MLD2N06CL

http://onsemi.com

3

THE SMARTDISCRETES CONCEPT

From a standard power MOSFET process, several active

and passive elements can be obtained that provide on−chip

protection to the basic power device. Such elements require

only a small increase in silicon area and/or the addition of one

masking layer to the process. The resulting device exhibits

significant improvements in ruggedness and reliability as

well as system cost reduction. The SMARTDISCRETES

device functions can now provide an economical alternative

to smart power ICs for power applications requiring low

on−resistance, high voltage and high current.

These devices are designed for applications that require a

rugged power switching device with short circuit protection

that can be directly interfaced to a microcontroller unit

(MCU). Ideal applications include automotive fuel injector

driver, incandescent lamp driver or other applications where

a high in−rush current or a shorted load condition could occur.

OPERATION IN THE CURRENT LIMIT MODE

The amount of time that an unprotected device can

withstand the current stress resulting from a shorted load

before its maximum junction temperature is exceeded is

dependent upon a number of factors that include the amount

of heatsinking that is provided, the size or rating of the

device, its initial junction temperature, and the supply

voltage. Without some form of current limiting, a shorted

load can raise a device’s junction temperature beyond the

maximum rated operating temperature in only a few

milliseconds.

Even with no heatsink, the MLD2N06CL can withstand

a shorted load powered by an automotive battery (10 to 14

V) for almost a second if its initial operating temperature is

under 100

°C. For longer periods of operation in the

current−limited mode, device heatsinking can extend

operation from several seconds to indefinitely depending on

the amount of heatsinking provided.

SHORT CIRCUIT PROTECTION AND THE EFFECT OF

TEMPERATURE

The on−chip circuitry of the MLD2N06CL offers an

integrated means of protecting the MOSFET component

from high in−rush current or a shorted load. As shown in the

schematic diagram, the current limiting feature is provided

by an NPN transistor and integral resistors R1 and R2. R2

senses the current through the MOSFET and forward biases

the NPN transistor’s base as the current increases. As the

NPN turns on, it begins to pull gate drive current through R1,

dropping the gate drive voltage across it, and thus lowering

the voltage across the gate−to−source of the power

MOSFET and limiting the current. The current limit is

temperature dependent as shown in Figure 3, and decreases

from about 2.3 amps at 25

°C to about 1.3 A at 150°C.

Since the MLD2N06CL continues to conduct current and

dissipate power during a shorted load condition, it is

important to provide sufficient heatsinking to limit the

device junction temperature to a maximum of 150

°C.

The metal current sense resistor R2 adds about 0.4 ohms

to the power MOSFET’s on−resistance, but the effect of

temperature on the combination is less than on a standard

MOSFET due to the lower temperature coefficient of R2.

The on−resistance variation with temperature for gate

voltages of 4 and 5 V is shown in Figure 5.

Back−to−back polysilicon diodes between gate and

source provide ESD protection to greater than 2 kV, HBM.

This on−chip protection feature eliminates the need for an

external Zener diode for systems with potentially heavy line

transients.