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MD0100

Supertex inc.

www.supertex.com

Doc.# DSFP-MD0100

C070113

Functional Description

The Supertex MD0100 can be considered as a normally

closed switch controlled by a switch control. (Refer to the

block diagram.) The switch control monitors the voltage

drop across terminals A and B. If the voltage difference is

greater than ±2.0V, the T/R switch will start to open. Once

in the open state, there is a small amount of current flowing

through the T/R switch, 200uA, to detect if the high voltage

is still present or not. The T/R switch will not close until the

voltage across terminal A and B drops within ±2.0V. A pair of

back-to-back diodes from the receive side of the switch to

ground are needed to complete the circuit and to allow the

initial peak current (about 60mA) to flow through the switch

so it can drop ±2.0V. If the diodes are not present, then there

is no current path and the voltage drop across terminal A

and B will be less than ±2.0V, so the switch will remain in the

closed position.

The other purpose of the diode is to clamp voltage spikes

to ±0.7V during transmit cycle. A low voltage diode, for ex-

ample 1.0V diode, is sufficient since it will only see ±0.7V. A

BAV99T dual diode from Diodes, Inc. was used in our test

circuit.

On Resistance

When the voltage across terminals A and B is within ±2.0V,

the switch is in the receive mode, and the R

15Ω. Once the voltage across terminals A and B is greater

than ±2.0V, the switch is in the transmit mode and blocks the

high voltage pulses from passing through to the receiver and

damaging it.

The MD0100 does not require any power supply. There are

only two active pins; one connects to the transmitter side

and the other one connects to the receiver side.

Switch Capacitance

The typical switch ON capacitance (C

measured from A to B or B to A when the switch is ON.

The switch OFF capacitance is a function of the voltage

across the T/R switch. The C

10V to 100V of the transmit voltage. Please see Figure 1 for

the C-V curve of the C

Figure 1: C

T

The T

provide a fast switch between the transmit mode and the

receive mode. The T

dent. The MD0100 turns ON and OFF faster when the rise

and fall times of the transmit pulse are faster. On the other

hand, the switch turns ON and OFF slower if the rise and fall

times of the transmit pulse are slower. The T

up are shown in Figure 2 and Figure 5 respectively.

Figure 2: Test set up for T

OFF

10

11

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14

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19

20

0

10

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60

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90

V

Tx

+10V

0V

10

Transmitter

MD0100

V

B

V

A

500ns