HC5523 Datasheet, PDF (8/18 Pages) Intersil Corporation – LSSGR/TR57 CO/Loop Carrier SLIC with Low Power Standby

HC5523
I_{TIP TIP}
RING
I_RING
+
-
I_TIP
I_RING
LOOP CURRENT
CIRCUIT
I_RSN
-
SATURATION GUARD
+
CIRCUIT
HC5523
A₁
I₁
-5V
A₂
I₂
-5V
FIGURE 13. DC LOOP CURRENT
-
+
R₁
-
+ ^17.3kΩ
V_TX
RSN
R_DC
-2.5V
R_RX
R_DC1
R_DC2
C_DC
R_SG
R_SG
-5V
V_BAT = -48V, I_L = 23mA, R_SG = 4.0kΩ
0
SATURATION
V_TIP
GUARD VOLTAGE
-10
CONSTANT CURRENT
FEED REGION
-20
RESISTIVE FEED
REGION
-30
-40
-50
0
SATURATION
GUARD VOLTAGE
1.2K
LOOP RESISTANCE (Ω)
FIGURE 14. V_TR vs R_L
V_RING
∞
Figure 15 shows the relationship between the saturation
guard voltage, the loop current and the loop resistance.
Notice from Figure 15 that for a loop resistance <1.2kΩ
(R_SG = 4.0kΩ) the SLIC is operating in the constant current
feed region and for resistances >1.2kΩ the SLIC is operating
in the resistive feed region. Operation in the resistive feed
region allows long loop and off-hook transmission by
keeping the tip and ring voltages off the rails. Operation in
this region is transparent to the customer.
50
V_BAT = -48V, R_SG = 4.0kΩ
40
30
CONSTANT CURRENT
FEED REGION
SATURATION GUARD
VOLTAGE, V_TR = 38V
20
V_BAT = -24V, R_SG = ∞
10 RESISTIVE FEED
REGION
0
0
10
20
30
LOOP CURRENT (mA)
SATURATION GUARD
VOLTAGE, V_TR = 13V
R_L 100kΩ
R_L 100kΩ
4kΩ
1.5kΩ
2kΩ
700Ω
<1.2kΩ R_SG = 4.0kΩ
<400Ω R_SG = ∞ Ω
FIGURE 15. V_TR vs I_L and R_L
The Saturation Guard circuit (Figure 13) monitors the tip to
ring voltage via the transconductance amplifier A₁. A₁
generates a current that is proportional to the tip to ring
voltage difference. I₁ is internally set to sink all of A₁’s
current until the tip to ring voltage exceeds 12.5V. When the
tip to ring voltage exceeds 12.5V (with no R_SG resistor) A₁
supplies more current than I₁ can sink. When this happens
A₂ amplifies its input current by a factor of 12 and the current
through R₁ becomes the difference between I₂ and the
output current from A₂. As the current from A₂ increases, the
voltage across R₁ decreases and the output voltage on R_DC
decreases. This results in a corresponding decrease in the
loop current. The R_SG pin provides the ability to increase the
saturation guard reference voltage beyond 12.5V. Equation 3
gives the relationship between the R_SG resistor value and
the programmable saturation guard reference voltage:
V_SGREF = 12.5 + -_R----_S----⁵_G-----^•-+---¹--₁-⁰--₇--⁵-₃----₀---₀--
where:
(EQ. 3)
V_SGREF = Saturation Guard reference voltage.
R_SG = Saturation Guard programming resistor.
When the Saturation guard reference voltage is exceeded,
the tip to ring voltage is calculated using Equation 4:
V_TR = R_L × -¹---⁶---^.R--⁶---_L⁶-----++----₍-⁵--_R---^•-_D---¹--_C-⁰---₁-⁵---+-^⁄---⁽-_R-^R----_D-^S---_C-^G---₂---+-₎---_⁄¹---₆-⁷--₀-³--₀-⁰----⁰----⁾
(EQ. 4)
where:
V_TR = Voltage differential between tip and ring.
R_L = Loop resistance.
For on-hook transmission R_L = ∞, Equation 4 reduces to:
V_TR
=
16.66 + ---------⁵-----^•----¹---⁰----⁵----------
R_SG + 17300
(EQ. 5)
63