Post Go back to editing

External bootstrap capacitors CB for the LTC3727-1

Category: Hardware
Product Number: LTC3727-1

Hello, 

I have a question regarding the external bootstrap capacitors CB for the topside MOSFET of the LTC3727-1 circuit. In the datasheet on page 18 it is stated that the value of the boost capacitor CB needs to be 100 times that of the total input capacitance of the topside MOSFET. When I had a look in the datasheet of the topside MOSFET that was used in the reference design in Figure 13 on page 30 (FAIRCHILD FDS6680A) (datasheet of the LTC3727-1), the input capacitance of this specific MOSFET is denoted as 1620pF. For CB, a value of 0.1uF is chosen. To my understanding the capacitor CB does not fulfill the stated requirement, as 1620pF = 0,162uF. So shouldn't CB be something around e.g. 0.22uF? 

I my application I am planning on using the PSMN2R9-30MLC as my topside (as well as low side) MOSFET. This MOSFET has an input capacitance of 2419pF according to its datasheet. So in my understanding the capacitor CB (C1 and C7 in my schematic) should have a value of around 0.27uF. Is this the right assumption or is the value of CB not that critical so I also could use a 0.1uF capacitor.

I attached a picture of my schematic in LTSpice. Any help on my question is greatly appreciated. 

Best

Nikita

Top Replies

  • FormerMember
    FormerMember
+1 verified

Hello,

You would need to use a value of around 0.27uF, that is correct. If your CB capacitance is too low, it might not be able to drive your top MOSFET correctly. As for the figure 18, it might be because…

Parents
  • FormerMember
    +1 FormerMember

Hello,

You would need to use a value of around 0.27uF, that is correct. If your CB capacitance is too low, it might not be able to drive your top MOSFET correctly. As for the figure 18, it might be because during the testing, it was found that the 0.1uF was already enough to drive it correctly.

Regards,

Jhun

  • Reply Children
    No Data