link to page 15 link to page 16 LT3753 APPLICATIONS INFORMATIONProgramming System Input Undervoltage LockoutSoft-Stop Shutdown(UVLO) Threshold and Hysteresis Soft-stop shutdown (similar to system undervoltage) can The LT3753 has an accurate 1.25V shutdown threshold be commanded by an external control signal. A MOSFET at the UVLO_VSEC pin. This threshold can be used in with a diode (or diodes) in series with the drain should be conjunction with an external resistor divider to define used to pull down the UVLO_VSEC pin below 1.25V but not the falling undervoltage lockout threshold (UVLO(–)) for below the micropower shutdown threshold of 0.6V(max). the converter’s system input voltage (VS) (Figure 4). A Typical VIN quiescent current after soft-stop is 165µA. pin hysteresis current of 5µA allows programming of the UVLO(+) threshold. Micropower Shutdown VS (UVLO(–)) [begin SOFT-STOP then shut down] If a micropower shutdown is required using an external control signal, an open-drain transistor can be directly ⎡ ⎛ R1 = 1.25 1+ ⎞ ⎤ connected to the UVLO_VSEC pin. The LT3753 has a ⎢ ⎥ micropower shutdown threshold of typically 0.4V at the ⎝⎜ R2+R3 ⎠⎟ ⎣ ⎦ UVLO_VSEC pin. VIN quiescent current in micropower VS (UVLO(+)) [begin SOFT-START] shutdown is 20µA. = VS (UVLO(–)) + (5µA • R1) It is important to note that the part enters soft-stop when Programming System Input Overvoltage Lockout the UVLO_V (OVLO) Threshold SEC pin falls back below 1.25V. During soft-stop the converter continues to switch as it folds back switching The LT3753 has an accurate 1.25V overvoltage shutdown frequency, volt-second clamp and COMP pin voltage. See threshold at the OVLO pin. This threshold can be used in Soft-Stop in the Applications Information section. When conjunction with an external resistor divider to define the the SS2 pin is finally discharged below its 150mV reset rising overvoltage lockout threshold (OVLO(+)) for the threshold the forward converter is shut down. converter’s system input voltage (VS) (Figure 5). When OVLO(+) is reached, the part stops switching immediately and a hard stop discharges the SS1 and SS2 pins. The SYSTEM INPUT (V falling threshold OVLO(–) is fixed internally at 1.215V and S) LT3753 allows the part to restart in soft-start mode. A single resis- R1 UVLO_VSEC – tor divider can be used from system input supply (VS) to 5µA define both the undervoltage and overvoltage thresholds R2 TO + for the system. Minimum value for R3 is 1k. If OVLO is 1.250V OVLO PIN R3 unused, place a 10k resistor from OVLO pin to ground. VS OVLO(+) [stop switching; HARD STOP] ⎡ ⎛ R1+R2 3753 F04 = 1.25 1+ ⎞ ⎤ ⎢ ⎥ ⎝⎜ R3 ⎠⎟ ⎣ ⎦ Figure 4. Programming Undervoltage Lockout (UVLO) VS OVLO(–) [begin SOFT-START] 1.215 = V ( ) • S OVLO + 1.25 Rev. B For more information www.analog.com 15 Document Outline Features Applications Typical Application Description Table of Contents Absolute Maximum Ratings Order Information Pin Configuration Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Timing Diagrams Operation Applications Information Package Description Revision History Typical Application Related Parts
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