SMH4803
Dropper Resistor Selection
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-
ating current under minimum supply voltage conditions,
but must not allow the maximum supply current to be
exceeded under maximum supply voltage conditions.
The dropper resistor value is calculated from:
(VS min –
VDD
max)
(IDD +
I
load)
APPLICATIONS CIRCUITS
Reversing Polarity of the Power Good Outputs
The open-drain Power Good outputs on the SMH4803 are
active LO. The output polarity may be changed to active
HI, when required, with a minor circuit change around the
high-voltage buffer transistor, see Figure 6. The 1N4148
blocking diode must be included to prevent high-voltage
damage to the SMH4803.
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
low-voltage comparator expands the EN/TS input to four
enable inputs. The comparators draw power from the 5V
output on the SMH4803 and use the 2.5V reference for
the switching threshold. EN1 to EN4 inputs can accept
either analog or CMOS logic level signals between Vss
and +5V. The comparator outputs are ANDed together
and drive the EN/TS input. A 1MΩ resistor adds hysteresis
around the comparators to prevent oscillation near the trip
point.
RD
=
Where Vsmin is the lowest operating supply voltage,
Vddmax is the upper limit of the SMH4803 supply voltage,
Idd is minimum current required for the SMH4803 to
operate, and Iload is any additional load current from the
2.5V and 5V outputs and between Vdd and Vss.
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 V
SS
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 V
DS
(ON) Threshold
The drain sense input on the SMH4803 monitors the
voltage at the drain of the external power MOSFET switch
with respect to V
SS
. When the MOSFET’s V
DS
is below the
user-defined value the switch is considered to be ON. The
V
DS
(ON) is adjusted using the resistor, R
T
, 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 V
DS
(ON) threshold is calculated from:
V
DS
= V
SENSE
– (I
SENSE
×
R
T
) –
V
DIODE
– (I
RS
×
R
S
)
Where V
DIODE
is the forward voltage drop of the protection
diode, and I
RS
is the current flowing through the circuit
breaker sense resistor R
S
. The V
DS
(ON) threshold varies
over temperature due to the temperature dependence of
V
DIODE
and I
SENSE
. The calculation below gives the
V
DS
(ON) threshold under the worst case condition of
+85°C ambient. Using a 68kΩ resistor for R
T
gives:
V
DS
(ON) threshold =
2.5V – (15µA x 68kΩ) – V
DIODE
=
2.5 – 1.0 – 0.5 = 1.0V
2041 8.4 6/15/00
SUMMIT MICROELECTRONICS
12
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
