To protect the MOSFET from exceeding its maximum junc-

tion temperature rating the power dissipation needs to be

limited The maximum power dissipation allowed (derated

for temperature) and the maximum drain to source ON re-

bient temperature needs to be determined When switched

ON the power dissipation in the MOSFET will be

therefore

With this restriction the actual load current and power dissi-

pation obtained will be a direct function of the actual

ture but the junction temperature of the power device will

never exceed its rated maximum

To limit the maximum load current requires an estimate of

required operating temperature range

The maximum current to the load will be

The maximum junction temperature of the MOSFET andor

the maximum current to the load can be limited by monitor-

ing and setting a maximum operational value for the drain to

load is inadvertently shorted to ground the power device

will automatically be turned-OFF

In all cases should the MOSFET be switched OFF by the

built in protection comparator the output sink current is

switched to only 10 mA to gradually turn OFF the power

device

Figure 3 illustrates how the threshold voltage for the internal

protection comparator is established

Two resistors connect the drain and source of the MOSFET

to the LM9061 The Sense input pin 1 monitors the source

voltage while the Threshold input pin 2 is connected to the

drain which is also connected to the constant load power

supply Both of these inputs are the two inputs to the protec-

tion comparator Should the voltage at the sense input ever

drop below the voltage at the threshold input the protection

comparator output goes high and initiates an automatic

latch-OFF function to protect the power device Therefore

the switching threshold voltage of the comparator directly

while conducting load current

The threshold voltage is set by the voltage drop across re-

ence current over temperature a stable band gap reference

voltage is provided to bias a constant current sink The ref-

erence current is set by

The reference current sink output is internally connected to

tor trips

It is important to note that the programmed reference cur-

rent serves a multiple purpose as it is used internally for

biasing and also has a direct effect on the internal charge

pump switching frequency The design of the LM9061 is

optimized for a reference current of approximately 80 mA

guaranteed performance characteristics it is recommended

The protection comparator is configured such that during

normal operation when the output of the comparator is low

the differential input stage of the comparator is switched in

TLH12317 – 11

FIGURE 3 Protection Comparator Biasing

8