MC33091AD View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MC33091AD

High-Side TMOS Driver

Motorola => Freescale 

MC33091A

APPLICATION

The following design approach will simplify application of

the MC33091A and will insure the components chosen to be

optimal for a specific application.

1. Characterize the load impedance and determine

the maximum load current possible for the load supply

voltage used.

2. Select a TMOS device capable of handling the

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.

3. Determine the maximum steady state VDS voltage the

TMOS device will experience under normal operating

conditions. Typically, this is the maximum load current

multiplied by the specified RDS(on) of the TMOS device.

Junction temperature considerations should be taken into

account for the RDS(on) value since it is significantly

temperature dependent. Normally, TMOS data sheets depict

the effect of junction temperature on RDS(on) and an RDS(on)

value at some considered maximum junction temperature

should be used. Various graphs relating to RDS(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 VDS voltage the TMOS device will

experience under normal operating conditions is thus:

VDS(norm) = IL(max)RDS(on)

(14)

4. Calculate the maximum power dissipation of the TMOS

device under normal operating conditions:

PD(max) = VDS(on)IL(max)

(15)

5. The calculated maximum power dissipation of the

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:

TRJA = (TJ(max) – TA(max))/PD(max)

(16)

Where TJ(max), the maximum allowable junction

temperature, is found on the TMOS data sheet and TA(max),

the maximum ambient temperature, is dictated by the

application itself.

6. The thermal resistance, TRJA, represents the maximum

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 TRJC, TRCS and

TRSA such that:

TRJA = TRJC + TRCS + TRSA

(17)

Where TRJC, TRCS and TRSA represent the junction–to–

case, case–to–heatsink and heatsink–to–ambient thermal

resistances respectively. TRCS and TRSA are the only

parameters the device user can influence.

The case–to–heatsink thermal resistance, TRCS, is

material dependent and can be expressed as:

TRCS = ρ t/A

(18)

Where “ρ” is the thermal resistivity of the heatsink material

(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 TRCS for standard heatsinks. For nonstandard heatsinks,

the user is required to calculate TRCS using some form of the

basic Equation 18.

The required heatsink–to–ambient thermal resistance,

TRSA, can easily be calculated once the terms of Equation 17

are known. Substituting TRJA of Equation 16 into Equation 17

and solving for TRSA produces:

TRSA = (TJ(max)–TA(max))/PD(max)–(TRJC+TRCS) (19)

Consulting the heatsink manufactures catalog will provide

TRCS information for various heatsinks under various

mounting conditions so as to allow easy calculation of TRSA

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.

7. Set the value of VDS(min) to something greater than the

normal operating drain to source voltage, VDS(norm), the

TMOS device will experience as calculated in Step 3 above

(Equation 14). From a practical standpoint, a value two or

three times VDS(norm) expected under normal operation will

prove to be a good starting point for VDS(min).

8. Select a value of RT less than 1.0 MΩ for minimal timing

error whose value is compatible with RX (RX will be selected

in Step 9 below). A recommended starting value to use for RT

would be 470 k. The consideration here is that the input

impedance of the threshold comparators are approximately

10 MΩ and if RT values greater than 1.0 MΩ are used,

significant timing errors may be experienced as a result of

input bias current variations of the threshold comparators.

9. Select a value of RX which is compatible with RT. The

value of RX should be between 50 k and 100 k. Recall in

Equation 5 that VDS(min) was determined by the combined

selection of RX and RT. Low values of RX will give large

values for K (K = 4.0 µA/V2 for RX = 50 k) causing ISQ to be

very sensitive to VDS variations (see Equation 1). This is

desirable if a minimum VDS trip point is needed in the 1.0 V

range since small VDS values will generate measurable

currents. However, at high VDS 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 ISQ exceeds several hundred microamps. See

Figure 22 for graphical aid in the selection of RT and RX.

12

MOTOROLA ANALOG IC DEVICE DATA

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