HC5515 Datasheet, PDF (8/17 Pages) Intersil Corporation – ITU CO/PABX SLIC with Low Power Standby

HC5515
V_TX
I_{TIP TIP}
RING
I_RING
+
-
I_TIP
I_RING
-
+
HC5515
LOOP CURRENT
CIRCUIT
SATURATION GUARD
CIRCUIT
A₁
I₁
-5V
A₂
I₂
-5V
I_RSN
-
+
R_1-
+ ^17.3kΩ
RSN
R_DC
-2.5V
R_RX
R_DC1
R_DC2
C_DC
R_SG
R_SG
-5V
FIGURE 12. DC LOOP CURRENT
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 13. V_TR vs R_L
V_RING
∞
Figure 14 shows the relationship between the saturation
guard voltage, the loop current and the loop resistance.
Notice from Figure 14 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 14. V_TR vs I_L and R_L
The Saturation Guard circuit (Figure 12) 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:
V_SGREF = Saturation Guard reference voltage, and
R_SG = Saturation Guard programming resistor.
(EQ. 3)
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, and
R_L = Loop resistance.
For on-hook transmission R_L = ∞, Equation 4 reduces to:
V_TR = 16.66 + _R-----_S----⁵_G-----^•-+---¹--₁-⁰--₇--⁵-₃----₀---₀--
(EQ. 5)
The value of R_SG should be calculated to allow maximum
loop length operation. This requires that the saturation guard
reference voltage be set as high as possible without clipping
the incoming or outgoing VF signal. A voltage margin of -4V
62