stlc1512 STMicroelectronics, stlc1512 Datasheet - Page 23

• Manufacturer Part Number: stlc1512
• Description: Northenlite?? G.lite Bicmos Analog Front-end Circuit
• Manufacturer: STMicroelectronics

The headroom for switching can be determined from the numbers in Table 4. The switching headroom is 0.70
V at low currents (i.e. while on the low supply rail) and 0.85 V at high currents (i.e. while on the high supply rail).
The most difficult number to arrive at is the voltage that will appear at the pins PAOP1,2 and PAON1,2. This is
a combination of the input voltage, the line impedance and the losses in the transformers.
For a 100 load the maximum signal on the line will be 10.7 V. Since we are generating an active 100 output
impedance the voltage on the line for any other load is given by:
where Z
There are various losses in the transformers that can be modeled as resistors. To calculate the effect of these
losses we must know the current through the load which is given by:
The loss through the line transformer can be modeled as a 2.6
10 reference resistors. Therefore to determine the voltage at the output of the switched transformer we have:
At this point there is some additional current that flows through the hybrid balance network. This current flows through
a resistance that is equivalent to 1270 . Therefore the current flowing out of the switched transformer is:
The switched transformer has losses that can be modeled as a 3.6 resistor and has a 4:1 turns ratio. Therefore
the voltage at the primary side of the transformer is given by:
Where V
required to drive a full scale signal into the desired line impedance (Z
combined to calculate the required headroom to drive a certain impedance.
Where V
can be rearranged to calculate the switching threshold. The headroom can be determined from the drop across
the diode from the low supply and the low current drive capability of the amplifier given in Table (0.70V).
Where V
and V
Substituting V
be scaled to the nominal value of 10.7V (full scale) to determine a switching threshold based on the full scale
level of the signal.
The headroom calculation is worst at maximum line impedance. There is also a supply rail requirement for the
high (5.0V) supply which is based on being able to supply enough current to drive an 80
is not a trivial calculation and has been based on simulations. The possibility exists that the requirements on the
minimum supply voltage may be able to be reduced in the future.
diode
o
supplymin
PAOx
PAOx
is the line impedance and V
is the voltage drop across the diode when it has the appropriate amount of current flowing through it.
headroom
is the voltage at the output pins of the power amp. This is essentially the amount of headroom
is the required headroom to drive V
is the minimum value for the lower supply, V
V
in for V
PA Ox
Appendix D - Headroom Calculation for Switching
V
PAOx
hea droo m
=
V
V
V
------
PA Ox
s wtxo ut
2
in Equation D.7 you can determine the allowable output voltage V
n
I
V
s wtxo ut
Z
-------------------------------------------------------------------------------------------------- -
line
li ne
o
=
+
is the voltage on the line.
=
=
=
20
V
-------------------------------------------------------------- -
I
V
l oad
swtx out
2 10.7
V
=
s up ply min
+
line
I
2.6
loa d
=
n
+
+
volts out onto a line with the impedance Z
V
------------
Z
+
Z
+
l ine
3.6
20
---------------------- -
100
o
o
3.6 I
4
V
----------------------
+
–
swtx ou t
+
1270
Z
0.70 V
100
Z
--------------------------------- -
+
o
2.6
headroom
o
s wtxo ut
Z
+
o
1270
–
20
I
loa d
resistor. There is also a drop across the two
dio de
+
is the headroom available on the low supply
2.6
o
). Equation D.1 to Equation D.5 can be
+
1
line impedance. This
o
. This equation
STLC1512
n
. This can
(EQ D.1)
(EQ D.2)
(EQ D.3)
(EQ D.4)
(EQ D.5)
(EQ D.6)
(EQ D.7)
23/26