DEMO MANUAL DC1935A LT4363-1/LT4363-2 High Voltage Surge Stopper with Current Limit DESCRIPTION Demonstration Circuit 1935A showcases the LT 4363 surge By removing 0 jumpers R1C and R4C, the operation may stopper in a 12V, 2A application. Inputs of up to 80VDC be modified for 28V systems with an output regulation and 1ms transients of up to 250V are limited to 16.3V point of 34.6V in overvoltage conditions. at the output. The MOSFET is protected against output Design files for this circuit board are available at overloads by current limiting. Sustained overvoltage or DEMO MANUAL DC1935A QUICK START PROCEDURE Danger High voltage testing should be performed by While the input of the DC1935A can handle 250V, several qualified personnel only. As a safety precaution at least points should be brought to mind. First, the dissipation two people should be present during high voltage testing. of several components rises to significant levels when 250V is applied. R4 dissipation rises to near 500mW, D1 dissipates over 10W and R7 dissipates nearly 40W. These BOARD LAYOUT components dissipate high, destructive power even though DC1935A is designed to withstand 250V peak input voltage the LT4363 regulates and then shuts down the output, for up to 1ms. This high voltage is stood off by CSNAB, saving the load and Q1 from destruction. The maximum R4A, R7, R10, Q1, Q2 and Q3. Maximum input voltage is safe DC input is 80V, limited by the dissipation of R7 and limited by Q1s 250V BV rating. The permissible time at DSS D1. Always remember that 250V should be applied only 250V is limited by R7, which dissipates slightly less than as a spike or transient, not exceeding 1ms duration. 40W and is capable of doing so for at least 1ms. Danger High voltage testing should be performed by The minimum spacing is limited by 1206 pad spacing qualified personnel only. As a safety precaution at least where the gap between solder pads is 2mm, or just under two people should be present during high voltage testing. 80mils. Thus, the spacing between the input plane and all other board traces is maintained at a minimum of 2mm. DC1935A-A AND DC1935AB STUFFING OPTIONS As a point of reference, a 2mm needle gap in air breaks down well above 1kV. The DC1935A-A is fitted with the LT4363-1 which latches off after a timer delay in the presence of overvoltage or overcurrent conditions. Once latched off the LT4363-1 ver- 250V CAPABILITY sions timer executes a cool-down cycle, after which the The LT4363 is rated to survive up to 100V. If input spikes output may be restarted by pulsing the SHDN pin low for did not exceed 100V, the V pin could be connected directly CC at least 100s, or by briefly disconnecting the input supply. to the input. Nevertheless, in some systems input voltage The DC1935A-B is fitted with the LT4363-2 which automati- spikes may reach levels much higher than 100V. Opera- cally retries after its cool-down cycle. Retry is inhibited by tion under these conditions is made possible by limiting the OV pin, if the input is greater than 15.5V. the voltage at the V pin with an external filter or clamp. CC The simplest clamp is a Zener diode or transient voltage OPERATION suppressor (D1) and a 1k series limiting resistance (R7). This arrangement is adequate for millisecond excursions The shutdown pin, SHDN, is floating so that when power of up to 250V. The voltage is limited by the breakdown is applied to the input, the LT4363 automatically turns on. of Q1 and the peak voltage rating of R7, and the time is The LT4363 protects the load from destruction by regulating limited by the pulse power capability of R7, D1, and Q1. the output voltage to a safe level during intervals of input The OV and UV pins are limited to 100V absolute maximum. overvoltage. DC1935A is designed to regulate the output Practical divider ratios attenuate peak inputs of 250V to at 16.3V. If the input voltage is less than 16.3V, power safe levels at the OV and UV pins. For the resistors used in passes through directly to the output. The output voltage DC1935A, the peak voltage at the UV pin is less than 60V is sensed by the R1/R2 divider and the FB pin. The GATE when the input spikes to 250V. If there is an input voltage/ pin controls Q1 to regulate the output voltage in the event divider ratio combination that leads to greater than 100V at the input rises above 16.3V. OV or UV, the top of the divider may be moved to the V CC Overcurrent is sensed by R and the SNS and OUT pins SNS clamp for protection, with a small penalty in accuracy, or of the LT4363. If the load current reaches 50mV/20m, a Zener clamp may be added to the affected pin. the GATE pin will control Q1 to regulate the output cur- rent at 2.5A. dc1935af 2