IC-MFN Datasheet, PDF (11/13 Pages) IC-Haus GmbH – 8-FOLD FAIL-SAFE N-FET DRIVER

iC-MFN
8-FOLD FAIL-SAFE N-FET DRIVER
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.
Rev A2, Page 11/13
t_t0..t1 [µs]
=
C_iss @(V_ds
=
hi)
×
V_th (FET )
−Isc(OUTx)hi
(1)
3.3V
Microcontroller
VB
iC−MFN
IN1
IN2
IN3
IN4
IN5
IN6
IN7
IN8
EN5
EN10
ENFS
VBR
Supply, Ground and
VB
Temperature Monitor
OUT1
OUT2
OUT3
OUT4
OUT5
OUT6
OUT7
OUT8
NOK
GNDR
GND
VB
RL
VD
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.
VB
t_t1..t2 [µs] = C_rss@(V_ds = hi) × _{−Isc(OUTx)hi} (2)
t_t2..t3 [µs]
=
C_iss @(V_ds
=
lo)
×
Vr(OUTx) − V_th(FET)
−Isc(OUTx)hi
(3)
t_on = t_t0..t1 + t_t1..t2 + t_t2..t3
(4)
C_iss = C_gs + C_gd = voltage dependent gate-source and
gate-drain capacitor [nF]
C_rss = C_gd = voltage dependent gate-drain capacitor
[nF]
Isc(OUTx)lo = short circuit current lo at OUTx [mA]
t_t0..t1 = dead time [µs]
t_t1..t2 = slope time at drain (Miller-Plateau) [µs]
t_t2..t3 = time to reach static gate voltage [µs]
t_on = overall turn on time [µs]
VB = power supply VB [V]
Vr(OUTx) = configured static turn on voltage at OUTx
[V]
V_th(FET) = threshold of the transistor [V]
V(OUTx)
Vr()
Vth(FET)
t
VD
VB
t0
t1
t
t2
t3
Figure 9: On switching of a transistor