Datasheet


  
SLRS028A − SEPTEMBER 1988 − REVISED NOVEMBER 2004
10
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
driving power MOSFETs
The drive requirements of power MOSFETs are much lower than comparable bipolar power transistors. The
input impedance of an FET consists of a reverse-biased PN junction that can be described as a large
capacitance in parallel with a very high resistance. For this reason, the commonly used open-collector driver
with a pullup resistor is not satisfactory for high-speed applications. In Figure 14a, an IRF151 power MOSFET
switching an inductive load is driven by an open-collector transistor driver with a 470-Ω pullup resistor. The input
capacitance (C
ISS
) specification for an IRF151 is 4000 pF maximum. The resulting long turn-on time, due to the
product of input capacitance and the pullup resistor, is shown in Figure 14b.
0
1
2
3
4
IRF151
0 0.5 1 1.5 2 2.5
3
t − Time − µs
470 Ω
1/2 SN75447
M
48 V
TLC555
48
7
6
21
5
3
5 V
(a)
(b)
V − V − Gate Voltage − V
OH
OL
Figure 14. Power MOSFET Drive Using SN75447
A faster, more efficient drive circuit uses an active pullup, as well as an active pulldown output configuration,
referred to as a totem-pole output. The SN75374 driver provides the high-speed totem-pole drive desired in an
application of this type (see Figure 15a). The resulting faster switching speeds are shown in Figure 15b.
IRF151
0 0.5 1 1.5 2 2.5 3
0
1
2
3
4
t − Time − µs
1/4 SN75374
M
48 V
TLC555
48
7
6
21
5
5 V
(a)
(b)
V − V − Gate Voltage − V
OH
OL
3
Figure 15. Power MOSFET Drive Using SN75374