SMH4812
Preliminary
The min/max current limits are easily met using the drop- Thedropperresistorvalueshouldbechosensuchthatthe
per resistor, except in circumstances where the input minimum and maximum IDD and VDD specifications of the
voltage may swing over a very wide range (e.g., input SMH4812 are maintained across the host supply’s valid
varies between 20V and 100V). In these circumstances it operating voltage range. First, subtract the minimum VDD
may be necessary to add an 11V zener diode between oftheSMH4812fromthelowendofthevoltage,anddivide
VDD and VSS to handle the wide current range. The zener by the minimum IDD value. Using this value of resistance
voltage should be below the nominal regulation voltage of as RD find the operating current that would result from
the SMH4812 so that it becomes the primary regulator.
running at the high end of the supply voltage to verify that
the resulting current is less than the maximum IDD current
allowed. If some range of supply voltage is chosen that
wouldcausethemaximumIDD specificationtobeviolated,
then an external zener diode with a breakdown voltage of
MOSFET VDS(ON) Threshold
The drain sense input on the SMH4812 monitors the
voltageatthedrainoftheexternalpowerMOSFETswitch
withrespecttoVSS.WhentheMOSFET’sVDS isbelowthe
user-definedthresholdtheMOSFETswitchisconsidered
≈12V should be used across VDD
.
to be ON. The VDS(ON)THRESHOLD is adjusted using the As an example of choosing the proper RD value, assume
resistor, RT, in series with the drain sense protection the host supply voltage will range from 36 to 72V. The
diode. This protection, or blocking, diode prevents high largest dropper resistor that can be used is: (36V-11V)/
voltage breakdown of the drain sense input when the 3mA = 8.3kΩ. Next, confirm that this value of RD also
MOSFETswitchisOFF. AlowleakageMMBD1401diode works at the high end: (72V-13V)/8.3kΩ = 7.08mA, which
offersprotectionupto100V. Forhighvoltageapplications is less than 10mA.
(up to 500V) the Central Semiconductor CMR1F-10M
The FS# input can also be used in conjunction with a
diode should be used. The VDS(ON)THRESHOLD is calcu-
secondary-side supervisory circuit providing a positive
lated from:
feedback loop during the power up sequence. As an
example, assume the SMH4812 is configured to turn on –
V
ON
= V
− ISENSE ×RT − V
(
)
(
)
,
DS
SENSE DIODE
THRESHOLD
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 (100BIN in register 5)
has been selected, then FS# must be driven high within
1.6msafterPG#goeslow, otherwisethePGoutputwillbe
disabled. Ideally, therewouldbeasecondary-sidesuper-
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 thesupplydoesn'tturnon, theRESET#willnotbe
released and the SMH4812 will disable the PG output.
where VDIODE is the forward voltage drop of the protection
diode. The VDS(ON)THRESHOLD varies over temperature
duetothetemperaturedependenceofVDIODE andISENSE
ThecalculationbelowgivestheVDS(ON)THRESHOLD under
the worst case condition of 85°C ambient. Using a 68kΩ
resistor for RT gives:
.
V
ON
= 2.5V − 15µA ×68kΩ − 0.5V = 1V
.
THRESHOLD
(
)
(
)
DS
The voltage drop across the MOSFET switch and sense
resistor, VDSS, is calculated from:
V
= I R +R
D S ON
,
(
)
DSS
where ID is the MOSFET drain current, RS is the circuit
breaker sense resistor and RON is the MOSFET on resis-
tance.
SUMMIT MICROELECTRONICS, Inc.
2055 4.0 12/22/00
12
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
