Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

Sync buck FET driver has too low dead time? (linear.com)

Status
Not open for further replies.
T

treez

Guest
Hello
This sync Buck FET driver has extremely low dead time of 13ns between top fet driven off and bottom fet being driven on (and vice versa) . This is far too low. Do you know why it is so low?
The simulator (LTspice) shows it giving serious shoot-thru current spikes of 100 Amps plus.

LTC4449 (sync Buck FET driver)
http://cds.linear.com/docs/en/datasheet/4449fa.pdf
 

Attachments

  • Sync Buck with LTC4449 driver.pdf
    24.3 KB · Views: 43
  • Sync Buck with LTC4449 driver.txt
    12.3 KB · Views: 51

Hi,

datasheet says:
Adaptive Shoot-Through Protection
Internal adaptive shoot-through protection circuitry
monitors the voltages on the external MOSFETs to ensure
that they do not conduct simultaneously. The LTC4449
does not allow the bottom MOSFET to turn on until the
gate-source voltage on the top MOSFET is suffi ciently
low, and vice-versa. This feature improves effi ciency by
eliminating cross-conduction current from fl owing from
the VIN supply through the MOSFETs to ground during a
switch transition.
Click to expand...

Therefore i think the simulation gives wrong results.

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
i possibly agree with you, but then again, the simulator might not be wrong....and there is a general rule for ltspice, if it doesnt work on ltspice, itll never work on the bench. I think i will check for a ti.com or other manuf sync buck driver with a less "knife-edge" type dead time.
 

Hi,

The datasheet says tgat the opposite fet only becomes conductive when the voltage moves. If the voltage moves, then the fet/diode must be (relatively) high ohmic. So it is safe to turn on the fet.
The dead time us not fixedto some ns, it adjusts itself.

Klaus
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
with a fet with quicker internal diode, but with a bigger gate drive series resistor, there is definetely shoot through....as this ltspice shows. It shows the ltc4449 isnt doing the datasheet thing of checking ones off before it turns the other on
 

Attachments

  • LTC3774 8 PHASE _1v5 12A_single_1.txt
    12.9 KB · Views: 50
  • LTC3774 8 PHASE _1v5 12A_single.pdf
    24.7 KB · Views: 69

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top