LT1640AL datasheet(15/36 Pages) LINER

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LTC4252-1/LTC4252-2

LTC4252A-1/LTC4252A-2

425212fd

APPLICATIONS INFORMATION

and R2 = 32.4k gives a typical operating range of 43.2V

to 82.5V. The undervoltage shutdown and overvoltage

recovery thresholds are then 39.2V and 74.4V. 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 100μA and

define an impedance at UV/OV of 30kΩ. In most applica-

tions, 30kΩ impedance coupled with 300mV UV hysteresis

makes the LTC4252 insensitive to noise. If more noise

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

from UV/OV to VEE.

Separate UV and OV pins are available in the 10-pin MS

package and can be used for a different operating range

such as 35.5V to 76V as shown in Figure 3. Other combi-

nations are possible with different resistor arrangements.

UV/OV COMPARATORS (LTC4252A)

A UV hysteretic comparator detects undervoltage condi-

tions at the UV pin, with the following thresholds:

UV low-to-high (VUV) = 3.08V

UV high-to-low (VUV – VUVHST) = 2.756V

An OV hysteretic comparator detects overvoltage condi-

tions at the OV pin, with the following thresholds:

OV low-to-high (VOV) = 5.09V

OV high-to-low (VOV – VOVHST) = 4.988V

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

standard telecom operating range of 43V to 71V when

connected together as in Figure 2. A divider (R1, R2) is

used to scale the supply voltage. Using R1 = 390k and R2

= 30.1k gives a typical operating range of 43V to 71V. The

undervoltage shutdown and overvoltage recovery thresh-

olds are then 38.5V and 69.6V respectively. 1% divider

resistors are recommended to preserve threshold accuracy.

The R1-R2 divider values shown in Figure 2 set a standing

currentofslightlymorethan100μAanddefineanimpedance

at UV/OV of 28kΩ. In most applications, 28kΩ impedance

coupled with 324mV UV hysteresis makes the LTC4252A

insensitive to noise. If more noise immunity is desired, add

a 1nF to 10nF filter capacitor from UV/OV to VEE.

The UV and OV pins can be used for a wider operat-

ing range such as 35.5V to 76V as shown in Figure 4.

Other combinations are possible with different resistor

arrangements.

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 other interlock

425212 F04

–48RTN

VEE

VIN

SENSE

TIMER

GATE

PWRGD

DRAIN

LTC4252A-1

464k

34k

10k

0.68μF

CSS

68nF

10nF

–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 4. – 48V/2.5A Application with Wider Input Operating Range

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