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參數(shù)資料
| 型號： | SMH4812S |
| 廠商： | Summit Microelectronics, Inc. |
| 元件分類： | 基準(zhǔn)電壓源/電流源 |
| 英文描述： | Distributed Power Hot-Swap Controller |
| 中文描述： | 分布式電源熱插拔控制器 |
| 文件頁數(shù)： | 12/19頁 |
| 文件大?。?/td> | 441K |
| 代理商： | SMH4812S |
12
SMH4812
Preliminary
2055 4.0 12/22/00
SUMMIT MICROELECTRONICS, Inc.
The min/max current limits are easily met using the drop-
per 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 SMH4812 so that it becomes the primary regulator.
MOSFET V
DS
(ON) Threshold
The drain sense input on the SMH4812 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
, 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
I
T
DIODE
V
THRESHOLD
V
ON
R
=
×
,
where V
DIODE
is the forward voltage drop of the protection
diode. 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 68k
resistor for R
T
gives:
(
)
(
DS
THRESHOLD
V
ON
2.5V
15 A 68k
0.5V
1V
=
=
.
The voltage drop across the MOSFET switch and sense
resistor, V
DSS
, is calculated from:
)
DSS
V
S
ON
I R
R
=
+
,
where I
D
is the MOSFET drain current, R
S
is the circuit
breaker sense resistor and R
ON
is the MOSFET on resis-
tance.
The dropper resistor value should be chosen such that the
minimum and maximum I
DD
and V
DD
specifications of the
SMH4812 are maintained across the host supply
’
s valid
operating voltage range. First, subtract the minimum V
DD
of the SMH4812 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 will range 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 10mA.
The FS# input can also be used in conjunction with a
secondary-side supervisory circuit providing a positive
feedback loop during the power up sequence. As an
example, assume the SMH4812 is configured to turn on
–
48V to a DC/DC converter with a 1.6ms delay. Further
assume all of the enable inputs are true and PG# has just
been sequenced on. If FS# option 4 (100
BIN
in register 5)
has been selected, then FS# must be driven high within
1.6ms after PG# goes low, otherwise the PG output will be
disabled. Ideally, there would be a secondary-side super-
visor similar to the SMS44 that would have its reset time-
out period programmed to be less than 1.6ms. After the
supply turns on the RESET# output of the SMS44 would
be released and FS# pulled high. However, if for any
reason the supply doesn't turn on, the RESET# will not be
released and the SMH4812 will disable the PG output.
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