HC5515 Datasheet, PDF (7/17 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5515
Electrical Specifications
PARAMETER
V_BAT to 2 or 4-Wire Port
T_A = -40^oC to 85^oC, V_CC = +5V ±5%, V_EE = -5V ±5%, V_BAT = -48V, AGND = BGND = 0V, R_DC1 = R_DC2 = 41.2kΩ,
R_D = 39kΩ, R_SG = 0Ω, R_F1 = R_F2 = 0Ω, C_HP = 10nF, C_DC = 1.5µF, Z_L = 600Ω, Unless Otherwise Specified. All pin
number references in the figures refer to the 28 lead PLCC package. (Continued)
CONDITIONS
MIN
TYP
MAX
UNITS
(Note 29, Figure 11)
-
40
-
dB
-48V SUPPLY
+5V SUPPLY
-5V SUPPLY
R_L
600Ω
100mV_RMS, 50Hz TO 4kHz
TIP
V_TX
27
19
R_T
600kΩ
PSRR = 20 log (V_{T X}/V_IN)
V_TX
RING RSN
28
16
R_RX
300kΩ
FIGURE 11. POWER SUPPLY REJECTION RATIO
Circuit Operation and Design Information
The HC5515 is a current feed voltage sense Subscriber Line
Interface Circuit (SLIC). This means that for short loop
applications the SLIC provides a programed constant current to
the tip and ring terminals while sensing the tip to ring voltage.
The following discussion separates the SLIC’s operation into
its DC and AC paths, then follows up with additional circuit
and design information.
Constant Loop Current (DC) Path
SLIC in the Active Mode
The DC path establishes a constant loop current that flows
out of tip and into the ring terminal. The loop current is
programmed by resistors R_DC1, R_DC2 and the voltage on
the R_DC pin (Figure 12). The R_DC voltage is determined by
the voltage across R₁ in the saturation guard circuit. Under
constant current feed conditions, the voltage drop across R₁
sets the R_DC voltage to -2.5V. This occurs when current
flows through R₁ into the current source I₂. The R_DC voltage
establishes a current (I_RSN) that is equal to V_RDC/(R_DC1
+R_DC2). This current is then multiplied by 1000, in the loop
current circuit, to become the tip and ring loop currents.
For the purpose of the following discussion, the saturation
guard voltage is defined as the maximum tip to ring voltage
at which the SLIC can provide a constant current for a given
battery and overhead voltage.
For loop resistances that result in a tip to ring voltage less than
the saturation guard voltage the loop current is defined as:
I_L = -_R----_D----_C----²-₁---^.-+⁵----^V-_R----_D-----_C----₂- × 1000
(EQ. 1)
where: I_L = Constant loop current, and
R_DC1 and R_DC2 = Loop current programming resistors.
Capacitor C_DC between R_DC1 and R_DC2 removes the VF
signals from the battery feed control loop. The value of C_DC
is determined by Equation 2:
C_DC
=
T
×


R^-----_D^--1--_C^-----₁^-
+
-_R----_D--¹--_C-----₂-_
(EQ. 2)
where T = 30ms.
NOTE: The minimum C_DC value is obtained if R_DC1 = R_DC2.
Figure 13 illustrates the relationship between the tip to ring
voltage and the loop resistance. For a 0Ω loop resistance
both tip and ring are at V_BAT/2. As the loop resistance
increases, so does the voltage differential between tip and
ring. When this differential voltage becomes equal to the
saturation guard voltage, the operation of the SLIC’s loop
feed changes from a constant current feed to a resistive
feed. The loop current in the resistive feed region is no
longer constant but varies as a function of the loop
resistance.
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