Si32172/3/5

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6.9. DTMF Detection (Si32175 Only)

In DTMF, two tones generate a DTMF digit. One tone is chosen from four possible row tones, and one tone is

chosen from four possible column tones. The sum of these tones constitutes one of 16 possible DTMF digits.

6.10. Pulse Metering (Si32175 Only)

The pulse metering system for the Si32175 is designed to inject a 12 or 16 kHz billing tone into the audio path with

maximum amplitude of 0.5 VRMS at TIP and RING into a 200  ac load impedance. The tone is generated in the

DSP via a table lookup that guarantees spectral purity by not allowing drift. The tone will ramp up until it reaches a

host-programmed threshold, at which point it will maintain that level until instructed to ramp down, thus creating a

trapezoidal envelope.

The amplitude is controlled by an automatic gain control circuit (AGC). While the tone is ramping up, the AGC

takes the feedback audio and applies it to a band pass filter, which is programmed for the 12 or 16 kHz frequency

of interest. When the peak is detected, the ramp is stopped.

See AN340 section 2.3.9 for additional details and considerations on Pulse Metering.

6.11. DC-DC Controller

The Si32172/3/5 devices integrate a dc-dc controller to control an external tracking dc-dc converter which

generates the high voltage supply (VBAT) to the SLIC. The tracking VBAT voltage generated from a single positive

dc input is optimized to minimize power consumption by closely tracking the SLIC state, even tracking the ringing

waveforms.

The dc-dc controller output DCDRV is driven by an internal charge pump which allows it to connect directly to the

gate of the MOSFET switch of the dc-dc converter. This eliminates the need for the MOSFET predrive circuit that is

typically required when other SLICs are used with a MOSFET with VTH greater than VDD. See Table 7.

6.12. Wideband Audio

Select Si32172/3 ICs support a software-selectable wideband (50 Hz–7 kHz) and narrowband (200 Hz–3.4 kHz)

audio codec. The wideband mode provides an expanded audio band at a 16-bit, 16 kHz sample rate for enhanced

audio quality while maintaining standard telephony audio compatibility. Wideband audio samples are transmitted

and received on the PCM interface using two consecutive 8 kHz frames.