MAX4529 Просмотр технического описания (PDF) - Maxim Integrated

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MAX4529

Low-Voltage, Bidirectional RF/Video Switch

Maxim Integrated 

Low-Voltage, Bidirectional

RF/Video Switch

The MAX4529 is a optimized for ±5V operation. Using

lower supply voltages or a single supply increases

switching time, on-resistance (and therefore on-state

attenuation), and nonlinearity.

Switch Off Condition

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

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

through the off-capacitances of the series MOSFETs,

but it is shunted to ground by N3. This forms a high-

pass filter whose exact characteristics depend on the

source and load impedances. In 50Ω systems, and

below 10MHz, the attenuation can exceed 80dB. This

value decreases with increasing frequency and

increasing circuit impedances. External capacitance

and board layout have a major role in determining over-

all performance.

Applications Information

Power-Supply Considerations

Overview

The MAX4529’s 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 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 the voltage on any pin exceeds V+ or V-,

one of these diodes will conduct. During normal opera-

tion these reverse-biased ESD diodes leak, forming the

only current drawn from 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 from the signal path to the V+ and

V- pins constitutes the analog signal-path leakage cur-

rent. All analog leakage current flows to the supply ter-

minals, not to the other switch terminal. This explains

how both sides of a given switch can show leakage

currents of either the same or opposite polarity.

When the switch is on, there is no connection between

the analog signal paths and GND. The analog signal

paths consist of an N-channel and P-channel MOSFET

with their sources and drains paralleled and their gates

driven out of phase with V+ and V- by the logic-level

translators.

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

translators, and set the input logic thresholds. The

logic-level translators convert the logic levels to

switched V+ and V- signals to drive the gates of the

analog switches. This drive signal is the only connec-

tion between the logic supplies and the analog sup-

plies. All pins have ESD protection to V+ and to V-.

Increasing V- has no effect on the logic-level thresh-

olds, but it does increase the drive to the P-channel

switches, reducing their on-resistance. V- also sets the

negative limit of the analog signal voltage.

The logic-level thresholds are CMOS and TTL compati-

ble when V+ is +5V. As V+ is raised, the threshold

increases slightly; when V+ reaches +12V, the level

threshold is about 3.1V, which is above the TTL output

high-level minimum of 2.8V, but still compatible with

CMOS outputs.

Bipolar-Supply Operation

The MAX4529 operates with bipolar supplies between

±2.7V and ±6V. The V+ and V- supplies need not be

symmetrical, but their sum cannot exceed the absolute

maximum rating of 13.0V. Do not connect the

MAX4529 V+ pin to +3V and connect the logic-level

input pins to TTL logic-level signals. TTL logic-level

outputs can exceed the absolute maximum ratings,

causing damage to the part and/or external circuits.

CAUTION:

The absolute maximum V+ to V- differential

voltage is 13.0V. Typical “±6-Volt” or “12-Volt”

supplies with ±10% tolerances can be as high

as 13.2V. This voltage can damage the

MAX4529. Even ±5% tolerance supplies may

have overshoot or noise spikes that exceed

13.0V.

Single-Supply Operation

The MAX4529 operates from a single supply between

+2.7V and +12V when V- is connected to GND. All of

the bipolar precautions must be observed. Note, how-

ever, that these parts are optimized for ±5V operation,

and most AC and DC characteristics are degraded sig-

nificantly when departing from ±5V. As the overall sup-

ply voltage (V+ to V-) is lowered, switching speed,

on-resistance, off isolation, and distortion are degraded

(see Typical Operating Characteristics).

Single-supply operation also limits signal levels and

interferes with grounded signals. When V- = 0V, AC sig-

nals are limited to -0.3V. Voltages below -0.3V can be

clipped by the internal ESD-protection diodes, and the

parts can be damaged if excessive current flows.

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