figure 15 , with r in = 2 r f , the input gain is 0.5 which cancels out the final gain stage gain of 2. the net gain for these two sections is then 1. the result is that the final gain is determined solely by the adjustable gain stage and will match whatever value is programmed into the gain register. ai13941 lpf 100hz-8khz 256hz 512hz 256/512 selec t t one on/off from internal r tc timing chain 300mw @ 8 ohms v dd 2 aout+ aout ? r f 10k 0.1uf r i1 20k r i2 20k +6db 0db 20 log (r f /r in ) = ? 6db (r f /r in = 0.5) (1x, @ midband) (0.022x to 4x) (2x) gain register bbbb 33db to +12db ? ? ? ? ? ? ? ? ? = ? ? ? ? ? ? ? ? = = in 2 in out 2 out in out p /r v /r v log 10 p p log 10 db in gain power a in out 2 in out 2 in 2 out p v v log 20 v v log 10 v v log 10 a = ? ? ? ? ? ? ? ? = ? ? ? ? ? ? ? ? =
m41t00 audio section specifications AN2583 16/18 3 m41t00 audio section specifications in summary, the audio section of the m41t00aud uses a bridged amplifier output stage which can deliver 300 mw into an 8 ohm load. the device produces less than 2% total harmonic distortion at this output level, typically 0.2%. signals are summed into the input by connecti ng multiple, parallel in put resistors. for r in = 20 k and c = 0.1 uf, the input will be attenuat ed below 80 hz. the low pass filter after the input amplifier will attenuate signals above 8 kh z thus forming a band pass of 80 ? 8000 hz. the end-to-end gain is controlled by the input and feedback resistors along with the software controlled gain stage. for r in = 2r f , software can adjust the gain between ?33 and +12 db. lastly, with a psrr of ?55 db, any noise present on the power supply will be reduced by at least 55 db at the output.
AN2583 revision history 17/18 4 revision history table 1. document revision history date revision changes 06-dec-2007 1 initial release.
AN2583 18/18 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a parti cular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by an authorized st representative, st products are not recommended, authorized or warranted for use in milita ry, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or registered trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2007 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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