iC-MFN

8-FOLD FAIL-SAFE N-FET DRIVER

Rev A2, Page 11/13

APPLICATION NOTES

Driving an N-channel MOSFET

One typical field of application for iC-MFN is in the op-

eration of N-FETs with microprocessor output signals,

as shown in Figure 8.

VB

VBR

VB

VB

3.3V

Microcontroller

RL

VD

IN8

IN3

IN4

IN5

IN6

IN7

EN5

EN10

ENFS

IN1

IN2

iC−MFN

OUT2

OUT1

OUT3

OUT4

OUT5

OUT6

OUT7

OUT8

NOK

GNDR

GND

Supply, Ground and

Temperature Monitor

Figure 8: Driving an N-channel MOSFET

Slowly switching of a transistor is done with a current

limited driver. Figure 9 shows the different phases of

a turn on process with resitive load. In Section t0 to

t1 the gate of the transistors is loaded to the threshold

voltage Vth(FET) and is a dead time. In section t1 to t2

the gate voltage keeps nearly constant (miller-plateau)

during the drain voltage slope. The slew rate depends

on the current of the driver and the gate-drain capaci-

tor of the transistor. In section t2 to t3 the gate voltage

reach the static value. The transistor thus goes low

ohmic and minimizes the power dissipation. The equa-

tions 1 to 4 are simplified and give an estimation of the

timing on the basis of data from the specifications of

the device iC-MFN and the used transistor. The turn off

looks similar to the turn on but with reverse run trough.

t

t

VD

V(OUTx)

VB

Vth(FET)

Vr()

t0

t1

t2

t3

Figure 9: On switching of a transistor

−Isc(OUTx)hi

(1)

VB

−Isc(OUTx)hi

(2)

Vr

−Isc(OUTx)hi

(3)

(4)

gate-drain capacitor [nF]

[nF]

Isc

(OUTx)lo = short circuit current lo at OUTx [mA]

VB

= power supply VB [V]

Vr

(OUTx) = configured static turn on voltage at OUTx

[V]