SMH4814 Datasheet, PDF (22/44 Pages) Summit Microelectronics, Inc. – Dual Feed Active-ORing Programmable Hot Swap Controller

APPLICATIONS INFORMATION (CONTINUED)
Dropper Resistor Selection
The SMH4814 is powered from the high-voltage
supply via a dropper resistor, R_D. 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.
The dropper resistor value is calculated from:
VS - VDD
R_D =
MIN
MAX
I -I
DD LOAD
where VS_MIN is the lowest operating supply voltage,
V_DDMAX is the upper limit of the SMH4814 supply
voltage, I_DD is minimum current required for the
SMH4814 to operate, and I_LOAD is any additional load
current from the 2.5V and 5V outputs and between
V_DD and V_SS.
Calculate the minimum wattage required for RD from:
P
R0
≥
(VS_{MA X} - V DD_MIN )²
R_D
where V_DDMIN is the lower limit of the SMH4814 supply
voltage, and VS_MAX is the highest operating supply
voltage.
The dropper resistor value should be chosen such that
the minimum and maximum I_DD and V_DD specifications
of the SMH4814 are maintained across the host
supply’s valid operating voltage range. First, subtract
the minimum V_DD of the SMH4814 from the low end of
the voltage, and divide by the minimum I_DD value.
Using this value of resistance as R_D 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 I_DD current allowed. If some
range of supply voltage is chosen that would cause
the maximum I_DD specification to be violated, then an
external zener diode with a breakdown voltage of
~12V should be used across V_DD.
As an example of choosing the proper R_D 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 R_D also works at the high end: (72V-13V)/8.3kΩ =
7.08mA, which is less than 8mA.
SMH4814
Preliminary Information
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 V_DD and V_SS 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.
MOSFET V_DS(ON) Threshold
The drain sense input on the SMH4814 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 threshold the MOSFET switch
is considered to be ON. The V_DS(ON)_THRESHOLD is
adjusted using the resistor, R_T.
The V_DS(ON)_THRESHOLD is calculated from:
V_DS (ON)_{T HRESHO LD} = V_SENSE - (I_{SE NSE} x R_T)
The V_DS(ON)_THRESHOLD varies over temperature due to
the temperature dependence of I_SENSE. The calculation
below gives the V_DS(ON)_THRESHOLD under the worst
case condition of 85°C ambient. Using a
100kΩ resistor for R_T gives:
V_DS(ON)_THRESHOLD = 2.5V - (15_µA x 100k_Ω) = 1V
The voltage drop across the MOSFET switch and
sense resistor, V_DSS, is calculated from:
V
DSS
=
I
D
(R
S
+
R
D
SON⁾
where I_D is the MOSFET drain current, R_S is the circuit
breaker sense resistor and R_DSON is the MOSFET on
resistance.
Summit Microelectronics, Inc
2080 2.0 07/21/05
22