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LMZ36002: Switching Frequency Selection and results of use out of allowable range

Ray Bloch
Prodigy 20 points
Part Number: LMZ36002

TI LMZ36002 is VERY COOL! Thanks TI!

The data sheet provides "allowable" switching frequencies under some described conditions; however, there is no information about the resulting behavior if conditions change and the part is used outside of the datasheet descriptions. 

See:

7.3.2 Switching Frequency (RT)

7.3.3 Recommended Operating Range

Can TI add some graphs of out of limit, exceeding "allowable", or crossing outside "allowable" limit behavior in "Typical Characteristics" graph section?

Can TI add further explanation of failure modes or reasons for limiting switching frequency selection range.

For example: We have a design for 24V input, 1.5A, 4V output, 850kHz (53.6k resistor). What happens if end user incorrectly installs with 36V input power?

explosion?

loss of efficiency?

output voltage regulation failure?

short term failure?

How do we safely test and observe resulting behavior? Probe Input, Output, ?PH

 Thank you and highest regards for TI engineering support!

  • 0
    TI__Guru 50120 points

    Hi Ray,

    Every converter has a specific minimum controllable on-time. This minimum on time is designed to account for the internal control circuit delays and needs to be specified in such a way that the inductor current is clean for sampling. This can be further understood in the SNVA736 app note.

    As the datasheet provides a recommended switching frequency for a certain PVIN and load condition in Table 4-5, I would expect the device to be unstable and may not provide a regulated output voltage  if the end user accidentally increased the PVIN from 24V to 36V for a 1.5V,4A,850kHz application. 

    You can test for device stability by proving the PH(phase node) pin. Slowly ramp the PVin up from 24V to 36V and observe PH and VOUT. Additionally you can use a frequency response analyzer to obtain a Bode plot waveform with a necessary 10-20Ohms injection resistor to check  for stability. 

    Regards,

    Jimmy