LTC4211CS8 datasheet(23/36 Pages) LINER

23 / 36 page

LTC4211

4211f

The RDS(ON) of the external pass transistor should be low

to make its drain-source voltage (VDS) a small percentage

of VCC. At a VCC = 2.5V, VDS + VRSENSE = 0.1V yields 4%

error at the output voltage. This restricts the choice of

MOSFETs to very low RDS(ON). At higher VCC voltages, the

VDS requirement can be relaxed in which case MOSFET

package dissipation (PD and TJ) may limit the value of

RDS(ON). Table 5 lists some power MOSFETs that can be

used with the LTC4211.

For reliable circuit operation, the maximum junction tem-

perature (TJ(MAX)) for a power MOSFET should not exceed

the manufacturer’s recommended value. This includes

normal mode operation, start-up, current-limit and

autoretry mode in a fault condition. Under normal condi-

tions the junction temperature of a power MOSFET is given

by Equation 13:

MOSFET Junction Temperature,

TJ(MAX) ≤ TA(MAX) + θJA • PD

(13)

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 LTC4211 can be used on either a printed circuit board

or on the backplane side of the connector, and examples

for both are shown in Figures 13 and 14. 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 RESET, 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 LTC4211’s ON pin

to detect whether the printed circuit board has been fully

seated in the backplane before the LTC4211 commences

a start-up cycle.

The first example is shown in the schematic on the front

page of this data sheet. In this case, the LTC4211 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 LTC4211 in an OFF

state. Once the connectors are mated, the resistive divider

is connected to VCC, VON > 1.316V and the LTC4211

begins a start-up cycle.

In Figure 13, an LTC4211 is illustrated in a basic configu-

ration on a PCB daughter card. The ON pin is connected to

VCC on the backplane through a 10k pull-up resistor once

the card is seated into the backplane. R2 bleeds off any

potential static charge which might exist on the backplane,

the connector or during card installation.

A third example is shown in Figure 14 where the LTC4211

is mounted on the backplane. In this example, a 2N2222

transistor and a pair of resistors (R4, R5) form the PCB

connection sense circuit. With the card out of the chassis,

Q2’s base is biased to VCC through R5, biasing Q2 ON and

driving the LTC4211’s ON pin low. The base of Q2 is also

wired to a socket on the backplane connector. When a card

is firmly seated into the backplane, the base of Q2 is then

grounded through a short pin connection on the card. Q2

is biased OFF, the LTC4211’s ON pin is pulled-up to VCC

and a start-up cycle begins.

In the previous three examples, the connection sense was

hard wired with no processor (low) interrupt capability. As

illustrated in Figure 15, the addition of an inexpensive

logic-level discrete MOSFET and a couple of resistors

offers processor interrupt control to the connection sense.

R4 keeps the gate of M2 at VCC until the card is firmly

mated to the backplane. A logic low for the ON/OFF signal

turns M2 OFF, allows the ON pin to pull high and turns on

the LTC4211.

OPERATIO

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