_______________Theory of Operation

Logic-Level Translators

The MAX4545/MAX4546/MAX4547 are constructed as

high-frequency “T” switches, as shown in Figure 1. The

logic-level input, IN_, is translated by amplifier A1 into a

V+ to V- logic signal that drives amplifier A2. (Amplifier

A2 is an inverter for normally closed switches.)

Amplifier A2 drives the gates of N-channel MOSFETs

N1 and N2 from V+ to V-, turning them fully on or off.

The same signal drives inverter A3 (which drives the

P-channel MOSFETs P1 and P2) from V+ to V-, turning

them fully on or off, and drives the N-channel MOSFET

N3 off and on.

The logic-level threshold is determined by V+ and

GND_. The voltage on GND_ is usually at ground

potential, but it may be set to any voltage between

(V+ - 2V) and V-. When the voltage between V+ and

GND_ is less than 2V, the level translators become very

slow and unreliable. Since individual switches in each

package have individual GND_ pins, they may be set to

different voltages. Normally, however, they should all

be connected to the ground plane.

Switch On Condition

When the switch is on, MOSFETs N1, N2, P1, and P2

are on and MOSFET N3 is off. The signal path is COM_

to NO_, and because both N-channel and P-channel

MOSFETs act as pure resistances, it is symmetrical

(i.e., signals may pass in either direction). The off

MOSFET, N3, has no DC conduction, but has a small

amount of capacitance to GND_. The four on

MOSFETs also have capacitance to ground that,

together with the series resistance, forms a lowpass fil-

ter. All of these capacitances are distributed evenly

along the series resistance, so they act as a transmis-

sion line rather than a simple R-C filter. This helps to

explain the exceptional 300MHz bandwidth when the

switches are on.

Quad/Dual, Low-Voltage,

Bidirectional RF/Video Switches

8

_______________________________________________________________________________________

______________________________________________________________Pin Description

NAME

FUNCTION*

MAX4545

1, 10, 11,

20

IN_

Digital Control Input

PIN

3, 6, 8, 13,

15, 18

GND_

RF and Logic Ground. Grounds are not internally connected to each other,

and should all be connected to a ground plane (see

Grounding section).

16

V+

Positive Supply-Voltage Input (analog and digital)

2, 9, 12, 19

COM_

Analog Switch Common** Terminals

—

NC_

Analog Switch Normally Closed** Terminals

4, 7, 14, 17

NO_

Analog Switch Normally Open** Terminals

5

V-

Negative Supply-Voltage Input. Connect to ground plane for single-supply

operation.

MAX4546

1, 16

3, 7, 10, 14

12

2, 8, 9, 15

6, 11

4, 13

5

MAX4547

1, 9

4, 6, 12, 14

7, 15

5, 13

8, 10

2, 16

3, 11

*

All pins have ESD diodes to V- and V+.

** NO_ (or NC_) and COM_ pins are identical and interchangeable. Either may be considered as an input or output; signals pass

equally well in either direction.

A1

A2

A3

A1

(NC)

S

S

P1

N3

D

D

D

N1

V-

GND_

IN_

V+

V+

V+

COM_

NO_

S

D

N2

S

S

P2

D

NORMALLY OPEN SWITCH CONSTRUCTION

COM_ - NO_

IN_

0

1

OFF

ON

BSD DIODES

ON GND_, IN_,

COM_, NO_, AND NC_

Figure 1. T-Switch Construction