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I'm trying to make sense of the datasheet of the LTC3440. I can't find how to calculate the maximum output current and on what it depends.

There is an IOUT(MAX)BURST formula for the burst mode on page 9, which seems say that for 3.7 V input and 3.3 V output the maximum current will be around 50 mA (which is quite low).

How to calculate the current in normal mode? There are a few drawings showing 3.3 V and 600 mA, but in what does that depend? The frequency maybe? I don't understand. Please help.

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    \$\begingroup\$ It's not a clear data sheet. I'd assume that the maximum current into or out of the device (through the MOSFETs) is limited to 600 mA. So, you just need to work out what mode you are operating in and 600 mA cannot be exceeded either as an input or as an output. However, this will produce a slightly conservative maximum current compared to some of the graphs that appear to show an output current of 800 mA (contrary to the 600 mA stated on page 1). \$\endgroup\$
    – Andy aka
    Commented Feb 13, 2023 at 18:35
  • \$\begingroup\$ Indeed it is. In the datasheet of TPS6303x it is written plain clear in the first page for both of the modes. \$\endgroup\$
    – verbessern
    Commented Feb 14, 2023 at 18:05
  • \$\begingroup\$ That may be true but it's the LTC3440 the op is using. \$\endgroup\$
    – Andy aka
    Commented Feb 14, 2023 at 18:59

2 Answers 2

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Typically, buck-boost regulators specify the maximum input current. If boosting voltage by 2, the maximum output current is reduced by half.

The feature list on the front page indicates output current to 600mA. This current is available only in buck mode and maybe for a small boost.

The characteristics on page 2 indicate the input current limit of 1A minimum.

The Supply Current Limit section on page 8 indicates that shutdown occurs at an input current of 2.7A.

Boosting a voltage from input to output requires a boost of current from output to input. So the maximum output current depends on the amount of boost.

In buck mode the output current must not exceed 600mA. The resulting input current will be reduced.

Of course the input current will be higher based on efficiency in either mode.

A calculation for boost mode might be $$I_{omax}=\frac{1A*\sqrt{eff}}{boost}$$

For buck mode, 600mA.

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  • \$\begingroup\$ Thank you, how much is eff and how much is boost for example? \$\endgroup\$
    – verbessern
    Commented Feb 14, 2023 at 18:00
  • \$\begingroup\$ Efficiency from charts at operating point. Boost is output/input. It’s a rough estimate then adjust from there. \$\endgroup\$
    – user319836
    Commented Feb 14, 2023 at 22:34
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At the bottom of page 3, in the section "Typical Performance Characteristics", three charts are shown, corresponding to three different switching frequencies. The charts are all very similar.

In these, efficiency peaks at around 50 mA load current. And some of the traces reach a maximum of 700 mA. Bottom line is, if the device you want to power normally consumes 50 mA or so, with occasional need for up to 600 mA, then this switcher is a good choice. If however, your load normally consumes 250 mA (not optimal efficiency for this device), or has occasional need for 800 mA (outside specifications), then you'll have to pick a different switcher.

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