Transceivers with ±15kV ESD Protection

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Detailed Description

The MAX3535E/MXL1535E isolated RS-485/RS-422 full-

tion between the RS-485/RS-422 isolation side and the

processor or logic side. These devices allow fast,

1000kbps communication across an isolation barrier even

when the common-mode voltages (i.e., the ground poten-

tials) on either side of the barrier are subject to large dif-

ferences. The isolation barrier consists of two parts. The

first part is a capacitive isolation barrier (integrated high-

voltage capacitors) that allows data transmission

between the logic side and the RS-485/RS-422 isolation

side. Data is sampled and encoded before it is transmit-

ted across the isolation barrier introducing sampling jitter

and further delay into the communication system.

The second part of the isolation barrier consists of an

external transformer with the required primary-to-sec-

ondary isolation, allowing the transmission of operating

power from the logic side across the isolation barrier to

the isolation side. Connect the primary of the external

transformer to the MAX3535E/MXL1535E’s 420kHz

transformer driver outputs ST1 and ST2. Since the

MXL1535E and the MAX3535E operate with different

supply-voltage requirements at their respective isolated

and logic sides, different isolation transformers must be

used with each device (see the Transformer Selection

section). The only external components needed to

complete the system are the isolation transformer, two

diodes, and two low-voltage, 10µF decoupling capaci-

tors (see the Typical Application Circuit).

The MAX3535E/MXL1535E include one differential dri-

ver, one receiver, and internal circuitry to send the RS-

485 signals and logic signals across the isolation barrier

(including the isolation capacitors). The MAX3535E/

MXL1535E receivers are 1/8 unit load, allowing up to 256

devices on a single bus.

The MAX3535E/MXL1535E feature fail-safe circuitry

ensuring the receiver output maintains a logic-high

state when the receiver inputs are open or shorted, or

when connected to a terminated transmission line with

all drivers disabled (see the Fail-Safe section).

The MAX3535E/MXL1535E feature driver slew-rate

select that minimizes electromagnetic interference

(EMI) and reduces reflections caused by improperly

terminated cables at data rates below 400kbps. The

driver outputs are short-circuit protected for sourcing or

sinking current and have overvoltage protection. Other

features include hot-swap capability, which holds the

driver off if the driver logic signals are floated after

power is applied. The MAX3535E/MXL1535E have

error-detection circuitry that alerts the processor when

there is a fault and disables the driver until the fault is

removed.

Fail Safe

The MAX3535E/MXL1535E guarantee a logic-high

receiver output when the receiver inputs are shorted or

open, or when connected to a terminated transmission

line with all drivers disabled. The receiver threshold is

fixed between -10mV and -200mV. If the differential

receiver input voltage (A - B) is greater than or equal to

-10mV, RO1 is logic-high (Table 2). In the case of a ter-

minated bus with all transmitters disabled, the receiv-

er’s differential input voltage is pulled to zero by the

termination. Due to the receiver thresholds of the

MAX3535E/MXL1535E, this results in a logic-high at

RO1 with a 10mV minimum noise margin.

Driver Output Protection

Two mechanisms prevent excessive output current and

power dissipation caused by faults or by bus con-

tention. The first, a foldback current limit on the output

stage, provides immediate protection against short cir-

cuits over the entire common-mode voltage range. The

second, a thermal-shutdown circuit, forces the driver

outputs into a high-impedance state if the die tempera-

ture exceeds +150°C.

Monitoring Faults on

RE

RE functions as both an input and an output. As an

input, RE controls the receiver output enable (RO1). As

an output, RE is used to indicate when there are faults

associated with the operation of the part. This dual

functionality is made possible by using an output driver

stage that can easily be overdriven by most logic

gates. When an external gate is not actively driving RE,

it is driven either high using a 100µA internal pullup

current (fault present), or low using a 60µA internal pull-

down current (no fault). When using RE to control the

receiver-enable output function, be sure to drive it

using a gate that has enough sink and source capabili-

ty to overcome the internal drive.