The LM7805 datasheet recommends a 0.33uF cap on the input and a 0.1uF cap on the output. The LD1117-3.3 datasheet recommends a 0.1uF cap on the input and a 10uF cap on the output.

I am building a circuit that will run off the 12V battery in a bus. I need two supplies - 5V and 3.3V, the overall current consumption being <200mA. I have an LM7805 to generate the 5V supply and it then drives an LD1117-.3.3, to generate the 3.3V supply. The circuit I have designed is shown in the figure below.power circuit schematic. My questions are as follows:

  1. Is it OK to also add a 470uF electrolytic cap on the input of LM7805? I am hoping for additional protection against fluctuations in the 12V supply from the bus battery.

  2. Is it OK to skip the 0.1uF on the input of the LD1117-3.3 since the output of the LM7805 already has a cap of this value?

  3. The LM7805 recommends a smaller cap on the output compared to that on the input, whereas the LD1117-3.3 recommends a larger cap on the output compared to that on the input - any insights in to why this is the case? I am just trying to understand whether there are any gotchas in connecting the output of the LM7805 to the input of the LD1117-3.3!

  4. Can the 0.33uf and 0.1uF be ceramic (SMT) caps?

  5. I have some off the shelf power regulator boards that use the LM7805 but have a very large electrolytic cap (1000uF) on the input and another electrolytic cap (10uF) on the output. Why would they deviate from the recommended values of the decoupling caps?

Thank you very much for your help.

  • 2
    \$\begingroup\$ At 200mA, using a linear converter to create 5V from 12V ends up dissipating ((12V - 5V) x 0.2A) = 1.4W of power. The LM7805 will get hot. If this is a concern, or if you simply want to be more power-efficient, you could use a switching regulator to create the 5V, then use your LD1117 to convert 5V to 3.3V. Also, you may want to look into automotive-grade regulator ICs. The power system in a bus (and other motor vehicles) is pretty dirty, with lots of voltage surges and other electrical noise... \$\endgroup\$
    – bitsmack
    Apr 20, 2015 at 17:10

2 Answers 2

  1. Yes, no problem. 1000uF is not much bigger physically.
  2. If they are close to each other, perhaps. Put it closer to the LM1117.
  3. Partly history of the applications, but the LM1117 actually needs a capacitor on the output with certain characteristics to keep it from oscillating like a banshee.
    With the LM7805 the caps are not generally necessary- they just improve performance (lower output impedance, improve transient response).
  4. They can all be ceramics (including the 10uF), however you would need to put a series resistor on the 10uF in accordance with datasheet ESR limits. It's probably worth the extra part to avoid using a tantalum.
  5. Most likely the 1000uF is also a filter cap. Going higher in value is usually better with capacitors and those are really common values for an off-mains supply for moderate current. 47uF/10V or 100uF/10V is not much physically larger for the output cap and I would tend to use that.

Edit: Regarding the output capacitor requirements of the LM1117, below is the information from the TI (nee National Semiconductor, the original designers) datasheet for their LM1117:

enter image description here

So something like a series 0.5 ohm resistor should be fine. Note that a 10uF ceramic capacitor has a (negative) voltage coefficent as well as a tolerance, so you may wish to use a higher voltage rated part or a higher nominal capacitance part to ensure the 10uF minimum requirement is met, just to be completely safe.

  • \$\begingroup\$ Thanks. The particular LDO I am using [LD1117-3.3 from ST] does not seem to specify any special requirements with regards to the 10uF capacitor (i.e, ESR). I do however remember seeing such a spec in a similar LDO manufactured by an unknown company. Could I have missed something here? \$\endgroup\$
    – NK2020
    Apr 20, 2015 at 17:13
  • \$\begingroup\$ @NK2020 Please see the edit above. \$\endgroup\$ Apr 20, 2015 at 17:19
  • \$\begingroup\$ Thanks. It looks like the TI part requires this specific ESR range, whereas the ST part (LD1117) DS [link: st.com/web/en/resource/technical/document/datasheet/… merely says, "Only a very common 10 µF minimum capacitor is needed for stability." with no reference to ESR in the datasheet. However, it does say at one point of time, that "Refer to the test circuits, TJ = 0 to 125 °C, CO = 10 µF, R = 120 Ω between GND and OUT pins, unless otherwise specified.". Does the 120 ohm here refer to ESR of C0? Thanks. \$\endgroup\$
    – NK2020
    Apr 20, 2015 at 17:26
  • \$\begingroup\$ I strongly suggest you use the TI recommendations (or a tantalum capacitor). The ST datasheet does not say it is stable with a ceramic part. It's probably derived from an older NS datasheet (from which the ST part was derived) before 10uF ceramic capacitors were widely available. TI have wisely updated their datasheet to warn the inexperienced about this pitfall. The 120 ohms means the minimum load current requirement is met. If you don't load the LM1117 sufficiently (5mA or more) the output voltage can rise out of regulation- I think that's clearly stated in the datasheet. \$\endgroup\$ Apr 20, 2015 at 17:37
  • \$\begingroup\$ Thanks. One last question: with regards to your answer to Q.1, can I take it to mean that it is OK to connect more than one cap in parallel on the input - also on the output, if I wanted? for example to connect a 10uF cap on the output of LM7805 in parallel to the 0.1uF cap? \$\endgroup\$
    – NK2020
    Apr 20, 2015 at 18:37
  1. It's OK
  2. I think it's OK, but I don't suggest that. If you need two supplies, 5V and 3.3V, add the output capacitor for LM7805 and input capacitor for LD1117-3.3, even though the value is the same.
  3. As for me, the value of the capacitor is decided by the value of the voltage. Generally, I prefer one 10uF and one 100nF as output capacitors for 5V and 3.3V both.
  4. Yes
  5. Please refer to the answer 3

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