What is the simplest and cheaper way to have 3.3V 0.5A from 4V?

I'm developing a board which uses a SIM800 so it must have 4V 2A power input from 24V external power supply (I'm using a LM2596S adj, able to supply 4V 3A). SIM800 has 4V Vdd and 3.3V logical input/output.

The rest of the board (microcontroller, etc) needs 3.3V, so I want to take it from the 4V (because if I take it from 24V I would need another step-down converter... and that means more components and cash).

The cheap AMS1117 has 1V dropout voltage, so it's not suitable. The cheap MCP1700 has only 250mA output current.

What's about to use diodes to get this voltage? There is another cheap regulator with less than 0.7V dropout voltage?

3.3V 500mA is needed (STM32 Cortex M0 + wiznet w5500 + some leds...). Efficiency is not a problem (AC power)


I will take diode voltage drop as the simplest solution versus cost.

  • 6
    \$\begingroup\$ A series silicon diode comes to mind (drop of 0.7 V). \$\endgroup\$
    – glen_geek
    Commented Jul 14, 2018 at 13:48
  • \$\begingroup\$ By the way, are you planning to have level shifters on the connections between your 3.3V microcontroller and the 4.0V SIM800? \$\endgroup\$ Commented Jul 14, 2018 at 13:52
  • \$\begingroup\$ Use two MCP1700's, one for the Cortex M0 and one for the w5500? \$\endgroup\$
    – Turbo J
    Commented Jul 14, 2018 at 13:59
  • \$\begingroup\$ While not what you asked, I expect you're going to have problems powering the SIM800 in the way you plan. \$\endgroup\$ Commented Jul 14, 2018 at 15:39
  • \$\begingroup\$ Why do you think that @ChrisStratton?? \$\endgroup\$
    – Fran H
    Commented Jul 14, 2018 at 15:44

2 Answers 2


A warning about using a diode to get 3.3V: the voltage drop does vary with current and temperature. For example a standard 1n4007 might drop 0.6V at 10mA @ 50C and 1V at 1A @ 0C.

  • \$\begingroup\$ and what's about Zenner ?? \$\endgroup\$
    – Fran H
    Commented Jul 14, 2018 at 15:23
  • \$\begingroup\$ @FranH Zener regulators don't work well when the load current can vary widely. \$\endgroup\$ Commented Jul 14, 2018 at 15:40
  • \$\begingroup\$ Of course, given that a good proportion of components that are described as requiring 3.3V are happy to run on anything from 3.0V to 3.6V, this is perhaps not as bad as it might appear at first... \$\endgroup\$
    – Jules
    Commented Jul 14, 2018 at 20:07

The term to look for is low drop out, or ultra low drop out linear regulator. Simple to use, reliable and i guess sufficient for your application. Always refer to graphs/tables provided in the datasheet rather than first page description. Please go through datasheet in detail, and also check thermal dissipation to see whether design need a small heatsink or not.

Only based on available inputs here is one example: too good but costly: - TPS79533DCQR
enter image description here

moderate priced - may not be best candidate: LP38693MPX-3.3 enter image description here

My frank suggestions:

There are atleast 100s of options for your application need. You are the right person to decide based on your application

  1. Is it battery operated? (consider leakage current or quiescent current)
  2. do you have space constraint?
  3. type of mounting expected
  4. regulation needed
  5. noise, ripple expectations
  6. price
  7. Enable disable function needed?
  8. is 500 mA max current or real application current?
  9. is 3.3 V mandatory, can the circuits operate at lower voltage for example 3 V?

I did a search for the regulator in digikey. Here is the link. you can continue from here. price is in the range 60 cents for one. you can explore from there.

  • \$\begingroup\$ The answers: No battery operated. Not too much space constraint. Surface mounting. Noise/ripple and voltage needed... enought to work with TTL 3.3V. Price as low as possible. 500mA is max current (250 mA operating current) \$\endgroup\$
    – Fran H
    Commented Jul 15, 2018 at 8:02
  • \$\begingroup\$ @FranH Then I would suggest to start filtering components as per price and keep an eye on Drop out voltage.. A bit higher quiescent current will be acceptable so that price will not be high because of that \$\endgroup\$
    – User323693
    Commented Jul 15, 2018 at 8:28

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