lm9061mx National Semiconductor Corporation, lm9061mx Datasheet - Page 11

• Manufacturer Part Number: lm9061mx
• Description: Power Mosfet Driver With Lossless Protection
• Manufacturer: National Semiconductor Corporation

Application Information
−V
continue conducting the load current until the energy in the
inductance has been dissipated (at time t7).
MOSFET PROTECTION CIRCUITRY
A unique feature of the LM9061 is the ability to sense exces-
sive power dissipation in the MOSFET and latch it OFF to
prevent permanent failure. Instead of sensing the actual
current flowing through the MOSFET to the load, which
typically requires a small valued power resistor in series with
the load, the LM9061 monitors the voltage drop from drain to
source, V
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
temperature, needs to be determined. When switched ON
the power dissipation in the MOSFET will be:
The V
therefore:
GS(ON)
DS
DS(ON)
DS
voltage to limit the maximum power dissipation is
because the MOSFET must turn itself back ON to
V
, across the MOSFET. This “lossless” technique
DS (MAX)
, with both at the maximum operating ambient
=
√
P
D (MAX)
x R
DS(ON) (MAX)
FIGURE 2. Turn ON and Turn OFF Waveforms
(Continued)
11
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
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.
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
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-
DS(ON)
of the MOSFET (the minimum R
1
⁄
4
W or less resistors.
01231709
www.national.com
DS(ON)
DS(ON)