To Scope
+V
DD
+5 V
V
IN
0V
+ V
DD
V
OUT
51
Ω
90%
50%
10%
510
Ω
R
L
V
OUT
to
Scope
t
d(on)
t
d(off)
90%
50%
10%
R
GEN
V
IN
0V
t
r
t
f
Figure 12. Switching Test Circuit
Speed
Because the on-resistance and input capacitance
are low, the DMOS switches are capable of sub-
nanosecond switching speeds. At these speeds
the external circuit rather than the FET itself is
often responsible for the rise and fall times that
can be obtained. Let's consider the switching
test circuit of Figure 12. At turn-on, the fall
time observed at the drain is a function of R
G
and of the input pulse amplitude and rise time.
The sooner C
GS
reaches V
GS(th)
, the sooner turn-
on will occur, and the lower the r
DS(on)
reached,
the faster C
DS
will be discharged.
The turn-off time (or the rise time of V
D
) is not
as much limited by the velocity at which C
GS
can be discharged by the gate control pulse, as it
is by the time it takes to charge up C
DS
and C
DG
via the load resistor R
L
. Table 1 shows typical
performances obtained. It is important to realize
that stray capacitance and parasitic inductances,
as well as scope probe capacitance, can
seriously affect the rise and fall times (switching
speed).
V
DD
(V)
5
5
10
2
R
L
(Ω)
330
680
680
t
d(on)
(ns)
0.6
0.6
0.7
t
r
(ns)
0.8
0.7
0.8
2
t
OFF
(ns)
4
8
8
15
1k
0.9
1.0
12
t
OFF
is dependant on R
L
and does not depend on the
device characteristics.
Table 1. Typical Switching Times
Drivers
The switch driver's function is to translate logic
control levels (either TTL, CMOS, or ECL) into
the appropriate voltages needed at the gate so
that the switch can be turned on or off. The
SD5000 can be operated as an inverter capable
of driving up to 20 V. This high-voltage rating,
together with its high speed, make it an
excellent driver for the other members of the
family. Figure 13 shows several driver circuits.
Since switching times depend on the C
GS
charge/discharge times, it is important to note
that the driver's current source/sink capability
plays a very important role in the process.
7
Linear Integrated Systems, Inc.
●
4042 Clipper Ct.
●
Fremont, CA 94538
●
Tel: 510 490-9160
●
Fax: 510 353-0261
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
