ISL41334IRZ datasheet(18/28 Pages) INTERSIL

18 / 28 page

FN6202.3

November 21, 2007

the logic device’s low VOH might not exceed the VIH of a 5V

powered dual protocol input. Connecting the VL pin to the

power supply of the logic device (as shown in Figure 12)

limits the ISL41334’s Rx output VOH to VL (see Figure 15),

and reduces the Tx and control input switching points to

values compatible with the logic device output levels.

Tailoring the logic pin input switching points and output levels

to the supply voltage of the UART, ASIC, or µcontroller

eliminates the need for a level shifter/translator between the

two ICs.

VL can be anywhere from VCC down to 1.65V, but the input

switching points may not provide enough noise margin when

VL < 1.8V. Table 5 indicates typical VIH and VIL values for

various VL values so the user can ascertain whether or not a

particular VL voltage meets his needs.

The VL supply current (IL) is typically less than 100µA, as

shown in Figures 20 and 21. All of the DC VL current is due

to inputs with internal pull-up resistors (DE, SP, LB, ON/OFF)

being driven to the low input state. The worst case IL current

occurs during SHDN (see Figure 20), due to the IL through

the ON/OFF pin pull-up resistor when that pin is driven low.

IIL through an input pull-up resistor is ~20µA, so the IL in

Figure 20 drops by about 40µA (at VL = 5V) when the two

SP inputs are high (middle vs top curve). IL is lowest in the

RS-232 mode, because only the ON/OFF pin should be

driven low. When all these inputs are driven high, IL drops to

<1µA, so to minimize power dissipation drive these inputs

high when unneeded (e.g., SP inputs aren’t used in RS-232

mode, so drive them high).

Active Low Rx Enable (RXEN)

In many RS-485 applications, especially half duplex

configurations, users like to accomplish “echo cancellation”

by disabling the corresponding receiver while its driver is

transmitting data. This function is available on the QFN

package via an active low RXEN pin for each port. The

active low function also simplifies direction control, by

allowing a single Tx/Rx direction control line. If an active high

RXEN were used, either two valuable I/O pins would be

used for direction control, or an external inverter is required

between DE and RXEN. Figure 13 details the advantage of

using the RXEN pin.

TABLE 5. VIH AND VIL vs VL FOR VCC = 5V

VL (V)

VIH (V)

VIL (V)

1.65V

0.79

0.50

1.8V

0.82

0.60

2.0V

0.87

0.69

2.5V

0.99

0.86

3.3V

1.19

1.05

FIGURE 12. USING VL PIN TO ADJUST LOGIC LEVELS

GND

RXD

TXD

VCC = +2V

UART/PROCESSOR

GND

VCC = +5V

ISL81334

VOH ≤ 2V

VOH = 5V

VIH ≥ 2V

ESD

DIODE

GND

RXD

TXD

VCC = +2V

UART/PROCESSOR

GND

VCC = +5V

ISL41334

VOH ≤ 2V

VOH = 2V

VIH = 0.9V

ESD

DIODE

FIGURE 13. USING ACTIVE LOW vs ACTIVE HIGH RX

ENABLE

0.1µF

VCC

GND

RXEN

DEN

+5V

ISL81387

Tx/Rx

ACTIVE HIGH RX ENABLE

0.1µF

VCC

GND

RXEN *

+5V

ISL41334

Tx/Rx

* QFN ONLY

ACTIVE LOW RX ENABLE

ISL81334, ISL41334

ISL41334IRZ Datasheet(PDF) 18 Page - Intersil Corporation

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