Frequency-synchronized positive and negative step-down DC/DC converters

Hi Pavel,

you already have +18V and -18V power supply, and just need conversion for +10V and -5V output, right? 

B R

Andy

Hi Andy,

Actually the project is still in the design stage. I have +24V. The plan to make +18/-18V synchronous freq is using e.g. 2x LM5575 which can be synchronized master -> slave and can run at >400kHz, IIUC.

But I would not mind a synchronous central power manager with external mosfets (should it make sense), the power scheme is something like

+24V -> +18V (1A) -> +10V (100mA) (or directly from +24V)

          -> -18V (1A) -> -5V (100mA)

          -> +5V (4A)

Thank a lot for your help, I very much appreciate it.

Regards,

Pavel.

Hi Pavel:

Below recommended solution:

1. LM61440 for 24V to 5V/4A

2. LM63615 or LMR16010 for 24V to 18V/1A

3. LMR16020 for  24V to -18V/1A

4. LMR16010 for 24V to 10V and -5V, with 2:1 coupled inductor

All these parts have freq sync features.

B R

Andy

Hi Andy,

Thanks a lot! Please can you direct me at more details for your recommendation LMR16010 for 24V to 10V and -5V, with 2:1 coupled inductor? Could one chip generate two stabilized voltages +10V/-5V?

With regards,

Pavel.

Hi Pavel,

Pls refer to below AN:

Application Report: SLVA369A–
Creating a Split-Rail Power Supply With a Wide Input Voltage Buck Regulator

LMR16010 is configured as normal step down converter for 24v to 10V, the inductor has two winding, the secondary will be -5V output.

B R

Andy

Andy, thank you very much. Do I understand correctly that the converter regulates the voltage diff between the two outputs in the dual-output circuit? Nevertheless if I wanted to avoid the (harder to source) 2:1 coupled inductor and regulate each voltage separately one LMR16010 could be used for each voltage, is that correct?

Can the RT/SYNC pins be joined for non-inverting as well as inverting converters? The sync pins seem to be based on the negative output voltage in the inverting circuits.

Is there a way to make one of the converters master and feed its clock to other slaved converters, or does it require a separate master sync clock? Since no master/slave operation is mentioned in the datasheet I assume that an external clock is required.

Thank you,

Pavel.

Hi Pavel,

If it's difficult to source 2:1 coupled inductor, then you need to use 2pcs LMR16010 converters for 10V and -5V.

You can set LM61440 or LM63615 at FPWM mode, then the SW node has fixed switching freq, this signal can be reformed as master clock to sync other converters

It require level shift circuit to inject clock to negative converter as ground is shifted,

B R

Andy 

Hi Andy,

Thanks a lot. Could just a capacitor be used as the level shifting circuit to RT/SYNC of the negative converter, as the AC coupling depicted in Figure 17 of https://www.ti.com/lit/ds/symlink/lmr16010.pdf , along with Rt to the respective GND for freewheeling at startup?

If so, then perhaps a simple 3.3V schmitt trigger hooked to SW node via some 3.3V voltage limiter directly to RT/SYNC of the positive converters and through 3.3V/0V analog switches flipping to a termination 50R before the 10pF coupling cap to the RT/SYNC + Rt of the negative converters? Is there perhaps any TI AN or design  which handles the synchronization of positive and negative converters?

Thank you very much for your help.

BR,

Pavel.

Hi Pavel,

First pls review if the suggested solution works for your application including part availability.

Clock coupling is easy and should not be a barrier.

B R
Andy

Hi Andy,

E.g. LMR16010 looks very good and while not obtainable in larger quantities now, they seem to be sourceable in single quantities for prototyping from third-party stocks.

The only reference I could find to the SW-> sync connection was in https://www.ti.com/lit/an/slvaeg8/slvaeg8.pdf Fig. 13 (pg. 11) But the Rfil/Cfil filter is not further specified - is its function to keep the minimum required clock pulse width in case the master converter is just lightly loaded and the PWM SW pulse is too short?

Thank you very much,

Pavel.