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Original question:

WEBENCH® Tools/LM5170-Q1: LM5170 support buck-boost converter when Vin is vary range (52V drop to 45V)

WEBENCH® Tools/LM5170-Q1: LM5170-Q1: LM5170 support boost mode while Vin is equal to Vo

Aska Lee
Intellectual 610 points
Part Number: LM5170-Q1
Other Parts Discussed in Thread: LM5170

Tool/software: WEBENCH® Design Tools

Hi

We got the EVM board of LM5170 (SNVSAQ6 48V to 12V).

Modified items: 1) LM5170 in boost mode with CV feedback circuit. 2) Inductor change to 15uH. 3) Mosfet (HQ and LQ) change to 100Vds rating. 

Test condition: 1.5A

Q1: If Vin is equal to Vo, is there any concern we need know?

Q2: It is normal operating (The Gate of HQ/LQ mosfet are swtiching) with Vin 35V, the Vout keep 48V. We want to test the Vin rise to 48V. We found the Gate of HQ is off while Vin=41V. It seems be diode emulation mode. Is it right for LM5170?

We didn't try it for heavy load due to we don't want get damaged EVM. Maybe it is not easy because there are dual phase with 2pcs high side mosfet on EVM. It means the boost current through 4pcs body diodes.

I knew the better solution should be buck-boost converter. I need know what kind limitation is for LM5170 if Vin is equal to Vo.

Appreciated for your reply.

Aska

  • 0
    TI__Guru* 78200 points

    Hi Aska,

    Thank you for considering the LM5170.  Here are the answers to your questions:

    A1: When the max Vin is equal to Vo, the converter will continue to operate in boost, it is because the internal voltage drops causes the effective Vin still to be lower than Vo.

    A2: The LM5170 has diode emulation mode. It makes a MOSFET behave like an ideal diode: only allow positive current to flow.  When the inductor current falls to zero, the sync FET (high side, controlled by HO) will turn off, to prevent the current flowing in the opposite direction.  Since your current is only 1.5A, the diode emulation mode is activated. 

    When  you load current increases, and the inductor current will not fall down to zero at the end of the switching cycle, so HO will appear to be complementary with LO.

    Thanks,

    Youhao Xi, Applications Engineering