SMH4803A
Preliminary
tions, but must not allow the maximum supply current to be
exceeded under maximum supply voltage conditions.
The dropper resistor value is calculated from:
VS
MIN
−
V
DD
MAX
I
DD
+
I
LOAD
The voltage drop across the MOSFET switch and sense
resistor, V
DSS
, is calculated from:
V
DSS
=
I
D
(
R
S
+
R
ON
)
,
where I
D
is the MOSFET drain current, R
S
is the circuit
breaker sense resistor, and R
ON
is the MOSFET on
resistance.
R
D
=
,
where VS
MIN
is the lowest operating supply voltage,
V
DDMAX
is the upper limit of the SMH4803A supply voltage,
I
DD
is minimum current required for the SMH4803A to
operate, and I
LOAD
is any additional load current from the
2.5V and 5V outputs and between V
DD
and V
SS
.
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 SMH4803A so that it becomes the primary regulator.
MOSFET V
DS
(ON) Threshold
The drain sense input on the SMH4803A 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:
V
DS
(
ON
)
THRESHOLD
=
V
SENSE
−
(
I
SENSE
×
R
T
)
−
V
DIODE
,
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 68kΩ
resistor for R
T
gives:
V
DS
(
ON
)
THRESHOLD
=
2.5V
−
(
15
µ
A
×
68k
Ω
)
−
0.5V
=
1V
.
12
2051 4.4 3/15/01
SUMMIT MICROELECTRONICS, Inc.
SUMMIT [ SUMMIT MICROELECTRONICS, INC. ]
