LTC4012 Bulk Charge

Question

Hello, 
 I am inquiring about the LTC4012 12.6V, 4A charger schematic found on the datasheet for this component. I had a question about pin 13 (PROG) and the FET that is attached to that. The gate of the transistor is attached to a net that is labeled " Bulk Charge ", however, I am a little confused on what that means in this context. I reviewed the data sheet and the section on bulk charge and understand that the LTC begins sourcing the current programmed by the external components connected to CSP, CSN, and PROG. 
 
 I am still uncertain on where that gate of the MOSFET connects too. Is it supposed to connect to the battery? or perhaps pin 3 (CHRG). Any suggestions would be greatly appreciated. 
 Thank you for your time.

ron.p · Accepted Answer

Hi TD147, 
 #CHRG pin is just an open-drain indicator pin. It shouldn&rsquo;t be connected to the gate of Q1. 
 The gate of Q1 is controlled by external circuit. For example, a micro controller can be used to control Q1 to get desired charge current. It is explained more on page 16 of the datasheet. Q1 can be turned on or off to set the charging current to either precharge or bulk charge.

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LTC4012 Bulk Charge

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