IL4822 / IL4622 / IL4685

12

NVE Corporation

11409 Valley View Road, Eden Prairie, MN 55344

(952) 829-9217

www.nve.com

YouTube.com/NveCorporation

iso-apps@ www.nve.com

Thermal Management

An IL46xx transceiver node can be operated at worst-case loading, data rate, and ambient temperature without exceeding the

maximum DC-to-DC convertor output current, maximum die temperature or package power rating. As shown in Figure 18, worst-

case power consumption for a node such as that shown in Figure 20 is less that 1.2 watts. Junction-to-ambient thermal resistance is

46 °C/W with a 2s2p circuit board, so the temperature rise is 55 °C and the die temperature is within the 140 °C maximum at the

85 °C maximum ambient.

If additional nodes are powered by the DC-to-DC convertor or additional power is needed from the DC-to-DC convertor, however,

care should be taken to ensure the die temperature does not exceed its 140 °C maximum operating temperature, and that the total

power dissipation does not exceed the 1.5 watt package maximum. The following sections summarize some thermal management

considerations in these situations.

Board Layout

If possible, use a double sided, double buried power plane (“2s2p”) board to maximize thermal performance. Thermal vias should

be used between the power plane and the board surfaces. All four IC ground pins should be connected, with wide traces to help

cool the leadframe.

Use Low-Power, Fractional-Load Transceivers

Transceivers such as the NVE IL3685P are fractional load to minimize the drive current required by transmitting nodes and, if

powered by the IL46xx, uses less bus power than other transceivers.

Avoid Termination Resistors with Shorter Bus Cables

Termination resistors minimize reflections, which can be important for long cable lengths. However, these resistors significantly

increase output drive current and may not be necessary with short bus cables.

Full Duplex Uses Less Power

Full-duplex RS-422 buses have only one transmitter per bus and therefore only need one termination resistor, typically 120 Ω.

Half-duplex RS-485 networks with long cables, however, are generally terminated on both ends because either end can receive

data. This doubles the power dissipated in the termination resistors.

No External “Fail-Safe” Resistors

The transceivers are designed to be “full fail-safe,” so “fail-safe” pull-up and pull-down bias resistors are generally unnecessary

and use power.

Minimize Data Rate

The transceiver draws more power at higher frequency, so the data rate should not be higher than necessary to minimize

transceiver power.

Limit Transmission Time

The transceivers use less power receiving than transmitting, and much less if there are termination resistors. Average power

dissipation can be reduced considerably by disabling the driver (DE = LOW) when not transmitting data.