SDT213 データシートの表示（PDF） - Linear Technology

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SDT213

N-CHANNEL LATERAL DMOS SWITCH

Linear Technology 

recommended for the output. Since CGD < CGS

this causes less charge injection noise on the

load.

The circuit shown in Figure 4 exhibits the rDS(on)

vs. analog signal voltage relationship shown in

Figure 5.

As can be seen from Figures 3a and 3b, the

body-source and body-drain pn junctions should

be kept reverse biased at all times-otherwise,

signal clipping and even device damage may

occur if unlimited currents are allowed to flow.

Body biasing is conveniently set, in most cases,

by connecting the substrate to V-.

20 V = On

-10 V = Off

RGEN

Switch Input

VS = ±10 V

Control

Input

G

S

D

B

-10 V

Switch

Output

VO

RL

CL

Figure 4. Normal Switch Configuration for a ±10

V Analog Switch

Main Switch Characteristics

rDS(on)

Channel on-resistance is controlled by the

electric field present across and along the

channel. Channel resistance is mainly

determined by the gate-to-source voltage

difference. When VGS exceeds the threshold

voltage (VGS(th)), the FET starts to turn on.

Numerous applications call for switching a point

to ground. In these cases the source and

substrate are connected to ground and a gate

voltage of 3 to 4 V is sufficient to ensure

switching action.

With a VGS in excess of +5 V, a low resistance

path exists between the source and the drain.

When the analog signal excursion is large (for

example ±10 V) the channel on-resistance

changes as a function of signal level. To

achieve minimum distortion, this channel on-

resistance modulation should be kept in mind,

and the amount of resistance in series with the

switch should be properly sized. For instance, if

the switch resistance varies between 20 Ω and

30 Ω over the signal range and the switch is in

series with a 200 Ω load, the result will be a

total ∆R = 4.5 %. Whereas, if the load is 100

kΩ, ∆R will only be 0.01 %.

200

160

(a)

120

(c)

80

(b)

40

0

-10

(a)

(b)

(c)

-5

0

5

10 15

VS (V)

VBODY = -10 V, VGATE = 20 V

VBODY = -10 V, VGATE = 15 V

VBODY = 0 V, VGATE = 20 V

Figure 5. On Resistance Characteristics

Threshold Voltage

The threshold voltage (VGS(th)) is a parameter

used to describe how much voltage is needed to

initiate channel conduction. Figure 6 shows the

applicable test configuration. In this circuit, it is

worth noting, for instance, that if the device has

a VGS(th) = 0.5 V, when V+ = 0.5 V, the channel

resistance will be:

RCHANNEL

=

0.5V

1µ A

=

500kΩ

3

Linear Integrated Systems, Inc. ● 4042 Clipper Ct. ● Fremont, CA 94538 ● Tel: 510 490-9160 ● Fax: 510 353-0261

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