I am working on a design and wondering about placing two switching regulators a Buck then a Boost in series. Is there any issue with placing the output of a buck circuit to the input of a boost circuit ...
The boost circuit will never be on without the buck being on first.

Is there any gotcha moments when placing a two switching supplies together.

I need a input of 2.8V -> 4.2V regulated for 3.3V and for 5V (board uses both voltages)

 

A better way would be to boost your battery to 5V first then buck down to 3.3. So, put them the other way to what you mentioned.

 

additional note is the 5V only needs about 20mA max the 3.3V needs > 400mA at max

 

As your battery is not above either of your required voltages all the time, you need to use a buck/boost setup anyway.
There is no point in using a buck first. That would then require 2 more boost converters to raise the now lower than min battery volts to 3.3 and another to 5V.
Boost the battery to 5V first, there is one supply. Then buck to 3.3V.
Or you would need a buck/boost combination switcher to have the battery converted to 3.3V, and a small boost to 5V. That may work better. You will need to look into the efficiencies of the switchers. I have not used a buck/boost supply myself.

 

I will change the design slightly
input 3.0V ->4.2V output 3.0V to 3.3V (Switching regulator has ability to run at 100% duty cycle so this should not be an issue)
3.0V to 3.3V is @>400mA

I also need an output of 5V@20mA I would like this to be behind the 3.3V regulator as to not draw more than 4uA when the 3.3V regulator is turned off ....

to get the 5.0V I am using a tps61222 however at >8mA to 10mA the voltage starts dropping... This should provide upwards of 200mA no issue.

on the 3.3V input to the tps61222 I can load it above the 400mA without an issue.

is putting the regulators in series causing this issue?

 

It may be you need a larger capacitor between them. That will be easy to try.

 

Consider a buck boost circuit, even if you have to use two of them.
Two switchers in tandem will have very poor efficiency.

 

Consider a buck boost circuit, even if you have to use two of them.
Two switchers in tandem will have very poor efficiency.

Can you explain more about poor performance of tandem switchers, why, what would cause this and can anything be done to help efficiency?

 

Can you explain more about poor performance of tandem switchers, why, what would cause this and can anything be done to help efficiency?

Yes. If you have one switcher that is 80 percent efficient and the next has 80 percent efficiency, then the overall efficiency is 80 percent of 80 percent which is 64 percent, very poor.
In the power supply industry this is referred to as "handling the power twice".
To contrast, a good buck boost will only switch the power one time only.
Imagine having 3 switchers in tandem all with 80 percent efficiency. That would result in an overall efficiency of just 51 percent! Nasty. If they were all 90 percent it would still go down to 73 percent, not good either.

The only thing that could help efficiency is to use higher efficient circuits, but even two 90 percent units in tandem will result in an overall efficiency of just 81 percent seems like a waste. A good buck boost may get 90 percent or at least 85 percent.

 

Yes. If you have one switcher that is 80 percent efficient and the next has 80 percent efficiency, then the overall efficiency is 80 percent of 80 percent which is 64 percent, very poor.
In the power supply industry this is referred to as "handling the power twice".
To contrast, a good buck boost will only switch the power one time only.
Imagine having 3 switchers in tandem all with 80 percent efficiency. That would result in an overall efficiency of just 51 percent! Nasty. If they were all 90 percent it would still go down to 73 percent, not good either.

The only thing that could help efficiency is to use higher efficient circuits, but even two 90 percent units in tandem will result in an overall efficiency of just 81 percent seems like a waste. A good buck boost may get 90 percent or at least 85 percent.

Thanks that is understood, unfortunately I need two separate regulated voltages several components require 3v several require 5v so I have no real choice but to handle twice.

 

Thanks that is understood, unfortunately I need two separate regulated voltages several components require 3v several require 5v so I have no real choice but to handle twice.

Well handling twice means twice in the same converter where the power flow is in tandem.
Just having two separate power supplies is not the same thing because that is in parallel.
So if you use two buck boost circuits you should see a much better efficiency. One would have a 3.3v output and the other will have a 5v output. So it is two *separate* switchers not two wired in tandem.

So with a quick analysis, if you have one buck circuit and two boost circuits connected to that each boost output efficiency would be around 64 percent if each switcher was 80 percent efficient.
To contrast, if you have two buck boost circuits each with a separate output, each one has 80 percent efficiency so the overall efficiency is still 80 percent.

 

output 3.0V to 3.3V

Do you mean (1) any voltage in that range is ok, or (2) that the output must be selectable/adjustable within that range?
Option 1 would need just a voltage limiter whereas option 2 would need a buck-boost regulator.