Datasheet

LM9061

SNOS738F –APRIL 1995–REVISED APRIL 1995

www.ti.com

Figure 16. Turn ON and Turn OFF Waveforms

To protect the MOSFET from exceeding its maximum junction temperature rating, the power dissipation needs to

be limited. The maximum power dissipation allowed (derated for temperature) and the maximum drain to source

ON resistance, R

DS(ON)

, with both at the maximum operating ambient temperature, needs to be determined.

When switched ON the power dissipation in the MOSFET will be:

(1)

The V

voltage to limit the maximum power dissipation is therefore:

DS (MAX)

= √P

D (MAX)

× R

DS(ON) (MAX)

(2)

With this restriction the actual load current and power dissipation obtained will be a direct function of the actual

DS(ON)

of the MOSFET at any particular ambient temperature but the junction temperature of the power device

will never exceed its rated maximum.

To limit the maximum load current requires an estimate of the minimum R

DS(ON)

of the MOSFET (the minimum

DS(ON)

of discrete MOSFETs is rarely specified) over the required operating temperature range.

The maximum current to the load will be:

(3)

The maximum junction temperature of the MOSFET and/or the maximum current to the load can be limited by

monitoring and setting a maximum operational value for the drain to source voltage drop, V

. In addition, in the

event that the 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 µA to gradually turn OFF the power device.

Figure 17 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 protection 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 controls the maximum V

allowed across the MOSFET while

conducting load current.

Product Folder Links: LM9061