STW5094A_05 Datasheet, PDF (11/52 Pages) STMicroelectronics – 18-bit Low Power Asynchronous Stereo Audio DAC with Integrated Power Amplifiers and Voice Codec

STw5094A
2.7 FM Input Path
The device is provided with stereo and mono single ended analog inputs, designed to amplify analog sig-
nals from an FM decoder but they can be considered as generic analog inputs. The stereo analog inputs
are FML and FMR pins, or alternatively MIC3 pin for mono input (sent to left and right channels). The se-
lection between FML-FMR or MIC3 is done with bit MFM in CR20.
The analog inputs are connected to a programmable gain stage that can amplify the signals in the range -20dB
to +18dB in 2 dB steps. The gain control is independent for Left and Right channel and is selected with bits
FMLA in CR10 (Left) and bits FMRA in CR11 (Right).
There are 2 ways to connect the FM inputs to the output drivers. The first is to select the FM Mode (bits
MD in CR21). The second is to activate Audio or Voice or Tone Only modes and to set to 1 bit FMS in
CR20. In the last case the signal coming from FM inputs will be summed to the Audio or Voice or Tones
signals respectively.
2.8 Output Drivers section
There are 3 Analog Output Drivers. The LSP ⁄ N differential driver delivers 300mW typical power with
0.1% T.H.D. (250mW minimum undistorted) on a 8Ω earpiece ⁄ loudspeaker (piezoceramic loads up to
50nF can also be driven with a series resistor), it has a 30dB range gain control (bits LSA in CR7). The 2
single ended drivers (HPL and HPR) deliver 40mW typical power with 0.1% T.H.D. (30mW minimum un-
distorted) on 16Ω stereo headphones, they have a 40dB range gain control (CR8 for HPL and CR9 for
HPR). It is possible to put the drivers in power-down and in power-up by programming bits PLS, PHL, PHR
in CR6. These settings are not dependent from the selected operative Mode.
The common mode voltage of all the drivers is selected with bits VCL in CR18 in the range 1.2V to 1.65V
with 150mV steps. This feature is useful to set the common mode voltage to VCCP/2 and therefore to ex-
tend the output range and increase the output power.
If HPL and HPR are enabled together in Voice Mode or Tone Only Mode the same signal is sent to both
Drivers. The active Drivers can be muted (keeping them in power-up state) using bit MUT in CR6. At pow-
er-up or after a change in PLS or PHL or PHR bits the outputs are muted for 10 ms to avoid unwanted
noise. The transient suppression filter is used to avoid clicks when the gain value is changed.
2.9 Common Mode Driver
The common mode voltage driver (VCMHP pin) simplifies the application for a stereo headset connection
saving two decoupling capacitors in series with HPL and HPR. The loads of the single ended drivers are
connected on one side to HPL and HPR respectively, and on the other to VCMHP, that has the same com-
mon mode voltage. The driver is enabled with bit VCE in CR18.
The output voltage of VCMHP is selected with bits VCL in CR18 in the range 1.2V to 1.65V with 150mV
steps.
2.10 Tone Generator
The Tone Generator can be activated (writing CR12) in all the STw5094A operating modes except FM
mode. In Voice and Audio modes the tones are summed to the signal. It is possible to generate 1 or 2
summed waveforms (either sinusoidal or square wave), their frequencies can be set in CR13 for the first
one (f1) and in CR14 for the second one (f2) accordingly to the values listed in Table 1 if the active master
clock is MCLK or AUXCLK. If the active master clock is AMCK the frequency values specified in Table 1
must be multiplied by a factor k_fAMCK that depends on the AMCK frequency value. The amplitude of the
generated waveform can be regulated in CR12 over a 33dB range. When both f1 and f2 are selected the
amplitude of f1 and f2 are lowered by 5dB and 7dB respectively with respect to the amplitude of a single
waveform. In this way the amplitude of the summed waveforms does not overload and there is a 2dB dif-
ference between f1 and f2 amplitude as required for DTMF generation. The Tone Generator output can
be sent to the Voice Transmit section (in Voice Mode), to the Power amplifiers, possibly mixed with audio
or voice, (in all the modes except FM mode) and to the buzzer output BZ (in all the modes except FM
mode).
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