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LMZM33606: LMZM33606 Cin selection when several power rails are required

Anton Stepanov
Prodigy 50 points
Part Number: LMZM33606
Other Parts Discussed in Thread: LMZM33604

Hi!

We would like to use LMZM33606 as main DC-DC buck converter.

Our design requires 4 independent power rails : 12V (3A), 5V (5A), 3.3V (5A), 3.3V (5A).

We are planing to use 4x LMZM33606 (or 1x LMZM33604 and 3x LMZM33606) sharing same power input connector.

Input power source will be 6S battery 25V (8S is also should be supported 34V).

I would like to clarify couple of questions:

1) Do we need to use same input capacitors to each LMZM33606  module ? 2x10uF ceramic + 33uF tantalum per LMZM device ?

Is it possible to combine it or to use less count of capacitors ? 

DS " 7.3.2 Input Capacitor Selection

The LMZM33606 requires a minimum of 20 μF of ceramic type input capacitance. Use only high-quality ceramic

type X5R or X7R capacitors with sufficient voltage rating. TI recommends an additional 33 μF of non-ceramic

capacitance for applications with transient load requirements."

2) What do you recommend against of inrush current when there will be many ceramic caps at high side of the device ?

Is there any good practice or "rule of thumb" schematics from TI ?

We would like to avoid overvoltage case described in "AN88".

Many thanks!

  • 0
    TI__Mastermind 47060 points

    Hi Anton,

    1. Each power module should have a local 20uF capacitor close to the VIN and GND to reduce return loop. The battery input can have a singular bulk high ESR capacitor for dampening the input voltage line. Then the input rail splits to 4 separate paths to the LMZM33606s with each power module having it's own 20uF input cap.

    2. The SS/TRK pin can be used to control the soft-start and effectively decrease inrush current during startup. Internally the device is set to 5ms softstart, you can then pick a capacitor such that the softstart time is above 5ms. Additionally the high ESR bulk input capacitor previously mentioned should also help with this too.

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Prodigy 50 points

    Hi Jimmy,

    1. All clear, thanks!

    2. Soft-start feature can effectively decrease inrush current of load (low-side).

    But I'm asking about capacitors charging process at high-side.

    We will have almost 100uF input ceramic capacitance in case of 4 LMZM power rails.

    And I'm worry that overvoltage transients can occurs.

    In case of 8S battery (35V max) >20% voltage increase during spikes will be more than max allowed voltage for LMZM (42V max).

    There was quite old application note from LT ( www.analog.com/.../an88f.pdf ) with problem description.

    So I'm looking for recommendation or good practice to aviod this. Could you share any from TI ?

    Many thanks!

  • 0 in reply to Anton Stepanov
    TI__Mastermind 47060 points

    Hi Anton,

    Thank you for the reference app note and I understand the concern here. 

    Please refer to this TI design (TIDA-01167) for input protection of common 12V-24V batteries. I believe this design reference for high side protection is applicable in your system.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 50 points

    Hi Jimmy,

    Many thanks for your help and clarifications!

  • 0 in reply to Anton Stepanov
    TI__Mastermind 47060 points

    You're welcome. Thank you for using the E2E forum.