LTC4210-2CS6#TRM【PDF 8/20 页】IC CONTROLLER HOT SWAP TSOT23-6

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Sheet目录41

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LTC4210-1/LTC4210-2

421012f

APPLICATIO S I FOR ATIO

Hot Circuit Insertion

When circuit boards are inserted into live backplanes, the

supply bypass capacitors can draw large transient cur-

rents from the backplane power bus as they charge. Such

transient currents can cause permanent damage to con-

nector pins, glitches on the system supply or reset other

boards in the system.

The LTC4210 is designed to turn a printed circuit boards

supply voltage ON and OFF in a controlled manner, allow-

ing the circuit board to be safely inserted into or removed

from a live backplane. The LTC4210 can reside either on

the backplane or on the daughter board for hot circuit

insertion applications.

Overview

The LTC4210 is designed to operate over a range of

supplies from 2.7V to 16.5V. Upon insertion, an undervolt-

age lockout circuit determines if sufficient supply voltage

is present. When the ON pin goes high an initial timing

cycle assures that the board is fully seated in the backplane

before the MOSFET is turned on. A single timer capacitor

sets the periods for all of the timer functions. After the

initial timing cycle the LTC4210 can either start up in

current limit or with a lower load current. Once the external

MOSFET is fully enhanced and the supply has ramped up,

the LTC4210 monitors the load current through an exter-

nal sense resistor. Overcurrent faults are actively limited

to 50mV/R

SENSE

for a specified circuit breaker timer limit.

The LTC4210-1 will automatically retry after a current limit

fault while the LTC4210-2 latches off. The LTC4210-1

timer function limits the retry duty cycle to 3.8% for

MOSFET cooling.

Undervoltage Lockout

An internal undervoltage lockout (UVLO) circuit resets the

LTC4210 if the V

supply is too low for normal operation.

The UVLO has a low-to-high threshold of 2.5V, a 100mV

hysteresis and a high-to-low glitch filter of 30祍. Above

2.5V supply voltage, the LTC4210 will start if the ON pin

conditions are met. A short supply dip below 2.4V for less

than 30祍 is ignored to allow for bus supply transients.

ON Function

The ON pin is the input to a comparator which has a low-

to-high threshold of 1.3V, an 80mV hysteresis and a high-

to-low glitch filter of 30祍. A low input on the ON pin resets

the LTC4210 TIMER status and turns off the external

MOSFET by pulling the GATE pin to ground. A low-to-high

transition on the ON pin starts an initial cycle followed by

a start-up cycle. A 10k pull-up resistor connecting the ON

pin to the supply is recommended. The 10k resistor shunts

any potential static charge on the backplane and reduces

the overvoltage stress at the ON pin during live insertion.

Alternatively, an external resistor divider at the ON pin can

be used to program an undervoltage lockout value higher

than the internal UVLO circuit. An RC filter can be added at

the ON pin to increase the delay time at card insertion if the

internal glitch filter delay is insufficient.

GATE Function

During hot insertion of the PCB, an abrupt application of

supply voltage charges the external MOSFET drain/gate

capacitance. This can cause an unwanted gate voltage

spike. An internal proprietary circuit holds GATE low

before the internal circuitry wakes up. This reduces the

MOSFET current surges substantially at insertion. The

GATE pin is held low in reset mode and during the initial

timing cycle. In the start-up cycle the GATE pin is pulled up

by a 10礎 current source. During an overcurrent fault

condition, the error amplifier servoes the GATE pin to

maintain a constant current to the load until the circuit

breaker trips. When the circuit breaker trips, the GATE pin

shuts down abruptly.

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