Power-Supply Considerations

Overview

The MAX349/MAX350 construction is typical of most

CMOS analog switches. It has three supply pins: V+, V-

and GND. V+ and V- are used to drive the internal

CMOS switches, and they set the limits of the analog

voltage on any switch. Reverse ESD-protection diodes

are internally connected between each analog signal

pin and both V+ and V-. If any analog signal exceeds

V+ or V-, one of these diodes will conduct. During normal

operation, these (and other) reverse-biased ESD diodes

leak, forming the only current drawn from V+ or V-.

Virtually all the analog leakage current is through the

ESD diodes. Although the ESD diodes on a given sig-

nal pin are identical, and therefore fairly well balanced,

they are reverse biased differently. Each is biased by

either V+ or V- and the analog signal. This means their

leakages vary as the signal varies. The

difference in the

two diode leakages to the V+ and V- pins constitutes

the analog signal-path leakage current. All analog leak-

age current flows to the supply terminals, not to the

other switch terminal. This is why both sides of a given

switch can show leakage currents of either the same or

opposite polarity.

There is no connection between the analog signal

paths and GND.

V+ and GND power the internal logic and logic-level

translators, and set both the input and output logic lim-

its. The logic-level translators convert the logic levels to

switched V+ and V- signals to drive the analog signal

gates. This drive signal is the only connection between

the logic supplies (and signals) and the analog sup-

plies. V+ and V- have ESD-protection diodes to GND.

The logic-level inputs and output have ESD protection

to V+ and to GND.

The logic-level thresholds are CMOS and TTL compati-

ble when V+ is +5V. As V+ rises, the threshold increases

slightly. Therefore, when V+ reaches +12V, the threshold

is about 3.1V; above the TTL-guaranteed high-level mini-

mum of 2.8V, but still compatible with CMOS outputs.

Serially Controlled, Low-Voltage,

8-Channel/Dual 4-Channel Multiplexers

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17

FOUR CLOCK

PULSES

DIN

SCLK

D4

SW4

D0

SW0

Figure 7. Differential Multiplexer Input Control

CS

SCLK

DIN

MAX349

MAX350

CS

SCLK

DIN

MAX349

MAX350

CS

SCLK

DIN

MAX349

MAX350

TO OTHER

SERIAL

DEVICES

DIN

SCLK

CS1

CS2

CS3

Figure 6. Addressable Serial Interface