SMH4803
Dropper Resistor Selection
APPLICATIONS CIRCUITS
The SMH4803 is powered from the high-voltage supply
via a dropper resistor, Rd. The dropper resistor must
provide the SMH4803 (and its loads) with sufficient oper-
Reversing Polarity of the Power Good Outputs
Theopen-drainPowerGoodoutputsontheSMH4803are
ating current under minimum supply voltage conditions, active LO. The output polarity may be changed to active
but must not allow the maximum supply current to be HI, when required, with a minor circuit change around the
exceeded under maximum supply voltage conditions.
high-voltage buffer transistor, see Figure 6. The 1N4148
blocking diode must be included to prevent high-voltage
damage to the SMH4803.
The dropper resistor value is calculated from:
Expanding Enable/Monitoring on the SMH4803
The 2.5V reference and 5V outputs on the SMH4803
make it easy to expand the number of enable or monitor-
ing inputs. The circuit in Figure 8 illustrates how a quad
(VS min – VDD max)
RD =
(IDD + Iload)
Where Vsmin is the lowest operating supply voltage, low-voltage comparator expands the EN/TS input to four
VddmaxistheupperlimitoftheSMH4803supplyvoltage, enable inputs. The comparators draw power from the 5V
Idd is minimum current required for the SMH4803 to output on the SMH4803 and use the 2.5V reference for
operate, and Iload is any additional load current from the the switching threshold. EN1 to EN4 inputs can accept
2.5V and 5V outputs and between Vdd and Vss.
either analog or CMOS logic level signals between Vss
and +5V. The comparator outputs are ANDed together
anddrivetheEN/TSinput.A1MΩresistoraddshysteresis
aroundthecomparatorstopreventoscillationnearthetrip
point.
The min/max current limits are easily met using the
dropper resistor except in circumstances where the input
voltage may swing over a very wide range, e.g. input
varies between 20V and 100V. In these circumstances it
may be necessary to add an 11V zener diode between
V
DD and VSS to handle the wide current range. The zener
voltage should be below the nominal regulation voltage of
the SMH4803 so that it becomes the primary regulator.
MOSFET VDS(ON) Threshold
The drain sense input on the SMH4803 monitors the
voltageatthedrainoftheexternalpowerMOSFETswitch
withrespecttoVSS.WhentheMOSFET’sVDS isbelowthe
user-definedvaluetheswitchisconsideredtobeON. The
VDS(ON) is adjusted using the resistor, RT, in series with
the drain sense protection diode. This protection or
blocking diode prevents high voltage breakdown of
the drain sense input when the MOSFET switch is
OFF. An inexpensive 1N4148 diode offers protection up
to 100V. The VDS(ON) threshold is calculated from:
VDS = VSENSE – (ISENSE × RT) – VDIODE – (IRS × RS)
WhereVDIODE istheforwardvoltagedropoftheprotection
diode, and IRS is the current flowing through the circuit
breaker sense resistor RS. The VDS(ON) threshold varies
over temperature due to the temperature dependence of
VDIODE and ISENSE
.
The calculation below gives the
VDS(ON) threshold under the worst case condition of
+85°C ambient. Using a 68kΩ resistor for RT gives:
VDS(ON) threshold =
2.5V – (15µA x 68kΩ) – VDIODE
2.5 – 1.0 – 0.5 = 1.0V
=
2041 8.4 6/15/00
SUMMIT MICROELECTRONICS
12
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
