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Unloaded LMZ14202H problem

Prodigy 40 points

Replies: 5

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we are developing a power module 36v in / 24v out 2 A . 
When the external 36V is turned off a battery system is activated with cca 26 volt supply voltage.
Our goal is to keep the regulator working until the supply voltage is higher than 20V.
Therefore we applied a voltage divider which defines the UVLO.
Here is the schema:

But we have noticed strange behavior.
We built five prototypes with the same components. From 5 boards 3 works well (meaning when
we turn on the supply voltage and the board is unloaded the output becomes 24V).


But on the next 2 boards when the board is unloaded the output never reaches 24V but grows up only to 5V.


We are confused because the five boards are totally identical / with same layout and components/

Thank you for quick reply

Best regards,



  • Hello Felix,

    The schematic section that you show for the module based regulator section looks okay. But I'm sorry that I don't understand the "cca" acronym and I don't understand the location of the 26V battery connections in regard to the schematic section provided.  Please clarify for my understanding.

    Taking a guess:   -   If the battery is essentially diode ORed with the 36V source to form a type of uninteruptable input supply be aware of several things.

    1) I'm not sure how well characterized this product is when operated in dropout. In other words, if you are attempting 26Vin to 24Vout at 2A. the product might not regulate all that well. It should however properly behave as a limiter to 24V maximum output.

    2) For input voltages less than dropout (approximately 27Vin), the output voltage will be lower than 24V. Or in other words, 24V will only be generated when the input voltage is greater than the minimum required input voltage.  i.e. it does not implement a buck-boost regulator. (20V input wil not result in 24V output)   It should however provide a high efficiency buck in the regions where it was designed to regulate.

    3) The ratio of the die size of the internal mosfets is designed for high step-down ratios such as 36V to 12V, or even 36V to 24V. low step down ratios such as 26V or 27V input to 24V ouput are more of a challenge.

    4) Be also aware that if the 26V battery voltage is applied to the 24V output of the regulator that the 26V battery voltage will be available at the 36V input side of the regulator by virtue of the internal mosfet substrate diodes. Blocking and steering diodes would need to be applied in that scenario.

    5) Your UVLO resistive divider network keeps the step down-regulator running for input voltages above 20V.  Your operating description sounds the opposite. The UVLO set point should really be above 27V or so for a regulated 24V output application.  

    6) The two boards that only generate 5V are puzzling, I agree.  What is the capacitor chemistry for the two 20 uF output capacitors? 40 uF total output is a bit on the low side for this product.

    Out of curiosity, is there a solar panel generating the 36V? and the solar panel charges the battery?


    Looking forward to helping resolve this application problem.


    Alan Martin

    Alan Martin  - Wide Vin Applications Wrangler (retired)

  • In reply to Alan Martin:

    Hello Alan

    Thank you for your reply :)
    Im sorry for this misunderstanding. I'll try to clear up our project,
    to avoid misunderstandings.

    This power module should be a part of existing motor drive system, which drives industrial firedoors.
    This module has one major function at this moment, powering up the locking system of the door
    /which is based on electromagnetic coil/.

    The regular operation is, when the whole system is powered up with 36V / from the grid /. In this case
    the input voltage for the power module is 36V the output 24V / 2A.
    In case of fire the main power source may fail but the firedoor has to be closed. Therefore a battery system is
    applied / charged by the system itself during regular operation / and the system is then run off of this 26V battery system
    /whose voltage might be lower depending on the charge and ambient state etc/.
    In this case our power module will be supplied with an input voltage of 26V.
    For simplicity, we chose this step down regulator, because we don't necessarily need 24V for the coil. The coil should be supplied
    with 18-24V. Maybe in the future we'll use a buck-boost regulator, but at this moment step down regulator is sufficient for us.

    We need to shut down the regulator when the input voltage achieves 20V to avoid damage to the battery.
    Therefore is the UVLO set to 20V /to keep the regulator working above 20V input voltage/.

    3 developed boards are working als we expected, meaning when we turn on the supply voltage, the unloaded output becomes 24V.
    Then we loaded the board with constant 1A. When we decrease the input voltage at 27V input we still have aprox. 24V at the output. Then of
    course the output voltage began to decrease. When the input voltage achieves 20V we still have aprox. 18V at output/by 1A constant load/.
    Under 20V input voltage the regulator switched off.

    But on the next 2 boards the unloaded output voltage never reaches 24V (only 5V).

    Our question is, why? The five boards are totally identical.

    The datasheet says,that it is good
    practice to use the enable divider and turn on the regulator when VIN is close to reaching its nominal value.
    It means, that for stable operation we need set the UVLO close to VIN? In other words, the LMZ14202H is unstable,
    when the UVLO is set below the designated output voltage?

    As output capacitor we are using ceramic capacitor / 50V X5R /

  • In reply to Juraj Mozes:

    Hello again,

    A co-worker and I will see if we can replicate the behaviour in our lab here. 

    You might try the following component changes on one of the two malfunctioning boards. (These are the changes we will try ourselves.)

    a) Increase the CSS value from 0.0047 to 0.01uF.  This will double the soft-start time (and also make transient response a bit slower when going from zero load to full load. Not a concern in your application.)


    b) Decrease the CFF value from 0.022uF to 0.01uF which will reduce the feed-forward  time constant.

    We think that the 0.92V Overvoltage protection trip point is being reached upon start-up and shutting down the switching action. However, this mode is supposed to recover once the feedback voltage drops back to 0.8V. In your case it is not recovering; so we want to look at it closely ourselves to make sure we don't miss some other factor.

    Thank you for your clear explanation of your system setup, by the way.



    Alan Martin  - Wide Vin Applications Wrangler (retired)

  • In reply to Alan Martin:

    Hello again,

    My previous suggestion of changing the two capacitors did not really improve things.  After a few experiments I came up with some different component values you should try.

    Change RON to 220k ohm

    Change Cff to 1000pF

    Leave CSS at 4700 pF

    Add a 300 ohm dummy load resistor across vout to keep a minimum load applied.

    Let us know how this works.


    Alan Martin  - Wide Vin Applications Wrangler (retired)

  • In reply to Alan Martin:

    Hello Alan

    i'll write about our 2 malfunctioning boards /because the other 3 are working well/

    in a related post
    we already read about this low load operation problem. And if we applied a greater than 25mA load, the IC worked. But if VIN drops to 24V or below the output starts oscillating:

    we applied your suggestions, but the result was the same. Moreover, 300ohm means an 80mA load, which means 1.92 watts, which means a fairly bulky resistor /unfortunately space is at premium and the resistor's high temperature is also undesirable/.

    The IC also worked when we plug our power module to the destination board / we have developed a motherboard, and our power module is a card which can be plugged into the motherboard/. The thing is that when the input voltage of the power module is increased instantly, everything works fine /unless we then decrease the input voltage to 24V or below, because then the oscillation starts/. But when the voltage increase is slow /due to the high input capacitance of our driving system/, we only get 5 volts on the output. Of course all of this applies only when the UVLO is set below 24V and the output is unloaded.

    Our question remains, is the LMZ14202H unstable when the UVLO is set lower than the regulated output voltage? Because if we set UVLO higher than 24V /we tried 27V / the output reaches 24V in all cases.

    Why is it possible that some of the ICs work and some don't?

    Thank you for reply

    best regards


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