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

iC-MFP
8-FOLD FAIL-SAFE P-FET DRIVER
APPLICATION NOTES
Rev A2, Page 11/13
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.
t_t0..t1 [µs]
=
C_iss @(V_ds
=
hi) ×
V_th (FET )
−Isc(NOUTx)lo
(1)
3.3V
Microcontroller
VB
iC−MFP
IN1
IN2
IN3
IN4
IN5
IN6
IN7
IN8
EN5
EN10
ENFS
VBR
Supply, Ground and
VB
Temperature Monitor
NOUT1
NOUT2
NOUT3
NOUT4
NOUT5
NOUT6
NOUT7
NOUT8
NOK
GNDR
GND
VB
VD
RL
Figure 8: Driving an P-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 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.
VB
t_t1..t2 [µs] = C_rss@(V_ds = hi) × _{−Isc(NOUTx)lo} (2)
t_{t2 ..t3}
[µs]
=
C_iss
@(V_ds
=
lo)
×
Vr(NOUTx) − V_th(FET
−Isc(NOUTx)lo
)
(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(NOUTx)lo = short circuit current lo at NOUTx [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(NOUTx) = configured static turn on voltage at
NOUTx [V]
V_th(FET) = threshold of the transistor [V]
V(NOUTx)
VB
Vth(FET)
Vr()
t
VD
VB
GND
t0
t1
t
t2
t3
Figure 9: On switching of a transistor