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LT1640AH 数据表(PDF) 11 Page - Linear Technology |
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LT1640AH 数据表(HTML) 11 Page - Linear Technology |
11 / 24 page 11 4251b12f LTC4251B/LTC4251B-1/ LTC4251B-2 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, 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 LTC4251B 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 LTC4251B and all associated components should be free of any other planes such as chassis ground, return, or secondary-side power and ground planes. 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 Kelvin terminal as illustrated in Figure 2, keeping trace lengths between VIN, CIN, DIN and VEE as short as possible. INTERNAL UNDERVOLTAGE LOCKOUT (UVLO) Internal circuitry monitors VIN for undervoltage. The exact thresholds are defined by VLKO and its hysteresis, VLKH. When VIN rises above 9.2V (VLKO) the chip is enabled; below 8.2V (VLKO-VLKH) it is disabled and GATE is pulled low. The UVLO function at VIN 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 VUVHI UV turning off at VUVLO APPLICATIONS INFORMATION OV turning off at VOVHI OV turning on at VOVLO The UV and OV trip point ratio for LTC4251B is designed to match the standard telecom operating range of 43V to 75V. The LTC4251B-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 resistors are recommended to preserve threshold accuracy. The same resistor values can be used for the LTC4251B-2. 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 30k. In most applica- tions, 30k impedance coupled with 300mV UV hysteresis makes the LTC4251B/LTC4251B-1/LTC4251B-2 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 trip point thresholds for the LTC4251B-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 100μA, and define an impedance at UV/OV of 32k. In most applica- tions, 32k impedance coupled with 260mV UV hysteresis makes the LTC4251B-1 insensitive to noise. If more noise immunity is desired, add a 1nF to 10nF filter capacitor from UV/OV to VEE. 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. |
类似零件编号 - LT1640AH |
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类似说明 - LT1640AH |
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