Other Parts Discussed in Thread: LM5175, , LM5176
Hi,
We're evaluating LM5176-Q1 based on the LM5175 EVM board. We have some questions about the pre-bias startup application.
Test is done under the following conditons:
1. Vin=13.5V, Vout=6.5V;
2. Rsns=10mohm for 5A average current limiting;
3. Lo=4.7uH, fs=340kHz, Rocp=3mohm. Pictures were capatured by toggling EN pin. Please checks the attached waveforms.
CH1: inductor current CH2: Vgs of boost highside mosfet CH3: Vgs of buck highside mosfet CH4: Vgs of buck lowside mosfet
In the pic "Prebias startup", the period concerned is highlighted in the red rectangle. During this period, boost highside mosfet is constant ON while buck lowside mofet is switching. And buck highside mosfet is constant OFF until real start-up kicks in. It seems that current in the inductor is monitered and clamped by turning off the buck lowside mosfet on time.
In the pic "Prebias startup ZOOM2", it shows the width of the first "effective" pulse of buck lowside fet is about 800ns. The reverse current is -1.1A.
In the pic "Prebias startup ZOOM3", it shows inductor current resonants when it's around 0 while buck lowside fet is OFF.
In the pic "Prebias startup ZOOM4", it shows real start-up kicks in. Buck highside fet is switching. The peak reverse current is -1.29A.
More tests were done:
A. Inductor changed from 4.7uH to 2.2uH, the first "effective" pulse of buck lowside fet decreases to 384ns with reverse current stays the same( -1.1A). BUT peak reverse current becomes -2A during start-up phase.
B:In addition, changing Rsns or Rocp did NOT affect the width of the first "effective" pulse and its reverse current(always -1.1A).
Here are the questions need to be clarified:
1. What's the purpose of this concerned period? and how long it would be? Buck lowside fet switching while Boost highside constant ON.
2. Is there any scheme used to clamp the reverse current during this period? If yes, could you pls point out the general theory? Seems it's un-relevant with Rocp and Rsns.
3. Is there a 2nd scheme used to clamp the reverse current durying other working condition, such as: real start-up, normally working, Input voltage shut down, etc.
Thanks a lot!
Looking forward to your reply!
