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LP8860-Q1: Boost Maximum Output Current

tateo yamashiro
Guru 10890 points
Part Number: LP8860-Q1

Hi,

We got a question from the customer about LP8860-Q1.
Could you help us?

[Question]
They want to make sure about Figure 5 in datasheet. Why the maximum boost current is limited by the input voltage and the output voltage? For example we think the IC can regulate over 500mA at Vin=5V and Vout=45V, when Imax is set 9A(BOOST_IMAX_SEL=111). They want to know this figure what represent.

Best Regards,
tateo

  • 0
    TI__Expert 4565 points
    Hi Yamashiro-san,

    Any boost DC/DC design has max switch duty it can support, and max voltage conversion ratio based on this max duty is 10 as shown in datasheet.
    This means boost can support 45V output with 4.5V input. But this is just a conversion ratio, not actual operation condition considering load current.
    If conversion ratio goes high, max switch duty will increase, and efficiency will be lower. If load current becomes high, efficiency will be even lower due to high power loss on each boost component. This will increase boost switch duty and limit max output current.
    For output current like 500mA or higher, actual conversion ratio will be ~5 or so, which means 5V input and 25V output. This is common fact for all boost DC/DC design, and max conversion ratio will also vary with component selection too.

    Hope this helps,
  • 0 in reply to Sung Ho Yoon
    Guru 10890 points

    Hi, Thank you for your support. We got additional question. Does this plots depend on the switching frequency? If the switching frequency is faster, the loss is increased. Therefore we assume the maximum boost output current is reduced. If there are other factors except for the loss, could you tell us it?

    Best Regards,
    tateo

  • 0 in reply to tateo yamashiro
    TI__Expert 4565 points

    Hi Yamashiro-san,

    Yes, max current depends on max SW current limit from boost design and conversion ratio.

    At light load, choosing correct inductor value may increase boost efficiency(lower loss) at certain conversion ratio, but in most of cases, higher SW freq will cause higher power loss and will limit max output current lower. So, reducing power loss on each component at high SW freq should be focused to maximize output current support such as using a SW FET with low Rdson and input capacitance, inductor with low DCR, Schottky diode with low Vf and fast reverse recovery time.

    Hope this helps,