LT8210 Burn out the mosfet

Question

Dear ADI: 
 I use lt8210 for buck boost charger for battery , But now I use 2.5m&Omega; RSENSE series to inductor for sample inductor current , And my inductor saturation current about 30A , When input is 39V, output is 54.7V, LT8210 work in boost state and CCM mode ,Then the MOSFET A and C is burn out , Maybe Overcurrent and inductor saturation, But i Check the datasheet page 5 about "Cycle-by-Cycle Inductor Current Limiting" ,The give message is that "VSNSP1 &ndash; VSNSN1 = 50mV" If 50mV is correct , When I use 2.5m&Omega; Rsense , The Maximum current just only 20A , I wanna know the VSNSP1 &ndash; VSNSN1= 50mV is right or not ,TKS!

fhlxUpdate · Accepted Answer

Dear Chris: Now i have Fixed startup issues， But now i have find the problem as following: 
 I use 4 mosfet, Every mosfet Qg about 50nC Then operation frequency is 200Khz, So i count out the current comsumption 
 I = 4*50nc*200K = 40mA, And external power 13V supply to EXTVCC and GATVCC about 10V ,When output current less than 0.5A/50.4V, GATVCC Ripple is very good， And LTC8210 efficient arrivate at 98.5% in CC mode, But if increase output current, For example 1A ,The lt8210's GATVCC is very unstabity, So many problem is out, can you help me find out the reason 
 I use external Switch Power IC power to EXTVCC pin ,can output 1A, When GATVCC unstabity, EXTVCC pin signal is very stablity)

fhlxUpdate · Answer

Dear Chris: 
 First burn out mosfet and LT8210 Chips, Mainly problem is GATVCC is untabilty, If load over 0.5-0.6A , GATVCC output voltage is unstable and the SW1 and SW2 ringing is very serious like a wave, I try to add capacitor to GATVCC pins from 10uF to 20uF, Never change the impact，Maybe GATVCC Insufficient driving ability To 40mA, Or other problem, Or VC loop compensation ????

ADIApproved · Answer

Hi fhlxUpdate, 
 The layout I think is OK. 
 I look at your schematic. You have a GATEVCC cap of 10uF and boost caps of 0.47 uF. Please check these values. If the ratio of GATEVCC cap to boost cap is not very high, then you can have difficulty getting a good GATEVCC voltage. As a rule of thumb, the GATEVCC cap should be 100x the boost cap. Please check to make sure that you have adequate GATEVCC capacitance. 
 Thank you, 
 Chris

ADIApproved · Answer

Hello fhlxUpdate, 
 My name is Chris Gass. I am the factory application engineer supporting the LT8210. Yes, the VSNSP1-VSNSN1 threshold is 50 mV. 
 Please let me know if you have further questions, 
 Chris

fhlxUpdate · Answer

Dear cgass: 
 The datasheet page 29" The bootstrap capacitors CBST1 and CBST2 need to store roughly 100 times the gate charge (QG) required by top switches A and D. In most situations, a 0.1&mu;F to 0.47&mu;F, X5R or X7R, 25V capacitor is adequate. The bypass capacitance from GATEVCC to ground should be at least ten times the value" 
 1. If use 10 times CAPS, only output 48V*2A is good select, If increase power, GATEVCC ripple is out 
 2. If use 100 Times CAPS, Can output 48V*10A, But GATEVCC Broken Easily in startup.

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LT8210 Burn out the mosfet