HV110
Application Circuits
The above factors take into account the quiescent
chip current above the 12V zener voltage as well as
the Signature resistors in the mid-range of the
Classification probe voltage range. As the current
ranges are wide, it is possible to ensure that the
Classification current is within the ranges of the
Table 2 on Page 4 under all conditions. It is
however recommended that a voltage reference be
used instead of a zener diode as the voltage
reference will maintain the voltage across it over a
wide current range.
Classification Option
Figure 9 above shows the schematic for a typical
classification scheme used with HV110. The PSE
identifies the Class of the PD, by measuring the
current flowing into Rclass during the Classification
Period.
The reference voltage at the base of the bipolar
transistor (the zener and resistor) should be chosen
to draw much less than 1mA so it does not interfere
with Classification levels. The zener temperature
coefficient is negative (if chosen below 4V) and the
Vbe temperature coefficient is negative providing a
first order temperature compensated reference
across Rclass. The (Vz-Vbe)/Rclass allows programming of
the classification current.
The diode from base to PWRGD turns off
classification as soon as PWRGD is pulled low to
save power. The zener in series with the HV110 will
keep the Classification components from drawing
current until after Discovery. Make sure that the
diode in series with the base of the bipolar transistor
can block the maximum expected differential
network voltage.
Choose Rclass corresponding to the current level of
the PD load, using the formula:
Rclass = (Vz-Vbe) / (Iclass- IQ -17.5V / 25k*)
Class
Rclass
Open
191 Ω
95.3 Ω
63.4 Ω
0
1
2
3
where Vz is the voltage rating of the Zener diode, Vbe
is the voltage across the b-e junction of the bipolar
transistor, Iclass is the classification Signature current
corresponding to the power rating of the PD as
shown in Table 2 and IQ Quiescent current of the
HV110 (1mA). (* Signature resistance)
Table 3. Rclass for different classes of PD
Figure 9. Classification Circuit with HV110
8
A061104
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