LTC4253 datasheet(14/36 Pages) LINER

14 / 36 page

LTC4252-1/LTC4252-2

LTC4252A-1/LTC4252A-2

425212fd

SHUNT REGULATOR

A fast responding shunt regulator clamps the VIN pin to 13V

(VZ). Power is derived from –48RTN by an external current

limiting resistor, RIN. A 1μF decoupling capacitor, CIN filters

supply transients and contributes a short delay at start-up.

To meet creepage requirements RIN may be split into two or

more series connected units. 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 LTC4252 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 LTC4252 and all associated components

should be free of any other planes such as chasis ground,

return, or secondary-side power and ground planes.

VIN may be biased with additional current up to 30mA to

accomodate external loading such as the PWRGD opto-

coupler shown in Figure 23. As an alternative to running

higher current, simply buffer VIN with an emitter follower

as shown in Figure 3. Another method shown in Figure

19 cascodes the PWRGD output.

VIN is rated handle 30mA within the thermal limits of the

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

protect VIN against damage from higher amplitude spikes,

clamp VIN to VEE with a 13V Zener diode. Star connect

VEE and all VEE-referred components to the sense resistor

APPLICATIONS INFORMATION

Kelvin terminal as illustrated in Figure 3, keeping trace

lengths between VIN, CIN, DIN and VEE as short as possible.

INTERNAL UNDERVOLTAGE LOCKOUT (UVLO)

A hysteretic comparator, UVLO, monitors VINforundervolt-

age. The thresholds are defined by VLKO and its hysteresis,

VLKH. When VIN rises above VLKO the chip is enabled; below

(VLKO – VLKH) it is disabled and GATE is pulled low. The

UVLO function at VIN should not be confused with the

UV/OV pin(s). These are completely separate functions.

UV/OV COMPARATORS (LTC4252)

An UV hysteretic comparator detects undervoltage condi-

tions at the UV pin, with the following thresholds:

UV low-to-high (VUVHI) = 3.225V

UV high-to-low (VUVLO) = 2.925V

An OV hysteretic comparator detects overvoltage condi-

tions at the OV pin, with the following thresholds:

OV low-to-high (VOVHI) = 6.150V

OV high-to-low (VOVLO) = 5.550V

The UV and OV trip point ratio is designed to match the

standard telecom operating range of 43V to 82V when con-

nected together as in the typical application. A divider (R1,

R2) is used to scale the supply voltage. Using R1 = 402k

425212 F03

–48RTN

VEE

VIN

SENSE

TIMER

GATE

PWRGD

DRAIN

LTC4252-1

432k

38.3k

4.75k

330nF

CSS

68nF

18nF

–48V

0.02Ω

IRF530S

10Ω

2.2k

RIN

10k

1/2W

10nF

CIN

1μF

100μF

–48RTN

(SHORT PIN)

RD 1M

LOAD

22k

* M0C207

Q2: MMBT5551LT1

**DIODES, INC

†

RECOMMENDED FOR HARSH ENVIRONMENTS

DIN

†

DDZ13B**

Figure 3. – 48V/2.5A Application with Different Input Operating Range

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