LTC3877

Question

Hi, 
 
 I am using the product LTC3877 & LTC3874 to generate a four phase core supply for the Arria10 GX FPGA . The design I have used is the same as in the Arria10 GX FPGA Development Kit schematics. However I require a few clarifications. 
 We are providing a 12V input to generate 0.9V core Voltage Supply (85A) . 
 What is the current limit set in the LTC3877 with 15mV Voltage on the ILIM pin . 
 As per the LTC3874, the Current limit programming should match with the LTC3877 (Master controller). However in the reference design the LTC3877 is 1/4 of INTVCC(ILIM pin connected to voltage divider) and the LTC3874 is connected to GND. Is this ok . 
 Also we are unable get LTC3877 part in LT power cad tool. 
 Kindly clarify at earliest. 
 
 Thanks, 
 Aravind D

mike.shriver · Accepted Answer

Hello Aravind, 
 Yes, the current limit of the LTC3877 should match or nearly match the current limit of the LTC3874. With the ILIM pin of the LTC3877 set to INTVCC/4, its current limit threshold will be 15mV. The current limit setting of the LTC3874 which best matches this threshold is 16mV. For this setting, setup the ILIM pins and LOWDCR pins as shown below. 
 ILIM = GND 
 LOWDCR = INTVCC 
 The LTC3874 current limit thresholds are shown below for reference. 
 
 The actual current limit depends on the current limit thresholds mentioned above plus the DCR and to a smaller extent the inductor value. Instructions on how to program the current limit and design the current sense filter are given in both data sheets. 
 An LTC3877 design tool is not available. But, the LTC3875 design tool can be used as a substitute. 
 Best regards, 
 Mike)

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