LM9061MXSLASHNOPB datasheet(13/33 Pages) TI1

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LM9061, LM9061-Q1

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SNOS738H – APRIL 1995 – REVISED JANAURY 2015

Feature Description (continued)

towards the VCC supply of the LM9061. The source current from pin 4 is typically 30 mA which quickly charges

CGD and CGS. As soon as the gate reaches the VGS(ON) threshold of the MOSFET, the switch turns ON and the

source voltage starts rising towards VCC. VGS remains equal to the threshold voltage until the source reaches

VCC. While VGS is constant only CGD is charging. When the source voltage reaches VCC, at time t2, the charge

pump 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 the Sense input pin of the LM9061

which is equal to the VCC supply in typical applications. The gate voltage rises above VCC in an exponential

fashion with a time constant dependent upon the sum of CGD and CGS. At this time however the load is fully

energized. At time t3, the charge pump reaches its maximum potential and the switch remains ON.

At time t4, the ON/OFF 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 µA current sink begins to discharge the gate input

capacitances to ground. The discharge rate (

ΔV/ΔT) is equal to 110 µA/ (CGD + CGS).

The load is still fully energized until time t5 when the gate voltage has reached a potential of the source voltage

(VCC) plus the VGS(ON) threshold voltage of the MOSFET. Between time t5 and t6, the VGS voltage remains

constant and the source voltage follows the gate voltage. With the voltage on CGD held constant the discharge

rate now becomes 110 µA/CGD.

At time t6 the source voltage reaches 0V. As the gate moves below the VGS(ON) threshold the MOSFET tries to

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 transient (flyback) across the load. The negative transient

will be clamped to

−VGS(ON) because the MOSFET must turn itself back ON to continue conducting the load

current until the energy in the inductance has been dissipated (at time t7).

7.3.4 Lossless Overcurrent Protection

A unique feature of the LM9061 is the ability to sense excessive 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 typically requires a small valued power resistor in series with the load, the LM9061 monitors the voltage

drop from drain to source, VDS, across the MOSFET. This “lossless” technique 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 particular power device for an application will minimize this concern. Another benefit of this

technique is that all applications use only standard inexpensive ¼W or less resistors.

To use this lossless protection technique requires knowledge 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.

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LM9061MX/NOPB Datasheet(PDF) 13 Page - Texas Instruments

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