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APPLICATION NOTE
SLIC L3000N/L3092 MAXIMUM LOOP RESISTANCE ANALYSIS
By W. Ro ssi
1. INTRODUCTION This evaluation was carried out in order to evaluate the maximum loop resistance allowed using the SLIC KIT L3000N/L3092. The evaluation is performed in conversation mode ; it shows how the maximum loop resistance (Rl) is influenced by the battery voltage (Vb), the feeding resistance (Rfs) and the common mode current (Icm). Figure 1 : SLIC Characteristic and Load Curve.
2. MAXIMUM LOOP RESISTANCE EVALUATION In fig. 1 you can see the L3000N + L3092 DC characteristic and the load curve. The load curve is obtained as the series of the loop resistance (Rl) and the sub-scriber telephone set. The subscriber telephone set is representedas the series of a 100 resistor and a 5V zener diode. If the operating point is on region (1) its coordinates
are : Il1 = Ilim Vl1 = 5 + (100+Rl)xIlim
(1)
Note : The slope of region (2) is 2xRfs where the feeding resistor Rfs is fixed by an external resistor. If the operating point is on region (2) you can find its coordinates solving the system of two equations : 1) Vl = (Vb-Vdr)-2xRfsxIl
AN497/0694
2) Vl = 5+(100+Rl)xIl obtaining : Il2 = (Vb-Vdr-5)/(100+Rl+2*Rfs) Vl2 = 5+(100+Rl)x(Vb-Vdr-5)/(100+Rl+2xRfs) (2) If you consider the DC characteristic of the device you can see that the longer is the line the lower is the voltage drop between the battery voltage (Vb) and the line voltage (Vl). It can happensthat for very
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APPLICATION NOTE
long line the voltage drop is not large enough to guarantee the fully AC performance of the device. In such conditionthe device is still working, but large signalcan appear slightly distorted on the line. If you want guarantee the optimum behavior of the device you must be sure that the operating point of the device (Il, Vl) satisfy the following condition : Vl Vb-Vd with Vd=5+100xIl+60xIl+2+Vdcm where : 5 : internal drop 100xIl : drop on sensing resistors (2x50 max) 60xIl : drop on external resistors (2x30) 2 : maximum AC signal peak Vdcm : (=100xIcm) drop for common mode current (Icm) You can obtain themaximum value for Rl (maximum loop length) imposing : Vl = Vb-Vd If the operating point is on region (1) solving the equation Vl1=Vb-Vd where Vl1 is given by the relation (1) you obtain : Rlmax= (Vb-12-260xIlim-100xIcm)/Ilim (3) If the operating point is on region (2) solving the equation Vl2=Vb-Vd where Vl2 is given by the relation (2) you obtain : Rlmax = ((100+2xRfs)x(Vb-12-100xIcm)-260x(VbVdr-5))/(7-Vdr+100xIcm) In the following you can find graphical representationsof Rlmax versus Icm in four different situations : 3. CONCLUSION The aboverelations show the possibility towork with good performances also in presence of common mode current. With a battery voltage of -48V, Rfs = 200 and no common mode current, the maximum loop resistance is over 3K ; in the same condition but with a common mode current of 20mA the maximum loop resistance is about 2K. Higher loop resistance can be obtained increasing Rfs (see fig. 2). The parameters of each curve are the battery voltage (Vb) and the feeding resistance (Rfs).
Figure 2 : Maximum Line Resistance Versus Common Mode Current (conversation mode).
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APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics.
(c) 1995 SGS-THOMSON Microelectronics - All Rights Reserved
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