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LM34927: LM34927 output current ability and output inductor

Brian Chou
Prodigy 60 points
Part Number: LM34927

I use the LM34927 to converter 24V to 12V to drive the 16pcs relays. The relay core resistance value is 240Ω. So the output current of 12V about need to 711mA. And I check the datasheet of the LM34927 in page6 has the current minimum limit is 0.7A. So I want to ask below the two questions.

1. Can I use the LM34927 to converter the 24V to 12V to drive the 16pcs realys about 700mA?

2. If question 1 answer is Yes, how to decide the output inductor value, does it have any limit?

  • 0
    TI__Expert 7080 points
    Hi Brian,

    Thanks for your interest in TI's product.
    I am afraid that the LM34927 can't drive 700mA load at 12V output voltage , since the current limit 0.7A and 12Vout.
    You can find the calculation formula in datasheet chapter 8.2.1.2.5 .
    Even assuming the non-isolator Iout1 = 0A , The minimum current limit threshold is 0.7V, if we want the design not trigger this threshold . the turn ratio of transformer should be lower than 1. but Vout=12V , and Vcc already tied to Vout1 , the allowed maximum Vcc voltage is 13V .
    So the lowest turn ration of transformer is 13/12=1.08. it means the inductor value should be very large for such a smaller Delta inductor current.
    So I afraid you have to choose TI's other product.

    Thanks.
  • 0 in reply to Jaden Ning
    Prodigy 60 points
    Hi Jaden,

    Thanks your reply.
    Because my design only use the non-isolator output 1. So I try to test the TI EVM BD. I remove the D1, change the T1 to the 47uH inductor(1.1A), change the R5 to the 1.47Kohm, change theR4 to the 13Kohm from the TI EVM BD. Then, I try to test the fully loaded experiment by rising the load current from the 0.3A to 0.9. The result you can see the below data. And as you say the current limit is 0.7A. But I see the current limit equally 0.7A @Vin=48 from the 6.5 Electrical Characteristics. And I see the 7.2 Functional Block Diagram SW terimal. I think the VIN=24V maybe higher than the Vin=48V? And, do the output inductor has any choice note?

    Vin Iin Vout Iout Efficiency Note
    24.1 0.17 11.96 0.3 87.58%
    24.1 0.23 11.92 0.4 86.02%
    24.1 0.28 11.89 0.5 88.10%
    24.1 0.34 11.86 0.6 86.84%
    24.1 0.4 11.82 0.7 85.83%
    24.1 0.46 11.79 0.8 85.08%
    24.1 0.5 11.55 0.9 86.27% OCP
  • 0 in reply to Brian Chou
    TI__Expert 7080 points

    Hi Brian,

    Thanks for the data sharing.

    Firstly I want to clarify that the current limit 0.7V is the minimum value in the datasheet.  the typical value is 1.02A and maximum is 1.3A .

    So your test result maybe can't reflect the minimum 0.7V current limit. And it's peak current of inductor , have no relationship with input voltage.

    About inductor choice , usually the peak to peak current of inductor equal to 30% output load current . that is just a start point . smaller peak to peak current means large value of inductance. vice versa.

    Thanks.