mc33091ap Freescale Semiconductor, Inc, mc33091ap Datasheet - Page 13

• Manufacturer Part Number: mc33091ap
• Description: High-side Tmos Driver
• Manufacturer: Freescale Semiconductor, Inc

for the application using Equations 8 and 9. This involves
determining the peak shorted load power dissipation of the
TMOS device and gate duty cycle. The duty cycle is based
on V DS(min) , the value of V DS under shorted conditions (i.e.
V S(max) ).
of Step 10 should be less than the maximum power
dissipation under normal operating conditions calculated in
Step 4. If this is not the case, there are two options.
TMOS device heatsink, in other words, use a larger or better
heatsink. This though, is not always practical to do
particularly if restricted by size.
If for instance V DS(min) is set to 4.0 V when only 2.0 V are
needed, the short circuit duty cycle will be over twice as large,
resulting in double the TMOS device power dissipated.
Keeping V DS(min) to a minimum, reduces the shorted load
average power.
determined either by trial and error or by characterizing the
V DS waveform for the load and selecting a capacitor value
that generates a minimum fault time curve (see Equation 4)
that encompasses the V DS versus time waveform. The value
of C T has no effect on the duty cycle itself as was pointed out
earlier. See Figure 23 for a graphical selection of C T .
Inductive Loads
ground potential. Turning off an inductive load will cause
the source of the TMOS device to go below ground due to
flyback voltage to the point where the TMOS device may
become biased on again allowing the inductive energy to be
dissipated through the load. An internal 14 V zener diode
clamp from the gate to source pin limits how far the source pin
can be pulled below ground. For high inductive loads, it may
be necessary to have an external 10 k current limiting resistor
in series with the source pin to limit the clamp current in the
event the source pin is pulled more than 14 V below ground.
Transient Faults
voltage transients directly. By correctly sizing resistor R S and
capacitor C S , the MC33091A can withstand load dump and
other automotive type transients. The V CC voltage is clamped
at approximately 30 V through the use of an internal zener
diode.
energized in reverse due to the parasitic body diode inherent
in the TMOS device. Under this condition, the drain is
grounded and the MC33091A clamps the gate at 0.7 V below
the battery potential. This turns the TMOS device on in
reverse and minimizes the voltage across the TMOS device
resulting in minimal power dissipation. Neither the
MC33091A nor the TMOS device will be damaged under
such a condition. In addition, if the load can tolerate a reverse
MOTOROLA ANALOG IC DEVICE DATA
10. Calculate the shorted load average power dissipation
11. The calculated shorted load average power dissipation
Option one is to reduce the thermal resistance of the
Option two is to set V DS(min) to the lowest practical value.
12. Choose a value of C T . The value of C T can be
The TMOS device is turned off by pulling the gate to near
The MC33091A is not able to withstand automotive
Under reverse battery conditions, the load will be
MC33091A
polarity, the load will not be damaged. Caution; some
sensitive applications may not tolerate a reverse polarity load
condition with reverse battery polarity.
reverse battery condition if the load itself is already shorted to
ground. The MC33091A will not incur damage under this
specialized reverse battery condition but the TMOS
device may be damaged since there could be significant
energy available from the battery to be dissipated in the
TMOS device.
28 V and with the proper TMOS device used, the system can
withstand a double battery condition.
positive transient voltages in excess of the rated FET
breakdown voltage. The zener voltage, in this case, should
be less than the FET breakdown voltage. The diode, D, is
necessary where reverse battery protection of the gate of the
FET is required.
EMI Concern
device used will determine the turn–on and turn–off times
experienced. In a practical sense, smaller TMOS devices
have smaller gate capacitances and give rise to higher slew
rates. By way of example, the turn–on of an MPT50N06
TMOS device might be of the order of 80 s while that of an
MPT8N10 might be 10 s (see Figure 25). The speed of
turn–on or turn–off can be calculated by assuming the charge
pump to supply approximately 100 A over the time the gate
capacitance will transition a V GS voltage of 0 V to 10 V. In
reality, the V GS voltage will be greater than 10 V, but the
additional increase in TMOS drain current will be minimal for
V GS voltages greater than 10 V.
need not be of concern in all but the most critical of
applications. Where limiting of EMI is of concern, the charge
pump of the MC33091A may be slew rate limited by adding
an external feedback capacitor from the gate–to–source of
the TMOS device for slow down adjustment of both turn–on
and turn–off times (see Figure 33). Figures 31 through 35
depict various methods of modifying the turn–on or turn–off
times.
components to decrease turn–off time and clamp the flyback
voltage associated with switching inductive loads. V GS(th)
used in the critical component selection criteria refers to the
gate–to–source threshold voltage of the FET used in the
application.
switching transition time since doing so can greatly increase
the average power dissipation of the TMOS device. The
resulting increase in power dissipation should be taken into
account when selecting the R T C T time constant values
in order to protect the TMOS device from any overcurrent
condition.
There is no protection of the TMOS device during a
The MC33091A will withstand a maximum V CC voltage of
Figure 36 depicts a method of protecting the FET from
The gate capacitance and thus the size of the TMOS
The charge pump current is sized so that turn–on time
Figure 35 depicts a method of using only six external
Caution should be exercised when slowing down the
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