MT91600
From (Equation 1)
R11 = k * (R1 + R2)
R11 = 120 * (220Ω + 220Ω)
∴
R11 = 52k8Ω
Power Sharing Resistor (R23)
To determine the value of R23, use the following equations:
R23(max)= |Vbat(min)| - 100 - (2*R2 + Lr + DCRP)
30mA
R23(min)= |Vbat(max)| - Pd(max) - R2
40mA
1.6mA
where,
Vbat(min/max) = the expected variation of Vbat.
R2 = the feed resistor.
Lr = maximum DC loop resistance.
DCRP = DC resistance of the phone set.
Pd(max) = the maximum power dissipation of the ring drive transistor Q3.
If R23(max) > R23(min), then set R23 to be the geometric center:
R23 = Square Root (R23(max) * R23(min))
If R23(max) < R23(min), then a violation has occurred. Pd(max) will have to be increased.
If R23 = negative value, power sharing is not required, i.e., R23=0
A numerical example:
Given:
R2 = 220Ω
Lr = 325Ω (2.5km of 28 gauge wire, averaged at 65Ω/km)
DCRP = 200Ω
Pd(max) = 1.5W
Vbat = -48V +/- 10% (i.e. -43V to -53V)
Therefore:
R23(max) = (43/30mA) - 100 - (2 * 220 + 325 + 200)
= 1433.3 - 100 - 965
R23(max) = 368.3Ω
R23(min) = (53/40mA) - (1.5/1.6mA) - 220
= 1325 - 937.5 - 220
R23(min) = 167.5
Ω
R23 = Square Root ( 368.3 * 167.5 )
R23 = 248.4Ω
Data Sheet
9
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