TURN ON AND TURN OFF CHARACTERISTICS

The actual rate of change of the voltage applied to the gate

of the power device is directly dependent on the input ca-

pacitances of the MOSFET used These times are important

to know if the power to the load is to be applied repetitively

as is the case with pulse width modulation drive Of concern

intervals in a typical application An inductive load is as-

sumed to illustrate the output transient voltage to be expect-

ed At time t1 the ONOFF input goes high The output

which drives the gate of the MOSFET immediately pulls the

source current from pin 4 is typically 30 mA which quickly

takes over the drive of the gate to ensure that the MOSFET

remains ON

The charge pump is basically a small internal capacitor that

acquires and transfers charge to the output pin The clock

rate is set internally at typically 300 kHz In effect the charge

pump acts as a switched capacitor resistor (approximately

67k) connected to a voltage that is clamped at 13V above

supply in typical applications The gate voltage rises above

the load is fully energized At time t3 the charge pump

reaches its maximum potential and the switch remains ON

At time t4 the ONOFF input goes low to turn OFF the

MOSFET and remove power from the load At this time the

charge pump is disconnected and an internal 110 mA cur-

rent sink begins to discharge the gate input capacitances to

ground The discharge rate (DV DT) is equal to 110 mA

The load is still fully energized until time t5 when the gate

the source voltage follows the gate voltage With the volt-

At time t6 the source voltage reaches 0V As the gate

turn OFF With an inductive load if the current in the load

has not collapsed to zero by time t6 the action of the

MOSFET turning OFF will create a negative voltage tran-

sient (flyback) across the load The negative transient will

itself back ON to 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 ex-

cessive 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 typi-

cally requires a small valued power resistor in series with

the load the LM9061 monitors the voltage drop from drain

nique 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 particu-

lar power device for an application will minimize this con-

cern Another benefit of this technique is that all applica-

tions use only standard inexpensive

W or less resistors

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

TLH12317 – 9

FIGURE 2 Turn ON and Turn OFF Waveforms

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