Silicon Laboratories SI2493/57/34/15/04 manual Analog Output, Modem Module VCC and Reset Filter

AN93

Motherboard		Murata BLM 18A
		G601 SN1
Connector			To Modem Chip VCC
VCC
			(Si24xx Pins 5, 21)
1.0 μF	0.1 μF	0.1 μF	1.0 μF
			10 kΩ
			To RESET
			(Si24xx Pin 12)
			2.2 μF
GND			GND
RESET

Figure 22. Modem Module VCC and RESET Filter

4.5. Analog Output

The call progress tone provided by AOUT and discussed in this section comes from a PWM output pin on the ISOmodem. AOUT is a 50% duty cycle, 32 kHz square wave, pulse-width modulated (PWM) by voice band audio, such as call progress tones.

The PWM signal should be processed by a high-pass filter (R2, R3, R4, C2,C3 and C4), and, with the aid of a bridge mode amplifier, provides low-cost 100 mW to 250 mW power with a 3 to 5 V supply. See the circuit in Figure 23. A slightly more expensive amplifier (LM4862) is available and, while still pin-compatible, provides twice as much power.

Figure 23. PWM Audio Processing and Amplifying Circuit

4.5.1. Interaction between the AOUT Circuit and the Required Modem Reset Time

When modifying the circuit shown in Figure 23, it is important to examine the reset timing and know that when external reset is applied to the modem, the AOUT pin still has time to rise to VCC due to the pullup installed on it. One has to assume that the modem has been operating prior to reset and has put AOUT into a PWM state that is 100% low.

This is important because the AOUT pin, which is shared with INT in some packages, is read by the strapping option logic in the modem at the end of the reset time to set the operational mode as shown in "2.1.3. Reset-Strap Options for 16-Pin SOIC Package" and "2.1.4. Reset-Strap Options for 24-Pin TSSOP Package" on page 13 and "2.1.5. Reset Strapping Options for QFN Parts" on page 15.

The value of the capacitors and resistors in the above circuit thus has an effect on the minimum required ISOmodem reset time.

54 Rev. 1.3