Application Hints
(Continued)
the load, the LM9061 monitors the voltage drop from drain to
source, V
, across the MOSFET. This “l(fā)ossless” technique
allows all of the energy available from the supply to be
conducted to the load as required. The only power loss is
that of the MOSFET itself and proper selection of a particular
power device for an application will minimize this concern.
Another benefit of this technique is that all applications use
only standard inexpensive
1
4
W or less resistors.
To utilize this lossless protection technique requires knowl-
edge of key characteristics of the power MOSFET used. In
any application the emphasis for protection can be placed on
either the power MOSFET or on the amount of current
delivered to the load, with the assumption that the selected
MOSFET can safely handle the maximum load current.
To protect the MOSFET from exceeding its maximum junc-
tion temperature rating, the power dissipation needs to be
limited. The maximum power dissipation allowed (derated for
temperature) and the maximum drain to source ON resis-
tance, R
, with both at the maximum operating ambient
temperature, needs to be determined. When switched ON
the power dissipation in the MOSFET will be:
The V
voltage to limit the maximum power dissipation is
therefore:
V
DS (MAX)
=
√
P
D (MAX)
x R
DS(ON) (MAX)
With this restriction the actual load current and power dissi-
pation obtained will be a direct function of the actual R
of the MOSFET at any particular ambient temperature but
the junction temperature of the power device will never
exceed its rated maximum.
To limit the maximum load current requires an estimate of
the minimum R
DS(ON)
of the MOSFET (the minimum R
DS(ON)
of discrete MOSFETs is rarely specified) over the required
operating temperature range.
The maximum current to the load will be:
The maximum junction temperature of the MOSFET and/or
the maximum current to the load can be limited by monitor-
ing and setting a maximum operational value for the drain to
source voltage drop, V
DS
. In addition, in the event that the
load is inadvertently shorted to ground, the power device will
automatically be turned-OFF.
In all cases, should the MOSFET be switched OFF by the
built in protection comparator, the output sink current is
switched to only 10 μA to gradually turn OFF the power
device.
Figure 3 illustrates how the threshold voltage for the internal
protection comparator is established.
Two resistors connect the drain and source of the MOSFET
to the LM9061. The Sense input, pin 1, monitors the source
voltage while the Threshold input, pin 2, is connected to the
drain, which is also connected to the constant load power
supply. Both of these inputs are the two inputs to the protec-
tion comparator. Should the voltage at the sense input ever
drop below the voltage at the threshold input, the protection
comparator output goes high and initiates an automatic
latch-OFF function to protect the power device. Therefore
01231709
FIGURE 2. Turn ON and Turn OFF Waveforms
L
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