SMH4811A
Preliminary
where VSMIN is the lowest operating supply voltage, offersprotectionupto100V. Forhighvoltageapplications
VDDMAX istheupperlimitoftheSMH4811Asupplyvoltage, (up to 500V) the Central Semiconductor CMR1F-10M
IDD is minimum current required for the SMH4811A to diode should be used. The VDS(ON)THRESHOLD is calcu-
operate, and ILOAD is any additional load current from the lated from:
2.5V and 5V outputs and between VDD and VSS
.
V
ON
= V
− ISENSE ×RT − V
(
)
(
)
,
DS
SENSE DIODE
THRESHOLD
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
VDD and VSS to handle the wide current range. The zener
voltage should be below the nominal regulation voltage of
the SMH4811A so that it becomes the primary regulator.
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:
.
MOSFET VDS(ON) Threshold
V
ON
= 2.5V − 15µA ×68kΩ − 0.5V = 1V
.
THRESHOLD
(
)
(
)
DS
The drain sense input on the SMH4811A monitors the
voltageatthedrainoftheexternalpowerMOSFETswitch
withrespecttoVSS.WhentheMOSFET’sVDS isbelowthe
user-definedthresholdtheMOSFETswitchisconsidered
to be ON. The VDS(ON)THRESHOLD is adjusted using the
resistor, RT, in series with the drain sense protection
diode. This protection, or blocking, diode prevents high
voltage breakdown of the drain sense input when the
MOSFETswitchisOFF. AlowleakageMMBD1401diode
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
resistance.
Note: Figures 5 through 8 — the *10Ω resistor must be located as close as possible to the MOSFET
SUMMIT MICROELECTRONICS, Inc.
2044 6.1 2/8/01
11
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
