iC-MFP
8-FOLD FAIL-SAFE P-FET DRIVER
Rev A2, Page 11/13
APPLICATION NOTES
Driving an P-channel MOSFET
One typical field of application for iC-MFP is in the op-
eration of P-FETs with microprocessor output signals,
as shown in Figure 8.
Vth(FET)
(1)
tt0..t1[µs] = Ciss@(Vds = hi)×
−Isc(NOUTx)lo
iC−MFP
IN1
IN2
IN3
IN4
IN5
IN6
IN7
IN8
NOUT1
NOUT2
NOUT3
NOUT4
NOUT5
NOUT6
NOUT7
NOUT8
VB
(2)
VB
3.3V
tt1..t2[µs] = Crss@(Vds = hi)×
−Isc(NOUTx)lo
VD
Microcontroller
RL
Vr(NOUTx)−Vth(FET)
EN5
tt2..t3[µs] = Ciss@(Vds = lo)×
EN10
ENFS
NOK
−Isc(NOUTx)lo
(3)
VBR
VB
GNDR
GND
VB
Supply, Ground and
Temperature Monitor
ton = tt0..t1 +tt1..t2 +tt2..t3
(4)
Figure 8: Driving an P-channel MOSFET
Ciss = Cgs +Cgd = voltage dependent gate-source and
gate-drain capacitor [nF]
Crss = Cgd = voltage dependent gate-drain capacitor
[nF]
Isc(NOUTx)lo = short circuit current lo at NOUTx [mA]
tt0..t1 = dead time [µs]
tt1..t2 = slope time at drain (Miller-Plateau) [µs]
tt2..t3 = time to reach static gate voltage [µs]
ton = overall turn on time [µs]
VB = power supply VB [V]
Vr(NOUTx) = configured static turn on voltage at
NOUTx [V]
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 thresh-
old 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
is depending on the current of the driver and the gate-
drain capacitor 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 equations 1 to 4 are simplified and give an estima-
tion of the timing on the basis of data from the specifi-
cations of the device iC-MFP and the used transistor.
The turn off looks similar to the turn on but with reverse
run trough.
Vth(FET) = threshold of the transistor [V]
V(NOUTx)
VB
Vth(FET)
Vr()
t
VD
VB
GND
t
t0
t1
t2
t3
Figure 9: On switching of a transistor
ICHAUS [ IC-HAUS GMBH ]
