參數(shù)資料
型號(hào)： SMH4804
廠商： Summit Microelectronics, Inc.
英文描述： -48V Programmable Hot Swap Sequencing Power Controller
中文描述： - 48V的可編程序熱插拔電源控制器
文件頁(yè)數(shù)： 21/41頁(yè)
文件大?。?/td> 574K
代理商： SMH4804
SMH4804
Timing Diagram
Summit Microelectronics
2050 3.7 10/30/02
21
not allow the maximum supply current to be exceeded
under maximum supply voltage conditions.
The dropper resistor value is calculated from:
where VS
MIN
is the lowest operating supply voltage,
V
DDMAX
is the upper limit of the SMH4804 supply voltage,
I
DD
is minimum current required for the SMH4804 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:
where V
DDMIN
is the lower limit of the SMH4804 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 SMH4803A
so that it becomes the primary regulator.
MOSFET V
DS
(ON) Threshold
The drain sense input on the SMH4804 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 calculated from:
where VDIODE 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 68 k
resistor for R
T
gives:
The voltage drop across the MOSFET switch and sense
resistor, V
DSS
, is calculated from:
where I
D
is the MOSFET drain current, RS is the circuit
breaker sense resistor and R
ON
is the MOSFET on
resistance.
The dropper resistor value should be chosen such that the
minimum and maximum I
DD
and V
DD
specifications of the
SMH4804 are maintained across the host supply’s valid
operating voltage range. First, subtract the minimum V
DD
of
the SMH4804 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.
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 SMH4804 is configured to turn on
–48V to three DC/DC converters and then sequentially turn
on the converters with a 1.6ms delay. Further assume all of
the enable inputs are true and PG4# 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 PG4# goes low, otherwise all of the PG[4:1]# outputs
are disabled.
Ideally, there would be a secondary-side supervisor similar
to the SMS44 that would have its reset time-out period
programmed to be less than 1.6ms. After the last supply
turns on the RESET# output of the SMS44 would be
released and FS# pulled high. However, if for any reason
R
D
= VS
MIN
- VDD
MAX
I
DD
- I
LOAD
P
R0
≥
(VS
MAX
- VDD
MIN
)
2
R
D
V
DS
(ON)
THRESHOLD
= V
SENSE
- (I
SENSE
x
R
T)
- V
DIODE
V
DS
(ON)
THRESHOLD
= 2.5V
- (15
μ
A x 68k
) - 0.5V = 1V
V
DSS
= I
D
(R
S
x
R
ON
)
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