# Power circuit with several voltage regulators — design rules

Please, share your professional rules you follow when designing a circuit with several voltage regulators (for example 5V, -5V, +2.5V, -2.5V, 3.3V).

Its understandable and can be read in datasheet how to operate with one voltage regulator (adding capacitors). But what should one additionally do to have different voltages stable and accurate?

I have decoupling caps at input and output of all LDOs. I use only LDOs, not switched. They are connected this way: (1) 9V => LP2992 => 5V (2) 5V => LP5907 => 3.3V (3) 5V => TLV700 => 2.5V (4) 5V => LP2664 => -5 (5) -5 => NCP4586 => -2.5

I come to this question when I realized that 3 of 5 voltage regulators in my device do its work perfectly, but two voltage outputs from other LDOs were wrong: 4V instead of 2.5, 1V instead of 3.3.

Also I have decoupling, which is shown on the image (DVDD = 3.3V, AVDD = 2.5, AVSS = -2.5).

• Please add details of (a) exactly the question you are asking - do you just want general tips? (b) what regulators you are using (with links to the datasheets) (c) an illustrative circuit schematic and (d) what you mean by "two voltage output s from other LDO`s were wrong". – stefandz Sep 8 '15 at 11:14
• I add more details to my question. – v_s_k Sep 9 '15 at 8:19
• Is AVSS in any way connected to GND ? You provide clarity regarding the decoupling but I doubt if the decoupling would have anything to do with the voltages being the wrong value. A schematic including the LDOs would help. – Bimpelrekkie Sep 9 '15 at 10:15
• Some of (perhaps all, I have not looked at all the datasheets) have specific output capacitor ESR requirements. Ceramics will work with the LP2992, but be careful of going below the stable region. In addition, there is a minimum capacitor size required (4.7uF). I suggest you look very carefully at the datasheets as not meeting those recommendations can cause the LDO regulator(s) to oscillate, with (usually) spectacular effects. – Peter Smith Oct 9 '15 at 9:12

As usual: it depends !

Always add decoupling caps at input and output of all LDOs !

A solution is to go down in voltage in steps: input voltage => 5 V => LDO => 3.3 V => LDO 2.5 V

But if you have a lot of digital circuits on 3.3 V and you want the 2.5 V as clean as possible then it might be better to do:

input voltage => 5 V => LDO => 3.3 V

and a separate

input voltage => 5 V => LDO => 2.5 V

It also depends on how much current is used in each supply, are all regulators linear regulators or are some switched ?

You mention something went wrong, it would help if you explained what did't work so we can focus on that.