design LTC3350 without external shunt design

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. 

Dear Marty

 thank your clear information.

my current setting is RSNSC 3m ohn,--> Max Ichg=32mV/RSNSC(mohm)=10A

and Rsunt is 5.1 ohm --> that means internal shunt current is 252mA.

Based on my use condition , I can accepted " charged current is reduced". when the caps are degraded.

Does that means, I can remove EXTERNAL SHUNT CURRENT without any risk. 

thank you

The internal shunt regulator is all that is needed to safely operate this application with the 100F caps and 10A charge current. The external shunt circuit is not needed.

The internal shunt regulator is all that is needed to safely operate this application with the 100F caps and 10A charge current. The external shunt circuit is not needed.

Dear Marty

I have some update. could you help to check.

my design is 100F caps x 2  parallel capacitor  (total value is 200F ) in 1 stack

total is 4 stack ( that means 100F x 8 )

is it  need external shunt circuit ?

Hi, 

Having a stack of 4 series, 2 parallel of 100F caps is still OK. The charge current will still be reduced and the shunt regulator will still discharge the high cell at ~250mA and protect the caps from over charging without external shunt FETs. This will just take 2x as long to reach the float voltage.