參數(shù)資料
型號(hào)： SMH4814SR05
廠商： Summit Microelectronics, Inc.
元件分類： 基準(zhǔn)電壓源/電流源
英文描述： Dual Feed Active-ORing Programmable Hot Swap Controller
中文描述： 雙飼料有源ORing提供可編程熱插拔控制器
文件頁(yè)數(shù)： 22/44頁(yè)
文件大?。?/td> 935K
代理商： SMH4814SR05
SMH4814
Preliminary Information
Summit Microelectronics, Inc
2080 2.0 07/21/05
22
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:
R
D
= VS
MIN
- VDD
MAX
I
DD
- I
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
MAX
- VDD
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 = 8.3k
. Next, confirm that this value
of R
D
also works at the high end: (72V-13V)/8.3k
=
7.08mA, which is less than 8mA.
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)
THRESHOLD
= V
SENSE
- (I
SENSE
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
100k
resistor for R
T
gives:
ambient.
Using
a
V
DS
(ON)
THRESHOLD
= 2.5V
- (15
μ
A x 100k
) = 1V
The voltage drop across the MOSFET switch and
sense resistor, V
DSS
, is calculated from:
V
DSS
= I
D
(R
S
+
R
DSON
)
where I
D
is the MOSFET drain current, R
S
is the circuit
breaker sense resistor and R
DSON
is the MOSFET on
resistance.
APPLICATIONS INFORMATION (CONTINUED)
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