HV110
The reference voltage at the base of the bipolar transistor
(the Zener and resistor) should be chosen to draw much less
than 1.0mA so it does not interfere with classification levels.
The Zener temperature coefficient is negative (if chosen be-
low 4.0V), 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.
Current
Choose RCLASS corresponding to the current level of the pow-
ered device load, using the formula:
DRAIN
RCLASS = (VZ-VBE) / (ICLASS - IQ -17.5V / 25k*)
where VZ is the voltage rating of the Zener diode, VBE is the
voltage across the B-E junction of the bipolar transistor,
Figure 4. Undercurrent shutdown when current falls ICLASS is the classification signature current corresponding to
from 12mA to 9mA
the power rating of the powered device as shown in Table 2
and IQ quiescent current of the HV110 (1mA). (* signature
resistance)
Input Current
The above factors take into account the quiescent chip cur-
rent above the 12V Zener voltage as well as the signature
resistors in the mid-range of the classification probe volt-
age 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 5 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 volt-
age across it over a wide current range.
DRAIN
Input Voltage
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 compo-
nents from drawing current until after Discovery. Make sure
that the diode in series with the base of the bipolar transistor
PWRGD
Figure 5. HV110 turning on into a 300uF CPORT, still main- can block the maximum expected differential network volt-
taining the maximum input current of 400mA (without age.
any additional components required)
Table 3. RCLASS for different classes of PD
Application Circuits
CLASS
RCLASS
Open
191Ω
95.3Ω
63.4Ω
0
1
2
3
Classification Option
Page 6 shows the schematic for a typical classification
scheme used with the HV110. The PSE identifies the class
of the powered device, by measuring the current flowing into
RCLASS during the classification period.
8
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