I'm doing my initial testing with the LM5104, and observing some odd asymmetry in the propagation delay during IN rising versus IN falling.
My configuration in this test:
- Vdd = Vhb = 12V
- 10ohm Gate resistors for HO and LO
- RT = 10k for a nominal delay time of 90ns (58ns min, 130ns max according to datasheet table 6.5)
- Cb = 470nF
- External load FETs: IRFR1205PBF
- IN: 100kHz, 0 to 3.3V, variable duty cycle
- My output half bridge supply voltage is 20V
- Very light load attached to my output, ~1kohm.
I can see that the specified propagation delays are typically as follows below:
- IN rising to LO falling: 25ns typical
- IN falling to HO falling: 25ns typical
- Trt should be ~90ns typical
Here is my capture:
CH1 (Blue) = IN
CH3 (Red) = HO
CH4 (Pink) = LO
So on the capture we see IN rise, and ~50ns later we see LO fall.
About 100ns after IN rises, we see HO go high, which seems rather fast considering Trt is supposed to be 90ns.
As for the ripple at the top of the HO trace I believe this may be in imperfect probing issue. My half bridge supply voltage is 20V, which is why we see HO pump up to about (20+12)=32V.
Then, on IN falling, we see HO go low about 50ns later, which is in line with the specs.
We however see LO rising roughly 350-400ns later!
Can someone help me to understand the significant mismatch in propagation delay of IN rising to HO rising (~100ns) versus IN falling to LO rising (~350-400ns)?
Is it possible the external load is too light to adequately discharge the HS pin and that is causing some issues?