LTC4251-2CS6#TRPBF【PDF 11/24 页】IC CTRLR HOTSWAP NEGVOLT SOT23-6

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Sheet目录42

7页 8页 9页 10页 [11页]12页 13页 14页 15页

425112fd

LTC4251/LTC4251-1/

LTC4251-2

For more information www.linear.com/4251

SHUNT REGULATOR

A fast responding shunt regulator clamps the V

pin to

13V (V

). Power is derived from 48RTN by an external

current limiting resistor, R

. A 1礔 decoupling capacitor,

filters supply transients and contributes a short delay

at start-up.

To meet creepage requirements R

may be split into two

or more series connected units, such as two 5.1k or three

3.3k resistors. This introduces a wider total spacing than is

possible with a single component while at the same time

ballasting the potential across the gap under each resistor.

The LTC4251 is fundamentally a low voltage device that

operates with 48V as its reference ground. To further

protect against arc discharge into its pins, the area in and

around the LTC4251 and all associated components should

be free of any other planes such as chassis ground, return,

or secondary-side power and ground planes.

is rated handle 30mA within the thermal limits of the

package, and is tested to survive a 100祍, 100mA pulse. To

protect V

against damage from higher amplitude spikes,

clamp V

to V

with a 13V Zener diode. Star connect

and all V

referred components to the sense resistor

Kelvin terminal as illustrated in Figure 2, keeping trace

lengths between V

, C

, D

and V

as short as possible.

INTERNAL UNDERVOLTAGE LOCKOUT (UVLO)

Internal circuitry monitors V

for undervoltage. The exact

thresholds are defined by V

LKO

and its hysteresis, V

LKH

When V

rises above 9.2V (V

LKO

) the chip is enabled;

below 8.2V (V

LKO

-V

LKH

) it is disabled and GATE is pulled

low. The UVLO function at V

should not be confused with

the UV/OV pin. These are completely separate functions.

UV/OV COMPARATORS

Two hysteretic comparators for detecting under- and

overvoltage conditions, with the following thresholds,

monitor the dual function UV/OV pin:

UV turning on at V

UVHI

UV turning off at V

UVLO

APPLICATIONS INFORMATION

OV turning off at V

OVHI

OV turning on at V

OVLO

The UV and OV trip point ratio for LTC4251 is designed to

match the standard telecom operating range of 43V to 75V .

The LTC4251-2 implements a UV threshold of 43V only.

A divider (R1, R2) is used to scale the supply voltage. Using

R1 = 402k and R2 = 32.4k gives a typical operating range

of 43.2V to 74.4V . The under- and overvoltage shutdown

thresholds are then 39.2V and 82.5V . 1% divider resis-

tors are recommended to preserve threshold accuracy.

The same resistor values can be used for the LTC4251-2.

The R1-R2 divider values shown in the Typical Application

set a standing current of slightly more than 100礎, and

define an impedance at UV/OV of 30k. In most applications,

30k impedance coupled with 300mV UV hysteresis makes

the LTC4251/LTC4251-1/LTC4251-2 insensitive to noise. If

more noise immunity is desired, add a 1nF to 10nF filter

capacitor from UV/OV to V

The UV and OV trip point thresholds for the LTC4251-1 are

designed to encompass the standard telecom operating

range of 36V to 72V .

A divider (R1, R2) is used to scale the supply voltage.

Using R1 = 442k and R2 = 34.8k gives a typical operating

range of 33.2V to 81V . The typical under- and overvoltage

shutdown thresholds are then 29.6V and 84.5V . 1% divider

resistors are recommended to preserve threshold accuracy.

The R1-R2 divider values shown in the Typical Application

set a standing current of slightly more than 100礎, and

define an impedance at UV/OV of 32k. In most applica-

tions, 32k impedance coupled with 260mV UV hysteresis

makes the LTC4251-1 insensitive to noise. If more noise

immunity is desired, add a 1nF to 10nF filter capacitor

from UV/OV to V

UV/OV OPERATION

A low input to the UV comparator will reset the chip and

pull the GATE and TIMER pins low. A low-to-high UV

transition will initiate an initial timing sequence if the three

remaining interlock conditions are met.

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