V+
D
1µA
V
GS(th)
V
GS(th)
S
1µA
V
SB
3.0
V
GS(th)
(V)
2.5
2.0
1.5
1.0
0.5
0
4
8
12
V
SB
(V)
16
20
Figure 6. Threshold Voltage Test Configuration
Body Effect
For a MOSFET with a uniformly doped
substrate, the threshold voltage is proportional
to the square root of the applied source-to-body
voltage. The SD5000 family has a non-uniform
substrate, and the V
GS(th)
behaves somewhat
differently. Figure 7 shows the typical V
GS(th)
variation as a function of the source-to-body
voltage VSB.
As the body voltage increases in the negative
direction, the threshold goes up. Consequently,
if V
GS
is small, the on-resistance of the channel
can be very high. Figure 8 shows the effects of
V
SB
and V
GS
on r
DS(on)
. Therefore, to maintain a
low on-resistance it is preferable to bias the
body to a voltage close to the negative peaks of
V
S
and use a gate voltage as high as possible.
Figure 7. Threshold vs Source to Body Voltage
V
D
1mA
D
V
GS
S
V
SB
300
r
DS(on)
(Ω)
240
180
120
60
5V
10 V
0
4
8
12
V
SB
(V)
16
20
V
GS
= 4 V
Charge Injection
Charge injection describes that phenomenon by
which a voltage excursion at the gate produces
an injection of electric charges via the gate-to-
drain and the gate-to-source capacitances into
the analog signal path. Another popular name
for this phenomenon is “switching spikes."
0
Figure 8. On Resistance vs Source to Body and
Gate to Source Voltages
4
Linear Integrated Systems, Inc.
●
4042 Clipper Ct.
●
Fremont, CA 94538
●
Tel: 510 490-9160
●
Fax: 510 353-0261
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
