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design LTC3350 without external shunt design

Category: Hardware
Product Number: LTC3350

my design is follow DC1937B.

and supercap size is 100Fx2  with 4 stack

But I don't have  EXTERNAL SHUNT CURRENT design.

I only use CAP1/2/3/4 to do balance and shunt current

how do I know, internal shurnt current is enought ? 

Parents
  • Hello, 

    The shunt regulator will basically short across the cap as it approaches the shunt regulator voltage with an internal FET. The resistance across the cap will be 2x RSHUNT since the current passes thru two resistors plus the internal RDSON of about 0.5 Ohms. If the shunt regulator is set for 2.7V and 2.7 Ohm resistors are used, then the regulation current is about 450mA. The charge current is also reduced so the voltage of the capacitor will not exceed the shunt regulation voltage. The charger will continue to  charge the other 3 caps at about 450mA until the float voltage is reached and the high capacitor voltage is reduced below the shunt regulation voltage. At this point the balancer will turn on to reduce the high capacitor and continue until all capacitors are within 10mV of each other. 

    The shunt regulator is a safety mechanism that hopefully will not be needed. The shunt regulator should be set for the maximum capacitor voltage which should be higher than the normal float voltage. If the capacitors becomes way off balanced for some reason then the shunt regulator will help prevent the cap from being charged over maximum voltage. Typically the capacitor should be close in value, have similar leakage current and ESR. Over time the ESR and capacitance can degrade increasing the risk that the shunt regulator might be needed. Especially if the float voltage will be cause the caps to be close to the shunt voltage. 

    The answer to the question is really application dependent. Most cases the external shunt resistor circuit is not needed. How fast does the stack need to be charge up? Worst case the other caps are at 0V and the one cap is at some higher voltage, usually less than the shunt regulation voltage. The stack will take the time to charge the remaining capacitors at about the shunt regulation current once the high cap reached the shunt voltage until the stack reaches the float voltage. Some worst case scenario for that application will need to be determined and see if that is acceptable for the application. 

Reply
  • Hello, 

    The shunt regulator will basically short across the cap as it approaches the shunt regulator voltage with an internal FET. The resistance across the cap will be 2x RSHUNT since the current passes thru two resistors plus the internal RDSON of about 0.5 Ohms. If the shunt regulator is set for 2.7V and 2.7 Ohm resistors are used, then the regulation current is about 450mA. The charge current is also reduced so the voltage of the capacitor will not exceed the shunt regulation voltage. The charger will continue to  charge the other 3 caps at about 450mA until the float voltage is reached and the high capacitor voltage is reduced below the shunt regulation voltage. At this point the balancer will turn on to reduce the high capacitor and continue until all capacitors are within 10mV of each other. 

    The shunt regulator is a safety mechanism that hopefully will not be needed. The shunt regulator should be set for the maximum capacitor voltage which should be higher than the normal float voltage. If the capacitors becomes way off balanced for some reason then the shunt regulator will help prevent the cap from being charged over maximum voltage. Typically the capacitor should be close in value, have similar leakage current and ESR. Over time the ESR and capacitance can degrade increasing the risk that the shunt regulator might be needed. Especially if the float voltage will be cause the caps to be close to the shunt voltage. 

    The answer to the question is really application dependent. Most cases the external shunt resistor circuit is not needed. How fast does the stack need to be charge up? Worst case the other caps are at 0V and the one cap is at some higher voltage, usually less than the shunt regulation voltage. The stack will take the time to charge the remaining capacitors at about the shunt regulation current once the high cap reached the shunt voltage until the stack reaches the float voltage. Some worst case scenario for that application will need to be determined and see if that is acceptable for the application. 

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