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參數(shù)資料
| 型號： | SMH4802 |
| 廠商： | Summit Microelectronics, Inc. |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Programmable -48V Hot-Swap Controller with Forced Shut Down |
| 中文描述： | 可編程的- 48V熱插拔控制器強制關(guān)機 |
| 文件頁數(shù)： | 12/20頁 |
| 文件大?。?/td> | 573K |
| 代理商： | SMH4802 |
12
SMH4802
2062 2.3 6/19/03
SUMMIT MICROELECTRONICS, Inc.
Preliminary Information
APPLICATIONS INFORMATION (
Continued
)
of 250μA for IDMAX is used to illustrate the following
calculations.
With V
OV
(2.5V) being the over-voltage trip point, R1 is
calculated by the formula:
OV
MAX
V
R1
ID
=
Substituting:
2.5V
250 A
R1
10k
=
=
Next the minimum current that flows through the resistive
divider, ID
MIN
, is calculated from the ratio of minimum and
maximum supply voltage levels
:
MAX
VS
MIN
MIN
MAX
ID
VS
ID
×
=
Substituting:
MIN
250 A 36V
2.5V
ID
125 A
=
=
Now the value of R3 is calculated from ID
MIN
:
MIN
ID
UV
MIN
VS
V
R3
×
=
V
UV
is the under-voltage trip point, also 2.5V. Substitut-
ing:
36V 2.5V
125 A
R3
286k
=
=
The closest standard 1% resistor value is 267k
Then R2 is calculated:
2.5V
125 A
R2
–10k
20k
–10k
10k
=
=
=
or
UV
MIN
V
R2
–R1
ID
=
Substituting:
2.5V
125 A
R2
–10k
20k
–10k
10k
=
=
=
An Excel spread sheet is available on Summit’s website
(
www.summitmicro.com
)
to simplify the resistor value
calculations and tolerance analysis for R1, R2, and R3.
Dropper Resistor Selection
The SMH4802 is powered from the high-voltage supply
via dropper resistor R
D
. The dropper resistor must
provide the SMH4802 (and its loads) with sufficient
operating current under minimum supply voltage condi-
tions, but must not allow the maximum supply current to
be exceeded under maximum supply voltage conditions.
The dropper resistor value is calculated from:
MAX
MIN
DD
D
DD
I
LOAD
VS
– V
–I
R
=
where VS
MIN
is the lowest operating supply voltage,
V
DDMAX
is the upper limit of the SMH4802 supply voltage,
I
DD
is minimum current required for the SMH4802 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:
(
)
MIN
2
MAX
DD
RO
D
VS
– V
R
P
≥
where V
DDMIN
is the lower limit of the SMH4802 supply
voltage, and VS
MAX
is the highest operating supply
voltage.
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
SMH4802 so that it becomes the primary regulator.
MOSFET V
DS
(ON) Threshold
The drain sense input on the SMH4802 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 consid-
ered to be ON. The V
DS
(ON)
THRESHOLD
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. A low leakage MMBD1401 diode
offers protection up to 100V. For high voltage applications
(up to 500V) the Central Semiconductor CMR1F-10M
diode should be used. The V
DS
(ON)
THRESHOLD
is calcu-
lated from:
(
)
(
)
DS
SENSE
V
SENSE
T
DIODE
THRESHOLD
V
ON
– I
–R
– V
=
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