L5970AD DCDC schematic doubt

[AlexMachine]

Hello,

I am investigating in using L5970AD DCDC converter for a 12VDC to 3V3 need.

It was advised to me by the ST DCDC online simulator.

It's a 20 year old circuit but it it matches the budget, the additional BOM and the efficiency so I am interested.

But I don't reach in answering some questions. If one of you can share some return of experience about this chip I am gladly listening.

I am in design phase, looking datasheets, and except error I don't find my answers :

- The VREF signal is noticed as 3V3. 3V3 is the output I want to create. On the Typical application schematic it seems not clear for me if connecting VREF to the output is okay, or if we need a precursor 3V3 ??? Should we supply a high impedance 3V3 from resistor divided input for example ? I couldn't reach advise about it, the is no paragraph about VREF. Is it an input or an output? What about generating 5V, do we need a 3V3 rail ?

 

Also, even if switching frequencies are different, why not group L5970AD and L5970D in one unique datasheet ?

 

Thank you for reading,

A++ Alex

 

 

[AlexMachine]

Reading back my post it seems obvious it is a 3V3 reference output I think. But the input can be as low as 1.2V. And when I see "Step-Down" regulator I am bugging. If VREF can output 3V3 for output references purposes while beeing supplied by 1.2V it is astonishing. In the pining table the "I/O" type column is appreciable. So there is an internal buck-boost to create a 3V3 ?

[AlexMachine]

Okay I spoke too quick.

Sorry I think you can delete this forum thread.

VREF is a 3V3 is a high impedance output for reference purpose.

L5970A with [4.4 - 35V] input is a buck

L5970AD with [1.2 - 35V] input has a power buck for 1Amp out and has something to build this 3V3 (charge pump, buck-boost...?).

 

Please keep on your super products.

What could be explained here is this elevator possibility for VREF in L5970AD when VCC < 3V3, while being step down.

Hope you don't loose time.

I am going to let VREF in the air!

Anyway super job. Waiting about ST WiFi MCU.

Alex

[Peter BENSCH]

@AlexMachine The L5970A is a buck regulator that can regulate an input voltage at VCC to a lower output voltage, the input voltage may be between 4.4V and 36V.

The VREF pin provides an auxiliary voltage of 3.3V, which can be used as a reference for external devices and can be loaded with a maximum of about 5mA.

The FB pin is the feedback input, which is used to check the output voltage. If there is a deviation from the internal reference of 1.235V, the buck can adjust accordingly. If you connect FB directly to the output, the buck regulates to an output voltage of 1.235V. For other voltages, a voltage divider must be used as shown in the data sheet.

L5970D and L5970AD have two different data sheets because they differ in a number of parameters, including the switching frequency.

Hope that answers your questions?

Regards
/Peter

[AlexMachine]

Okay thank you both for your replies.

I made a mistake : neither D or AD version can receive under 4V4 input! 

@Richard Li why would you put a test point on VREF? Okay if there is room. Otherwise would you use it to quickly test the bloc?

Always good to have a 3v3 available but not beeing protected (5mA) it can be a trap. And beeing poluted by 1Amp switching, not good ref I would say. My 1Amp 3V3 main out will be better ^^^^. Let's tell me what you think if you wish, I am not sure.

I imagine using this VREF for ADC/DAC reference or high Z comparator input. What could be other use? From here for precision I wouldn't use this kind of signal without LDO, ref chip or severe filtering, and you? Maybe low power little signal Opamp supply?

 

Thank to you Peter and Richard. @Peter BENSCH yes I got my answers. Do you think it's a good choice or can you advise a newer ref? The price is interesting, and the SOIC package is nice for little series circuits, fabless soldering and signal access, but efficiency doesn't benefit of new technologies right? Effiency is about 80% for 12 to 3V3. I guess one could reach like 92% with new chips. Knowing that L5970 is 80 cents of euros for 50 parts. 

As we know that is often about cost, size and efficiency 

 

Why do we speak english here ? ^^^ 

Anyway very impressive to maintain a 20 years old circuit. Waiting for comments but I go for it cause the trade off matches my app and there is stock!

 

Anyway I never saw a DCDC which needs reference input, it's non sense. The DCDC converter is always using internal reference, I think! It's one of its main jobs. I was in the moon when asking ^^

DCDC controllers maybe can need it. Never needed it for now. I guess they are made for high power purposes with external transistors.

 

Good sunday 

 

[Richard Li]

One value not clear for me, it is page 3, Sync function input maximum high input - Vref, so if this input signal higher than 3.3V, you may need add external Vref,

No detail for datasheet page 4 Vref buffer, So I don't know this signal can use for bi-direction or not.

Peter work for ST, his answer Vref only for output.

This Vref isn't normally we talk feedback reference voltage, this like use for inside threshold for input signal.

This why I suggestion you add test pointer in case this Vref can work for bi-direction.    

[AlexMachine]

Let's question any rabbit ^^

 

L5979AD VSYNC description

 

3. Pin Description

2 SYNC Master/Slave Synchronization. When it is open, a signal synchronous with the turn-off of the internal power is present at the pin. When connected to an external signal at a frequency higher than the internal one, then the device is synchronized by the external signal.

Connecting together the SYNC pin of two devices, the one with the higher frequency works as master and the other one, works as slave.

 

I don't see relation between VREF and SYNC except error. 

 

SYNC is 4V max before fume.

[AlexMachine]

The signal type column is helpfull.

 

SYNC is outputing the switching frequency exept if you feed it with a brother SYNC signal as I understand.

The one with lower frequency becames an input. No.... I lack something

 

I dont use SYNC in my app cause 1 amp is sufficient but it is interesting

[AlexMachine]

For what purpose is this 3V3 VREF created?

 

[AlexMachine]

@Peter BENSCH ok you had the solution mark but we are still wanting to discuss with you ^^^^^

[Peter BENSCH]

No, the mark as Accept as solution was set by yourself, you can also reset it with “Not the Solution”.

[AlexMachine]

Okay yes I see it.

I believe SYNC is either NC or to an other SYNC so no risk. Maybe some TVS diode for maniac design? ^^^^   Maybe you want to feed SYNC with an other signal?

 

[Richard Li]

In datasheet, if you want different frequency, you can use SYNC feed external signal (table 3) 

[AlexMachine]

I understand DCDC regulation with duty cycle but when it is frequency, I just trust chip designers for now

[Richard Li]

High frequency can use lower value inductor, it can reduce design area, so most DCDC try increase frequency, of cause it is more difficult compare lower frequency.

For example, higher frequency stability is more challenge.

[AlexMachine]

Depends, because high frequency is more loss in semiC no?

Interesting to separate spectrums from supply EMI and low freq app like audio, to ease filtering.

For L5970 SYNC must be <3V3.

But this 3V3 is the fifth weel of the charette.

PGOOD

What shows the app note?

Regards

[AlexMachine]

Okay interesting. 

I imagine changing nominal frequency can help in reducing component size or displace EMI spectrum?

 

Please Richard, how would you use this VREF? It is poluted and High Z. 

[Richard Li]

I didn't use it, I did attached two examples in early ST document, please look my early attachment.

[Richard Li]

Attached again, page 15

[AlexMachine]

A VREF of 3.6V would be more interesting so we can drop with better regulation (LDO, Ref chip).

I wonder what to do with this 3v3 and I would have liked more clearance in datasheet with a signal type colum in pinout table, to be constructive.

 

For a 5mA poluted ref I prefer a zener or a resistor divider.

 

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