Silicon Laboratories SI2493/57/34/15/04 manual DAA Line-Side Device, Hookswitch and DC Termination

AN93

3. DAA (Line-Side) Device

The Si3018/10 DAA or line-side device, contains an ADC, a DAC, control circuitry, and an isolation capacitor interface. The Si3018/10 and surrounding circuitry provide all functionality for telephone line interface requirement compliance, including a full-wave rectifier bridge, hookswitch, dc termination, ac termination, ring detection, loop voltage and current monitoring, and call-progress monitoring. The Si3018/10 external circuitry is largely responsible for EMI, EMC, safety, and surge performance.

3.1. Hookswitch and DC Termination

The DAA has programmable settings for the dc impedance, current limiting, minimum operational loop current, and Tip-to-Ring voltage. The dc impedance of the DAA is normally represented by a 50 Ω slope as shown in Figure 13, but can be changed to an 800 Ω slope by setting the DCR bit. This higher dc termination presents a higher resistance to the line as the loop current increases.

	12	FCC DCT Mode
(V)	11
DAA
	10
Across	9
	8
Voltage
	7
	6
	.01	.02 .03 .04 .05 .06 .07 .08 .09	.1	.11

Loop Current (A)

Figure 13. FCC Mode I/V Characteristics

DCV[1:0] = 11, MINI[1:0] = 00

For applications requiring current limiting per the legacy TBR21 standard, the ILIM bit may be set to select this mode. In this mode, the dc I/V curve is changed to a 2000 Ω slope above 40 mA, as shown in Figure 14. This allows the DAA to operate with a 50 V, 230 Ω feed, which results in the highest current possible in the old TBR21 standard.

	45		TBR21 DCT Mode
(V)	40
DAA	35
	30
Across
	25
	20
Voltage
	15
	10
	5
	.015	.02	.025 .03 .035 .04 .045	.05	.055	.06

Loop Current (A)

Figure 14. TBR21 (Legacy) Mode I/V Characteristics

DCV[1:0] = 11, MINI[1:0] = 00

The MINI[1:0] bits select the minimum operational loop current for the DAA, and the DCV[1:0] bits adjust the DCT pin voltage, which affects the Tip-to-Ring voltage of the DAA. These bits allow important trade-offs to be made between signal headroom and minimum operational loop current. Increasing the Tip-Ring voltage increases signal headroom, whereas decreasing the voltage allows compliance to PTT standards in low-voltage countries, such as Japan or Malaysia. Increasing the minimum operational loop current above 10 mA also increases signal headroom and prevents degradation of the signal level in low-voltage countries.

38 Rev. 1.3