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LMZM33606: Fsw and capacitor values in normal and inverting applications?

S. Gokduman
Prodigy 195 points
Part Number: LMZM33606
Other Parts Discussed in Thread: LMZM33602,

Hello,


I'm planning to use LMZM3360x series power modules in both Buck  and IBB configurations. Hence i need a psu without noise and ripple, i will use LDOs after them. Here is general info,

19V Input  =>  LMZM33606, +12.6V, 4.4A out
19V Input  =>  LMZM33606, -12,6V 3.2A out
19V Input  =>  LMZM33602, +5,6V 1,1A out

Here are my questions,

1) I know suggested max operation frequency is 1.2MHz and datasheet says above this, effiency will drop. But how much drop there is gonna be? I want to increase Fsw to 2MHz cause LDOs' PSRR values are not good at 1MHz ( around 15dB).

Webench does not allow me to do it but when i change Rt value, i can get 2MHz. Does this effect other components' values? Can i get these amount of current with 2MHz switching frequency? 

2) I've checked a few designs in TI forum and Webench values for input and output capacitors are lower than these designs. For example, in evaluation board there are 39uF Alu + 2x 4.7uF ceramic input caps  and 4x 47uF + 150uF Tantalum output caps. But when i try to use Webench, it suggests using 2x10uF input caps (de-rated 8,6uF) and 4x22uF output caps ( de-rated 56uF).

These values are vastly different from evaluation board. Should i add Aluminum or Tantalum caps at the input and output? Also maybe increaseing ceramic caps values?  What should be my caps values in total for this applications? 

Thank you in advance.

  • 0
    TI__Guru 50120 points

    Hi,

    Let me look into this and see if there are any efficiency data for 2.2MHz switching frequency. Regarding the output capacitance, the minimum output capacitance is detailed in Table 3 of the datasheet. I'd recommend designing for an output capacitance over the minimum as this will help give some stability headroom and allow for better transient response performance. In this case it is recommended to have at a minimum 50uF of derated ceramic output capacitance for Vout between 12V-20V. Any additional output capacitance can be ceramic, polymer, or a combination of both.  

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    TI__Guru 50120 points
    Hi,

    Just wanted to give you an update that I am still looking into the efficiency data for higher switching frequency. When I get more information from the designer I will get back to you.

    Regards,
    Jimmy
  • 0 in reply to Jimmy Hua
    TI__Guru 50120 points
    Hi,

    Unfortunately I didn't get a response from the designer on this expected efficiency drop. I plan on taking the LMZM33606EVM and configure it for positive output to measure the efficiency at 1MHz and 2MHz. That way I can at least provide you with some bench data comparison and show the drop in efficiency. Thank you for your patience. I'll get back to you once I've obtained the bench efficiency data.

    Regards,
    Jimmy
  • 0 in reply to Jimmy Hua
    TI__Guru 50120 points

    Hi,

    I took the EVM and ran a test for 19Vin-> 5.6Vout @ 1A loading. For 1MHz frequency the efficiency was roughly 87% while at 2.2MHz frequency the efficiency dropped to 79%. Clearly there is a drop in efficiency from 1MHz to 2.2MHz. If you plan on using an external frequency synchronization of 2.2MHz, it is best to test for stability. In your case of 19V the maximum output voltage may fall below the 12.6V in your application. You may build the circuit following your application and run validation test to ensure stability. Table 6 provides guidance on the switching frequency and operating output voltage. 

    Regards,

    Jimmy