1
\$\begingroup\$

What is the better way of dropping the voltage from 3.8V do 3.3V - using an LDO (e.g LP2985IM5-3.3) or the Schottky diode? It's about power supply for a microcontroller. I plan to use the ST1S10PHR step-down from 12V to 3.8V to power the GSM radio while I need 3.3V to power the MSP430 mcu. The input of the LDO or the Schottky would be the Vout of the ST1S10PHR step-down.

The problem I see with the Schottky diode is that the Vforward is a function of Iforward and this dependency is quite significant (e.g. for BAS70H: If=1mA -> Vf=410mV; If=15mA -> Vf=1V).

For the LDO LP2985IM5-3.3 I haven't found any particular limitation, however I met some chip that required the Vin to be Vout+1.5V. That is impossible in my case.

\$\endgroup\$
  • 1
    \$\begingroup\$ As you say a Schottky is a no-go, use a LDO. \$\endgroup\$ – Gunnish Nov 8 '13 at 21:39
3
\$\begingroup\$

You have 500 mV headroom, so a LDO should be possible. A diode is not a exact voltage drop. The output of a LDO will be much better regulated. Unless this is a very high volume product and the extra few cents actually matters, use a LDO.

\$\endgroup\$
  • 1
    \$\begingroup\$ 500mV is actually quite a lot. I don't expect the microcontroller to consume more than 100-150mA, and for that current, there is a large variety of LDOs available that have a guaranteed voltage drop below 500mV (and usually a typical value around 200-300mV). I have a small table of cheaper Microchip LDOs, so here's just a short list from my table: TC1071, TC1187, TC1173/4, MCP1725, MCP1755, MCP1801/2, MCP1824/5. \$\endgroup\$ – Laszlo Valko Nov 8 '13 at 22:10
  • 1
    \$\begingroup\$ @tml: I wouldn't call something that requires 1.5 V headroom a "LDO". "LDO" stands for low dropout. 1.5 V isn't really low dropout. Lots of LDOs can work with 500 mV headroom, including the dirt cheap MCP1700. \$\endgroup\$ – Olin Lathrop Nov 8 '13 at 22:42
  • \$\begingroup\$ Ok, so this is exactly what I was looking for: voltage drop, thanks! So the LP2985-N has 300 mV @ 150 mA load which makes it appropriate for my application. \$\endgroup\$ – tml Nov 8 '13 at 22:43

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.