LTC4367 datasheet(24/32 Pages) AD

24 / 32 page

LTC7862

Rev 0

For more information www.analog.com

Thermal Considerations

The sustained or static power loss in normal pass-through

operation must be limited to ensure suitable maximum

component temperatures during all normal operating con-

ditions. The temperature rise of a switching surge stopper

is best measured empirically. Power loss for the pass-

through power path is I2RSW-ON and may be calculated

according to the equation below.

I2RSW-ON = I2 • (RSENSE + RDS(ON)(TOPMOSFET) + RINDUCTOR)

The dynamic or transient power loss during switching

operation is of concern with respect to component tem-

perature rise and is principally managed by the timer func-

tion which sets a maximum time in this mode. Thermal

mass and thermal resistance play key roles in determining

the peak temperatures of components at the point in time

when the timer cycles off and shuts down.

Worst-case operation for a single fault shorted output is

typically with the input voltage in the high normal operat-

ing range and the output shorted. For this condition, the

bottom synchronous MOSFET is typically the hottest com-

ponent, as it conducts nearly all the peak current at a high

duty cycle. Worst-case operation for a single fault input

voltage surge is with the input at the maximum expected

input voltage and the maximum operational load current.

An input voltage surge and output short is a double fault

and may not be required. Specific fault testing and design

margin is determined by system requirements.

Thermal evaluation and timer setting can be most easily

done empirically by observing key component tempera-

tures dynamically in various fault conditions. Observe

peak temperatures with an instrument with sufficient

bandwidth to track temperatures, such as an infrared (IR)

camera. One with video capability is ideal.

Set a maximum temperature rise goal based on compo-

nent maximum junction temperature ratings, maximum

expected ambient temperature, and allowed junction to

case temperature rise. Start at lower input voltages and/

or longer TMR timer settings and change after empirical

system verification and measurement.

APPLICATIONS INFORMATION

Fault Timer (TMR)

The LTC7862 is a switching surge stopper. The LTC7862

switches only during startup, during an overcurrent fault,

and/or during an input overvoltage surge. The primary

function of a switching surge stopper is to limit the output

to a programmed maximum voltage during an input volt-

age surge. Limiting the output voltage “surge stops” the

input voltage surge and prevents it from propagating to

the system and potentially causing damage.

The switching surge stopper external components are

typically optimized to minimize the total resistance in nor-

mal (dropout) operation when the top MOSFET is on con-

tinuously. The LTC7862’s fault timer is used to limit the

time spent switching. The fault timer is programmed to

allow the switching surge stopper components to operate

within a safe temperature range with the increased power

losses incurred during switching. Since the LTC7862 timer

may stop switching before reaching thermal equilibrium,

the power rating can be significantly increased compared

to what could be achieved without a timer. Likewise, for

a given output power rating, a smaller, cooler, and less

costly solution can be achieved by using the timer.

Connecting a capacitor from the TMR pin to ground

sets the amount of time that the LTC7862 is allowed to

switch. This same capacitor also sets a cooldown period

before the LTC7862 is allowed to switch again. During

normal operation, a 1.1µA current source pulls down on

the TMR pin. While the LTC7862 is switching, a 40µA

current source pulls up on the TMR pin. If the TMR pin is

charged up to 2.19V, the LTC7862 stops switching with

both external MOSFETs held off (TG and BG held low).

This limits the switching time to:

TSWITCHING = 2.19V • CTMR /40µA

If the LTC7862 returns to normal operation before the

timer reaches 2.19V, the 1.1µA pull-down current source

discharges TMR, and the LTC7862 is allowed to switch

immediately if conditions warrant.

LTC4367 数据表(PDF) 24 Page - Analog Devices

LTC4367 数据表(HTML) 24 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

数据表下载