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LMR38010: LMR38010 -HS AND LS SWITCH CURRENT LIMIT

ALPHONS PULICKAN
Prodigy 50 points
Part Number: LMR38010

Tool/software:

  1. Given that the minimum value for the HS switch limit is 1.3 A and the low side switch limit is 0.9 A (FOR LMR38010),  if we have a load current of 1A and a total current change ( ΔI)  of 0.4 A, it will always exceed the LS switch current limit. This inherently restricts our designs to having a total change 0.2 A(ΔI=0.2A) , correct. ? This will consequently increase my inductance value.

          2.If the load current is less than 1A and the LS current limit is fixed, it will always trigger the LS current limit, causing the LS witch to turn off. In this case, how can we achieve a light load condition with a fixed LS  current  limit. How forced PWM works ? What is the significance of this LS peak current ? 

  • +1
    TI__Mastermind 23460 points

    Hi Alphons,

    1) The high-side current limit is referring to the peak current limit (the up-slope of the inductor current), and the low-side current limit is referring to the valley current limit (the down-slope of the inductor current). In your case, with a load current of 1A, your valley current is 0.8A, which is not exceeding the LS switch current limit of 0.9A.  

    2) If your load current is less than 1A, it is unlikely for you to hit the low-side current limit if the inductance is small enough.

    The significance of the LS peak current is to prevent the inductor current from running away if the inductor is 1) saturated, and 2) a high-duty cycle i.e. during short circuit.

    There is a delay time before the device realizes the circuit is in current limit, so in that time period the high-side switch is on, the inductor current can rise very quickly.

    Once the device engages current limit protection, the high-side switch turns off and the low-side switch turns on. If the down-slope of the inductor current is very low, then the inductor current will not dip much, and when the high-side switch turns on again, there is another drastic increase in current, which will damage the device. The way to limit the damage is to force the low-side switch on until the current is below the LS current limit.

    Thanks,

    Richard

  • 0 in reply to Richard Fu22
    Prodigy 50 points

    Hi Thanks for the explanation. Are you saying max valley current shouldn't exceed 0.9 A, and for the PFM variant , valley current can range between 0.25 A and 0..9 A, while for the FPWM variant, it can range between -0.5 A and 0.9 A during normal operation ?

  • 0 in reply to ALPHONS PULICKAN
    TI__Mastermind 23460 points

    Hi Alphons,

    Partly.

    Regardless of your device variant, as long as your valley current isn't exceeding 0.9A, you should be ok.

    For PFM, there is some confusion with the 0.25A minimum peak current limit. This value refers to the peak current limit that the device is checking when the device is operating at light load in order to maximize efficiency, it is not referring to a overcurrent protection feature. 

    For FPWM variant, you are correct.

    Thanks,

    Richard