Detailed Description

The MAX4815/MAX4816/MAX4817 are high-bandwidth,

low-on-resistance, quad-SPST analog switches targeted

to serve as integrated T1/E1 analog protection switches

for 1+1 and N+1 line-card redundancy applications.

These devices are designed to replace electromechani-

cal relays to save board space, reduce power consump-

tion, and simplify PC board routing. The devices allow

the user to live insert the boards with no adverse effects.

The MAX4815/MAX4816/MAX4817 support ±3.3V or

±5V dual-supply operation, which is required for E1/T1

signal switching in the line-side of the interface trans-

former. Internal voltage multipliers supply the switches

yielding excellent linearity and low on-resistance, typi-

cally 3.7

Ω, within the E1/T1 analog signal range. This

high-bandwidth, typically 550MHz, family of devices is

optimized for low return loss and matched pulse tem-

plate performance in E1/T1 short-haul and long-haul

applications.

Analog Signal Levels

The on-resistance of the MAX4815/MAX4816/MAX4817 is

very low and stable as the analog signals are swept from

V- to V+ (see the Typical Operating Characteristics).

Fault Protection

The fault protection of the MAX4815/MAX4816/

MAX4817 allows the devices to handle input signals of

more than twice the supply voltage without clamping

the signal, latching up, or disturbing other cards in the

system. The device detects when the input voltage

drops below the negative supply. As soon as a fault

condition is detected, the switch is immediately turned

off for 128 clock cycles (typically 128µs). At the end of

the 128µs timeout, the switch is turned back on for one

clock cycle. At the end of the one clock cycle, if the sig-

nal is within the operating range, the switch will remain

on. Otherwise, the device will turn the switch off again

for 128 clock cycles. This will repeat until the signal is

within the operating range. In T1/E1 redundancy appli-

removed or disconnected for any reason, as shown in

Figure 1. Without a load resistor, the output voltage

when using a 1:2 transformer can be as high as ±11V.

Hot Insertion

The MAX4815/MAX4816/MAX4817 tolerate hot inser-

tions, thus are not damaged when inserted into a live

backplane. Competing devices can exhibit low imped-

ance when plugged into a live backplane that can

cause high power dissipation leading to damage of the

device itself. The MAX4815/MAX4816/MAX4817 have

relatively high input impedance when V+ and V- are

unconnected or connected to GND. Therefore, the

devices are not destroyed by a hot insertion. In order to

guarantee data integrity, the V+ and V- supplies must

be properly biased.

Applications Information

T1/E1 N+1 Redundancy

The MAX4815/MAX4816/MAX4817 are designed for

adjacent line-card protection applications. Figures 6 and

7 show a basic architecture for twisted-pair interface

(120

Ω E1, or 100Ω T1). Coaxial cable interface (75Ω E1)

can be illustrated with the same figures but without the

single-ended-to-differential conversion stage. A single

protection card can replace up to N line cards in a N+1

redundancy scheme. Figure 6 shows the MAX4815/

MAX4816/MAX4817 sitting in the line cards where they

can reroute any of the input/output signals to a protection

line card. Figure 7 shows the MAX4815/MAX4816/

MAX4817 sitting in a protection-switching card where the

switches are always powered. These figures do not show

the surge protection elements and resistors for line termi-

nation/impedance matching.

The low on-resistance and high bandwidth of the

MAX4815/MAX4816/MAX4817 yield good pulse tem-

plate and return-loss performance (see the Typical

Operating Characteristics). The pulse template tests for

E1 (twisted pair interface 120

Ω and coaxial interface

75

Ω) and T1 (twisted pair interface 100Ω) were tested

using the Dallas DS2155 single-chip transceiver evalu-

ation board, and twelve switches in parallel with one

switch closed and the other eleven open. The internal

transmit termination feature must be disabled when

using this circuit. To use the same transmit resistors for

E1 twisted pair and coaxial cables, the transmit line

build out control register (TLBC) is set to the value 6Ah.

This sets the driver voltage so the output pulse has the

right amplitude for both 120

Ω (twisted pair) and 75Ω

(coaxial) loads. The analog switches were powered

with dual power supplies at ±5V.

High-Bandwidth, T1/E1, SPST Analog Switches

_______________________________________________________________________________________

9

MAX4815

MAX4816

MAX4817

5V

±10%

LIU

Tx

TRING

TTIP

10V

±10%

1:2

+

-

Vo

NC_

COM_

Figure 1. Fault Protection