MC33091AD View Datasheet(PDF) - Motorola => Freescale

Part Name

Description

Manufacturer

MC33091AD

High-Side TMOS Driver

Motorola => Freescale 

MC33091A

10. Calculate the shorted load average power dissipation

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 VDS(min), the value of VDS under shorted conditions (i.e.

VS(max)).

11. The calculated shorted load average power dissipation

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.

Option one is to reduce the thermal resistance of the

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.

Option two is to set VDS(min) to the lowest practical value.

If for instance VDS(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 VDS(min) to a minimum, reduces the shorted load

average power.

12. Choose a value of CT. The value of CT can be

determined either by trial and error or by characterizing the

VDS waveform for the load and selecting a capacitor value

that generates a minimum fault time curve (see Equation 4)

that encompasses the VDS versus time waveform. The value

of CT has no effect on the duty cycle itself as was pointed out

earlier. See Figure 23 for a graphical selection of CT.

Inductive Loads

The TMOS device is turned off by pulling the gate to near

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

The MC33091A is not able to withstand automotive

voltage transients directly. By correctly sizing resistor RS and

capacitor CS, the MC33091A can withstand load dump and

other automotive type transients. The VCC voltage is clamped

at approximately 30 V through the use of an internal zener

diode.

Under reverse battery conditions, the load will be

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

polarity, the load will not be damaged. Caution; some

sensitive applications may not tolerate a reverse polarity load

condition with reverse battery polarity.

There is no protection of the TMOS device during a

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.

The MC33091A will withstand a maximum VCC voltage of

28 V and with the proper TMOS device used, the system can

withstand a double battery condition.

Figure 36 depicts a method of protecting the FET from

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

The gate capacitance and thus the size of the TMOS

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 µs while that of an

MPT8N10 might be 10 µs (see Figure 25). The speed of

turn–on or turn–off can be calculated by assuming the charge

pump to supply approximately 100 µA over the time the gate

capacitance will transition a VGS voltage of 0 V to 10 V. In

reality, the VGS voltage will be greater than 10 V, but the

additional increase in TMOS drain current will be minimal for

VGS voltages greater than 10 V.

The charge pump current is sized so that turn–on time

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.

Figure 35 depicts a method of using only six external

components to decrease turn–off time and clamp the flyback

voltage associated with switching inductive loads. VGS(th)

used in the critical component selection criteria refers to the

gate–to–source threshold voltage of the FET used in the

application.

Caution should be exercised when slowing down the

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 RTCT time constant values

in order to protect the TMOS device from any overcurrent

condition.

MOTOROLA ANALOG IC DEVICE DATA

13

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