LTC4212CMS datasheet(22/24 Pages) LINER

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LTC4212

4212f

Table 5 lists some current sense resistors that can be used

with the circuit breaker. Table 6 lists some power MOSFETs

that are available. Table 7 lists the web sites of several

manufacturers. Since this information is subject to change,

please verify the part numbers with the manufacturer.

OPERATIO

where

PD = (ILOAD)2 • RDS(ON)

θJA = junction-to-ambient thermal resistance

TA(MAX) = maximum ambient temperature

If a short circuit happens during start-up, the external

MOSFET can experience a big single pulse energy. This is

especially true if the applications only employed a small

gate capacitor or no gate capacitor at all. Consult the safe

operating area (SOA) curve of the selected MOSFET to

ensure that the TJ(MAX) is not exceeded during start-up.

Using Staggered Pin Connectors

The LTC4212 can be used on either a printed circuit board

or on the backplane side of the connector. Printed circuit

board edge connectors with staggered pins are recom-

mended as the insertion and removal of circuit boards do

sequence the pin connections. Supply voltage and ground

connections on the printed circuit board should be wired

to the edge connector’s long pins or blades. Control and

status signals (like FAULT and ON) passing through the

card’s edge connector should be wired to short length pins

or blades.

PCB Connection Sense

There are a number of ways to use the LTC4212’s ON pin

to detect whether the printed circuit board has been fully

seated in the backplane before the LTC4212 commences

a start-up cycle.

An example is shown in the schematic on the front page

of this data sheet. In this case, the LTC4212 is mounted

on the PCB and a 20k/10k resistive divider is connected to

the ON pin. On the edge connector, R1 is wired to a short

pin. Until the connectors are fully mated, the ON pin is held

low, keeping the LTC4212 in an off state. Once the

connectors are mated, the resistive divider is connected

to VCC, VON > 1.316V and the LTC4212 begins a start-up

cycle.

PCB Layout Considerations

For proper operation of the LTC4212’s circuit breaker

function, a 4-wire Kelvin connection to the sense resistors

is highly recommended. In Hot Swap applications where

load currents can reach 10A or more, narrow PCB tracks

exhibit more resistance than wider tracks and operate at

more elevated temperatures. Since the sheet resistance of

1 ounce copper foil is approximately 0.54m

Ω/square,

track resistances add up quickly in high current applica-

tions. Thus, to keep PCB track resistance and temperature

rise to a minimum, PCB track width must be appropriately

sized. Consult Appendix A of LTC Application Note 69 for

details on sizing and calculating trace resistances as a

function of copper thickness.

In the majority of applications, it will be necessary to use

plated-through vias to make circuit connections from

component layers to power and ground layers internal to

the PC board. For 1 ounce copper foil plating, a good

starting point is 1A of DC current per via, making sure the

via is properly dimensioned so that solder completely fills

any void. For other plating thicknesses, check with your

PCB fabrication facility.

APPE DIX

Table 5. Sense Resistor Selection Guide

CURRENT LIMIT VALUE

PART NUMBER

DESCRIPTION

MANUFACTURER

LR120601R050

0.05

Ω 0.5W 1% Resistor

IRC-TT

LR120601R025

0.025

Ω 0.5W 1% Resistor

IRC-TT

2.5A

LR120601R020

0.02

Ω 0.5W 1% Resistor

IRC-TT

3.3A

WSL2512R015F

0.015

Ω 1W 1% Resistor

Vishay-Dale

LR251201R010F

0.01

Ω 1.5W 1% Resistor

IRC-TT

10A

WSR2R005F

0.005

Ω 2W 1% Resistor

Vishay-Dale

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