help needed switching buck regulator circuit theory.

[qtommer]

hey everyone,im kind of a newbie in electronics and i was wondering if anyone could help me understand the following circuit diagram which is a switching buck regulator using the LM 2678

**broken link removed**

ive read the datasheet but i still dont have a FIRM idea of how the circuit works.I was hoping someone could kindly put it in layman terms for me to help me understand the circuit better.

my questions are:
1.Is this a dc-dc converter?
2.Is the switching of this regulator done INSIDE the IC or is it done using external circuitry (or is both my assumptions wrong in this case.Do correct me with explanations)
3.If the switching is done using external circuitry,how is it done?
4.From the datasheet provided ,it is stated that the 47µF capacitor at the input is used to minimize a singing signal due to high current operation with fast transient changes on the circuit which causes the inductance of the input wires to generate ringing signals.
Does this mean that high current operation will generate an appreciable kind of noise?

5.What is the function of the boost capacitor ?
6.What is the purpose of feedback in a switching regulator?
7.What is the purpose of the zener diode at the output of this regulator?


im new to the forum so if the picture doesnt show here's the link
https://www.electro-tech-online.com/custompdfs/2009/01/10088603.pdf

Help needed would really be appreciated.

 

[blueroomelectronics]

National Semiconductor have a design program that works out all the values and even gives you a schematic. Google Simple Switcher
And yes it's a DC-DC converter.

 

[OutToLunch]

1.Is this a dc-dc converter?

yes - you need to look up buck regulators and get a good understanding of how they work. There is a lot of info on the net already...

2.Is the switching of this regulator done INSIDE the IC or is it done using external circuitry (or is both my assumptions wrong in this case.Do correct me with explanations)

both inside and out. the control switch, which would have to be an N-Channel MOSFET since there is a boost pin, is contained inside the IC while the freewheeling diode (the zener) is a discrete component on the outside.

3.If the switching is done using external circuitry,how is it done?

see (1) and (2) above

4.From the datasheet provided ,it is stated that the 47µF capacitor at the input is used to minimize a singing signal due to high current operation with fast transient changes on the circuit which causes the inductance of the input wires to generate ringing signals.
Does this mean that high current operation will generate an appreciable kind of noise?

The 0.47uF capacitor is used to supply the current to the control switch when it first turns on - the cap is ceramic and should be placed as close to the IC pins as possible to minimize parasitic inductances. It provides current to the switch faster than the bulk capacitors can.

5.What is the function of the boost capacitor ?

It provides gate drive bias for the internal n-channel MOSFET. It acts like a charge pump capacitor on the phase node (phase node=input of inductor)

6.What is the purpose of feedback in a switching regulator?

regulation and stability

7.What is the purpose of the zener diode at the output of this regulator?

it is the freewheeling diode. This is a standard buck regulator. In a synchronous buck regulator it can be replaced by a MOSFET. When the control switch turns off, the current in the inductor flows through the diode.

 

[mneary]

One correction: 2) and 7) The diode above that was called a 'Zener' is actually a 'Schottky'. In this application, the Schottky diode is expected to be fast, with a very low forward voltage. This reduces losses.

 

[qtommer]

followups

Thank you so much for your explanations especially to OutToLunch for your detailed explanation=)

I pretty much have a better understanding of this circuit operation and understand the datasheet much more clearly now but there are some parts Ive come across which I dont really understand and I was hoping that someone could help me understand this:
The following paragraphs in italic are extracts from the datasheet:

Datasheet Link:
https://www.electro-tech-online.com/custompdfs/2009/02/LM2678.pdf


This is the part on the output capacitor and inductor:

Impacting frequency stability of the overall control loop, the
output capacitance, in conjunction with the inductor, creates
a double pole inside the feedback loop. In addition the capacitance
and the ESR value create a zero. These frequency
response effects together with the internal frequency compensation
circuitry of the LM2678 modify the gain and phase
shift of the closed loop system.

My questions are:
1.What is a double pole?
2.Why does the capacitance and the ESR create a zero and why do want this to happen?

These frequency
response effects together with the internal frequency compensation
circuitry of the LM2678 modify the gain and phase
shift of the closed loop system.

3.Is this something to do with the PWM of the switching regulator or is it just for frequency stability?

As a general rule for stable switching regulator circuits it is
desired to have the unity gain bandwidth of the circuit to be
limited to no more than one-sixth of the controller switching
frequency. With the fixed 260KHz switching frequency of the
LM2678, the output capacitor is selected to provide a unity
gain bandwidth of 40KHz maximum. Each recommended capacitor
value has been chosen to achieve this result.

4.What does unity gain bandwidth mean?

I would really appreciate it if someone could help me on this.Thanks=)