TPS54060: Creating an ±15V supplies that track eachother

Hi John,

Input power capability is 24Vx200mA=4.8W, set conversion eff=75%, output power is 4.8x75%=3.6W, so +/-15V only can output 120mA.

TPS64060 is OK for this conversion, pls refer to Application Note

SNVA829–June 2018

How to Design a Simple Constant Current/Constant Voltage Buck Converter

B R

Andy

Hi Andy,

Thanks for the super quick response and sorry if I was not more clear. This is not a constant current output, it is a split rail output where the load can be as high as 200mA. The question is how to make the two tails +15V and -15V track each other on power up and power down. I don't see anything in the SNVA829 on how to implement tracking between two rails. Do we have any documentation on how to handle this?

John

Hi John,

For the circuit in SNVA829, the coupled inductor deliver energy to +V and -V output equally, so if the load current of two rail is similar, then they will have same slew rate at power up and down.

B R

Andy

Hi Andy,

Sorry if I am missing something but I thing L1 in Figure 3 is the coupled inductor but it doesn't show a second rail or talk about having split rails at all. I also search "coupled" and don't see anything. 

https://www.ti.com/lit/an/snva829/snva829.pdf

Is there a better app not that could show my customer how to implement this? 

John

Hi John,

Sorry, the correct AN is below:

SLVA369A–October 2009–Revised October 2012
Creating a Split-Rail Power Supply With a Wide Input Voltage Buck Regulator

B R

Andy

Thanks Andy! 

One more question. Have you seen any designs where someone successfully placed an inverting op amp between one buck output and the other SS/TR pin? Say from the inverting output to the positive tracking pin?

My concern is that I think you would need another voltage rail in the system to power the op amps because at start up the negative rail would turn on but the op amp wouldn't be able to drive a positive voltage until without a positive supply. Meaning the positive buck would never get a raising voltage on the tracking input. But if you had an op amp that could handle the 24V input to the negative rail, I think this could work. 

What do you think? Thanks for the help and sending the new app note. Makes sense now

John

Sorry for the delay in responding, I wasn't trying to get more information from the customer. One more follow up question. What I described above will work for one supply tracking the other, but is there a way to have both supplies track each other? Lets say if the positive voltage rail had a fault and the voltage dropped, in my example above, the negative rail would stay at -15V since it isn't tracking the first. I just don't understand how power up and power down would work if neither rail "lead" in powering up. 

Have you seen something like that before. 

Hi John,

I think there should be one output working as master channel to control the ramp up/down. the circuit may not startup if both supplies track each other.

Tracking ramp up is easy to understand, if master rail drop, then slave rail converter operating at forced PWM mode can also regulate the output to decrease quickly to tracking the master's drop.

To improve reliability, you can add a over voltage protection circuit by monitoring the average voltage of positive and negative rail,

B R

Andy

Hi Andy,

Thanks for all you help and sorry for the delay in responding. I don't know how to explain how the FPWM mode can help the device better regulate the output rail when it is ramping down? I understand that the device will not change it's switching frequency like it would in PFM mode but I don't understand why this helps the control ramp down of the output voltage. I was trying to find a white paper describing this better and couldn't find one. 

John

Hi John,

For FPWM mode, if output is higher than target, the inductor average current will be regulated to flow from output cap into converter Low side FET to ground, this will discharge the output quickly to track the reference.

B R

Andy