mc33091ap Freescale Semiconductor, Inc, mc33091ap Datasheet - Page 12

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

the MC33091A and will insure the components chosen to be
optimal for a specific application.
the maximum load current possible for the load supply
voltage used.
maximum load current. Though the MC33091A will equally
drive our competitors products, it is hoped you will select one
of the many TMOS devices listed in Motorola’s Power
MOSFET Transistor Data Book .
TMOS device will experience under normal operating
conditions. Typically, this is the maximum load current
multiplied by the specified R DS(on) of the TMOS device.
Junction temperature considerations should be taken into
account for the R DS(on) value since it is significantly
temperature dependent. Normally, TMOS data sheets depict
the effect of junction temperature on R DS(on) and an R DS(on)
value at some considered maximum junction temperature
should be used. Various graphs relating to R DS(on) are
depicted in Motorola TMOS data sheets. Though Motorola
TMOS devices typically specify a maximum allowable
junction temperature of 150 C, in a practical sense, the user
should strive to keep junction temperature as low as possible
so as to enhance the applications long term reliability. The
maximum steady state V DS voltage the TMOS device will
experience under normal operating conditions is thus:
device under normal operating conditions:
TMOS device dictates the required thermal impedance for
the application. Knowing this, the selection of an appropriate
heatsink to maintain the junction temperature below the
maximum specified by the TMOS manufacture for operation
can be made. The required overall thermal impedance is:
temperature, is found on the TMOS data sheet and T A(max) ,
the maximum ambient temperature, is dictated by the
application itself.
overall or total thermal resistance, from junction to the
surrounding ambient, allowable to insure the TMOS
manufactures maximum junction temperature will not be
exceeded. In general, this overall thermal resistance can be
considered as being made up of several separate minor
thermal resistance interfaces comprised of TR JC , TR CS and
TR SA such that:
case, case–to–heatsink and heatsink–to–ambient thermal
resistances respectively. TR CS and TR SA are the only
parameters the device user can influence.
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The following design approach will simplify application of
1. Characterize the load impedance and determine
2. Select a TMOS device capable of handling the
3. Determine the maximum steady state V DS voltage the
4. Calculate the maximum power dissipation of the TMOS
5. The calculated maximum power dissipation of the
Where T J(max) , the maximum allowable junction
6. The thermal resistance, TR JA , represents the maximum
Where TR JC , TR CS and TR SA represent the junction–to–
TR JA = (T J(max) – T A(max) )/P D(max)
TR JA = TR JC + TR CS + TR SA
V DS(norm) = I L(max) R DS(on)
P D(max) = V DS(on) I L(max)
APPLICATION
MC33091A
(14)
(15)
(16)
(17)
material dependent and can be expressed as:
(expressed in C/Watt/Unit Thickness), “t” is the thickness of
heatsink material, and “A” is the contact area of the
case–to–heatsink. Heatsink manufactures specify the value
of TR CS for standard heatsinks. For nonstandard heatsinks,
the user is required to calculate TR CS using some form of the
basic Equation 18.
TR SA , can easily be calculated once the terms of Equation 17
are known. Substituting TR JA of Equation 16 into Equation 17
and solving for TR SA produces:
TR CS information for various heatsinks under various
mounting conditions so as to allow easy calculation of TR SA
in units of C/W (or when multiplied by the power dissipation
produces the heatsink mounting surface temperature rise).
Furthermore, heatsink manufactures typically specify for
various heatsinks, heatsink efficiency in the form of mounting
surface temperature rise above the ambient conditions for
various power dissipation levels. The user should insure that
the heatsink selected will provide a surface temperature rise
somewhat less than the maximum capability of the heatsink
so that the device junction temperature will not be exceeded.
The user should consult the heatsink manufacturers catalog
for this information.
normal operating drain to source voltage, V DS(norm) , the
TMOS device will experience as calculated in Step 3 above
(Equation 14). From a practical standpoint, a value two or
three times V DS(norm) expected under normal operation will
prove to be a good starting point for V DS(min) .
error whose value is compatible with R X (R X will be selected
in Step 9 below). A recommended starting value to use for R T
would be 470 k. The consideration here is that the input
impedance of the threshold comparators are approximately
10 M
significant timing errors may be experienced as a result of
input bias current variations of the threshold comparators.
value of R X should be between 50 k and 100 k. Recall in
Equation 5 that V DS(min) was determined by the combined
selection of R X and R T . Low values of R X will give large
values for K (K = 4.0 A/V 2 for R X = 50 k) causing I SQ to be
very sensitive to V DS variations (see Equation 1). This is
desirable if a minimum V DS trip point is needed in the 1.0 V
range since small V DS values will generate measurable
currents. However, at high V DS values, TMOS device
currents become excessively large and the current squaring
function begins to deviate slightly from the predicted value
due to high level injection effects occurring in the output PNP
of the current squaring circuit. These effects can be seen
when I SQ exceeds several hundred microamps. See
Figure 22 for graphical aid in the selection of R T and R X .
TR SA = (T J(max) –T A(max) )/P D(max) –(TR JC +TR CS ) (19)
The case–to–heatsink thermal resistance, TR CS , is
Where “ ” is the thermal resistivity of the heatsink material
The required heatsink–to–ambient thermal resistance,
Consulting the heatsink manufactures catalog will provide
7. Set the value of V DS(min) to something greater than the
8. Select a value of R T less than 1.0 M for minimal timing
9. Select a value of R X which is compatible with R T . The
and if R T values greater than 1.0 M
MOTOROLA ANALOG IC DEVICE DATA
TR CS =
t/A
are used,
(18)