SMH4814
Preliminary Information
APPLICATIONS INFORMATION (CONTINUED)
Dropper Resistor Selection
The SMH4814 is powered from the high-voltage
supply via a dropper resistor, RD. The dropper resistor
must provide the SMH4814 (and its loads) with
sufficient operating current under minimum supply
voltage conditions, but must not allow the maximum
supply current to be exceeded under maximum supply
voltage conditions.
In circumstances where the input voltage may swing
over a wide range (e.g., from 20V to 100V) the
maximum current may be exceeded. In these
circumstances it may be necessary to add an 11V
Zener diode between VDD and VSS to handle the wide
current range. The Zener voltage should be below the
nominal regulation voltage of the SMH4814 so that it
becomes the primary regulator.
The dropper resistor value is calculated from:
MOSFET VDS(ON) Threshold
VSMIN - VDDMAX
RD =
IDD - ILOAD
The drain sense input on the SMH4814 monitors the
voltage at the drain of the external power MOSFET
switch with respect to VSS. When the MOSFET’s VDS is
below the user-defined threshold the MOSFET switch
is considered to be ON. The VDS(ON)THRESHOLD is
adjusted using the resistor, RT.
where VSMIN is the lowest operating supply voltage,
VDDMAX is the upper limit of the SMH4814 supply
voltage, IDD is minimum current required for the
SMH4814 to operate, and ILOAD is any additional load
current from the 2.5V and 5V outputs and between
V
DD and VSS.
The VDS(ON)THRESHOLD is calculated from:
Calculate the minimum wattage required for RD from:
VDS (ON)THRESHO LD= VSENSE - (ISENSE x RT)
2
(VSMAX - VDDMIN
RD
)
PR0
≥
The VDS(ON)THRESHOLD varies over temperature due to
the temperature dependence of ISENSE. The calculation
below gives the VDS(ON)THRESHOLD under the worst
where VDDMIN is the lower limit of the SMH4814 supply
voltage, and VSMAX is the highest operating supply
voltage.
case condition of 85°C ambient. Using
a
100kΩ resistor for RT gives:
The dropper resistor value should be chosen such that
the minimum and maximum IDD and VDD specifications
of the SMH4814 are maintained across the host
supply’s valid operating voltage range. First, subtract
the minimum VDD of the SMH4814 from the low end of
the voltage, and divide by the minimum IDD value.
Using this value of resistance as RD find the operating
current that would result from running at the high end
of the supply voltage to verify that the resulting current
is less than the maximum IDD current allowed. If some
range of supply voltage is chosen that would cause
the maximum IDD specification to be violated, then an
external zener diode with a breakdown voltage of
~12V should be used across VDD.
VDS(ON)THRESHOLD = 2.5V- (15 Ax 100k ) = 1V
µ
Ω
The voltage drop across the MOSFET switch and
sense resistor, VDSS, is calculated from:
VDSS = ID (RS + RDSON
)
where ID is the MOSFET drain current, RS is the circuit
breaker sense resistor and RDSON is the MOSFET on
resistance.
As an example of choosing the proper RD value,
assume the host supply voltage ranges from 36 to
72V. The largest dropper resistor that can be used is:
(36V-11V)/3mA = 8.3kΩ. Next, confirm that this value
of RD also works at the high end: (72V-13V)/8.3kΩ =
7.08mA, which is less than 8mA.
Summit Microelectronics, Inc
2080 2.0 07/21/05
22
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
