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LMZ35003

Max Orlarey
Prodigy 60 points
Other Parts Discussed in Thread: LMZ35003

Hello:

             I have a few question on the LMZ35003 Power module. I am considering using this to replace a 78SR105SC Power Trends Switching Regulator

Module.  The input voltage would be 7 to 48 VDC, Output to be 5VDC. I would like to run at 800 HZ, as the other module ran at around 650 to 700 Hz.

 Current draw on the 5 VDC, would be around 175 mA.  On the 78SR105 Module we had a Pi filter on input, and output sides of the module.

The recommend circuit on the application report SNVA698 Page 4 Figure 5 ,data sheet, for the LMZ35003 shows only a Pi filter on the output.

Would having a Pi filter on input be beneficial on lowering electrical noise?  Is there any problems with drawing lower current amounts on the output 175 Ma

compared to 2.5 Amps the module is capable of providing?

My thought is to use the typical schematic, for 5 VDC output. (See Page 17 of the Data Sheet for LMZ35003, Figure 30) I would change the 31.6 K Ohm

resistor to 40.2K on the INH/UVLO so that the input voltage would be 6.5 VDC. I would change the RT/CLK from 1100K ,to get the 800 HZ Switching

 Frequency.  Which would be better for ripple. I would add the PI filter as shown on page 4, Figure  of the Application Report SNVA698.

This is where my question is on the Pi filter, if it would be beneficial, to duplicate the PI filter on the output ,shown in figure 5, and also place it on the input

side of the LMZ35003 module?

Any thoughts on the addition of the PI filter, and increasing the switching Frequency, would be greatly appreciated.

Thank you

Max

 

  • 0
    Expert 2585 points
    Hi, Max,

    I guess you mean 800kHz for switching frequency of the LMZ35003 and 800Hz is mistyping. 800kHz you could set by choosing Rrt=267kOhm. In the mean time for Vout=5.0V it is recommended to choose fsw=500kHz (Rrt=1100kOhm) because in thus case you could have input range close to maximum.

    Concerning Pi filter at the input of the LMZ35003, if you talking about output ripple it shouldn't give significant impact. But if you are constrained by requirement to the input ripple it will give you some points.


    Concerning using LMZ35003 at 175mA (light load), please refer to the Light-Load Behavior section (p21 of the datasheet). From figure 38 you will see that for Vout=5V and output current 175mA you could have pulse skipping mode for Vin above 27V.
  • 0 in reply to Oleksiy Pazyuk
    Prodigy 60 points

    Yes, 800 KHz.. How much of the input would I lose at 800 KHz or at 700 KHz?

    Would I have less pulse skipping at a higher frequency, for a giving Vin voltage?

    Thank you

    Max