HC55185_06 Datasheet, PDF (9/20 Pages) Intersil Corporation – VoIP Ringing SLIC Family

HC55185
Design Equations
Loop Supervision Thresholds
SWITCH HOOK DETECT
The switch hook detect threshold is set by a single external
resistor, R_SH. Equation 3 is used to calculate the value of R_SH.
R_SH = 600 ⁄ I_SH
(EQ. 3)
The term I_SH is the desired DC loop current threshold. The
loop current threshold programming range is from 5mA to
15mA.
GROUND KEY DETECT
The ground key detector senses a DC current imbalance
between the Tip and Ring terminals when the ring terminal is
connected to ground. The ground key detect threshold is not
externally programmable and is internally fixed to 12mA
regardless of the switch hook threshold.
RING TRIP DETECT
The ring trip detect threshold is set by a single external
resistor, R_RT. I_RT should be set between the peak ringing
current and the peak off hook current while still ringing.
R_RT = 1800 ⁄ I_RT
(EQ. 4)
In addition, the ring trip current must be set below the
transient current limit, including tolerances. The capacitor
C_RT, in parallel with R_RT, will set the ring trip response time.
Loop Current Limit
The loop current limit of the device is programmed by the
external resistor R_IL. The value of R_IL can be calculated
using Equation 5:
R_IL
=
1_----7_---6_----0_-
I_LIM
(EQ. 5)
The term I_LIM is the desired loop current limit. The loop
current limit programming range is from 15mA to 45mA.
Impedance Matching
The impedance of the device is programmed with the
external component R_S. R_S is the gain setting resistor for
the feedback amplifier that provides impedance matching. If
complex impedance matching is required, then a complex
network can be substituted for R_S.
RESISTIVE IMPEDANCE SYNTHESIS
The source impedance of the device, Z_O , can be calculated
in Equation 6.
R_S = 133.3(Z_O)
(EQ. 6)
The required impedance is defined by the terminating
impedance and protection resistors as shown in Equation 7.
Z_O = Z_L – 2R_P
(EQ. 7)
4-WIRE TO 2-WIRE GAIN
The 4-wire to 2-wire gain is defined as the receive gain. It is
a function of the terminating impedance, synthesized
impedance and protection resistors. Equation 8 calculates
the receive gain, G₄₂.
G₄₂
=
–2
⎛
⎜
⎝
Z^----_O^-------+^-----2^--Z--R^--L---_P^-----+^------Z^----_L^-_{⎠⎟⎞}
(EQ. 8)
When the device source impedance and protection resistors
equals the terminating impedance, the receive gain equals
unity.
2-WIRE TO 4-WIRE GAIN
The 2-wire to 4-wire gain (G₂₄) is the gain from tip and ring to
the VTX output. The transmit gain is calculated in Equation 9.
G₂₄
=
⎛
–⎜
⎝
Z^----_O^-------+^-----2^--Z--R^--O---_P^-----+^------Z^----_L^-_{⎠⎟⎞}
(EQ. 9)
When the protection resistors are set to zero, the transmit
gain is -6dB.
TRANSHYBRID GAIN
The transhybrid gain is defined as the 4-wire to 4-wire gain
(G₄₄).
G₄₄
=
⎛
–⎜
⎝
Z^----_O^------+-----2^-Z---R^-O---_P^-----+-----Z^----_L^-_{⎠⎟⎞}
(EQ. 10)
When the protection resistors are set to zero, the transhybrid
gain is -6dB.
COMPLEX IMPEDANCE SYNTHESIS
Substituting the impedance programming resistor, R_S, with a
complex programming network provides complex
impedance synthesis.
2-WIRE
NETWORK
PROGRAMMING
NETWORK
C₂
C_Parallel
R₁
R_Series
R₂
R_Parallel
FIGURE 2. COMPLEX PROGRAMMING NETWORK
The reference designators in the programming network
match the evaluation board. The component R_S has a
different design equation than the R_S used for resistive
impedance synthesis. The design equations for each
component are provided below.
R_Series = 133.3 × (R₁ – 2(R_P))
(EQ. 11)
R_Parallel = 133.3 × R₂
C_Parallel = C₂ ⁄ 1^· 33.3
(EQ. 12)
(EQ. 13)
9
FN4831.14
December 18, 2006