# Boost Converter Supply

Discussion in 'The Projects Forum' started by djsfantasi, May 10, 2015.

1. ### djsfantasi Thread Starter AAC Fanatic!

Apr 11, 2010
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I have a Pololu 9V Step Up Regulator. It's specced at 1.4A maximum input current. I am supplying it with a 5.2VDC 8A power supply. The boost converter is in parallel with servo motors (2A), an Arduino Mega (70mA) and a custom circuit (40mA). The 9VDC the Pololu circuit produces us going to a 5V regulator powering an SSC32 (30mA)

My question is, do I need to be concerned with the 1.4 maximum input current? Is so, how do I limit it?

2. ### Papabravo Expert

Feb 24, 2006
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You limit the input current by limiting the output current. In other words don't ask a boost converter to supply more power than it can handle.

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3. ### MikeML AAC Fanatic!

Oct 2, 2009
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The maximum allowed input to the smps is 1.4A. At 5.2V input, the max allowed input power is P =IE =1.4*5.2 = 7.28W.

The smps is likely about 85% efficient, so the max power it can deliver to a 9V load is 0.85*7.28 = 6.2W.

The max current it can deliver to a load and stay within the allowed limit is I=P/E = 6.2/9 = 0.69A.

Since your load only requires 30mA at 9V, the smps will be loafing, so the output power is 0.03*9 = 270mW. The input to the smps will be 270/0.85 = 320mW (due to losses), so it will draw only 320mW/5.2V = 62mA from the 5.2V supply.

I would have used the adjustable regulator set to ~5.5V rather than the fixed 9V one. That would waste much less power than using the 9V step-up. I've given you all you need to figure the power saving in both the step-up and the on-card 5V regulator if the intermediate voltage is only 5.5V.

Last edited: May 10, 2015
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Apr 11, 2010
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