TPS54560: Inverting Power Supply Circuit

Mert Kantarcioglu

Part Number: TPS54560

Dear All,

We have designed Inverting Power Supply Circuit using TPS54560. I need to get +18V and -18V 4Amps from 36V Vin. The circuit for +18VDC works as we expected, but for -18V we could not get 4Amps, the output voltage drops according to load. In addition to schematic we add 1uF ceramic cap between Vin(2) and GND(7) pins. Do you have any idea for the reason?

Schematics, layout and Webench output are given below. All components choosen according webench part list.

/cfs-file/__key/communityserver-discussions-components-files/196/EK_2D00_5_5F00_TPS54560_5F00_SS_5F00_SIM.pdf

over 7 years ago

0 JohnTucker over 7 years ago

TI__Guru**** 191445 points

I don't believe you can get 4 A from that -18 V circuit. You have to derate the current by a factor of 1-D.

0 YangZhang over 7 years ago

TI__Genius 11085 points

If you use a buck converter as inverting buck-boost, the load current at the negative output will be much less than the rated current of the buck converter. Inductor current only conducts to the output capacitor during the diode conduction time, which is (1-D)Ts time period. If loss is ignored, a quick estimation of the max negative current can be found by
Inverting Ioutmax = Ioutmax(buck)*VIN/(VIN+|VOUT|).
With your 18V to -18V, you will get about 2.5A out of the -18V rail.
-Yang

0 Mert Kantarcioglu over 7 years ago in reply to YangZhang

Intellectual 390 points

Dear YangZhang,

Thank you for detailed explanation. I would like to just clarify one more issue.

Since Vi=36V according to your formula;

inverting Ioutmax = 7.5A (Ioutmax, really data sheet max) * 36V / 54V = 5A or 5 (Ioutmax, normal max) * 36V / 54V = 3.33A

Is it possible to get 5A or 3.33A from this inverting buck? Because 3 Amps will be OK for me at worst case. If yes, how could I do it? I have made new calculations according to application note below, but issue is messy. I have stucked.

/cfs-file/__key/communityserver-discussions-components-files/196/EK_2D00_4_5F00_TPS54560_5F00_AN.pdf

0 YangZhang over 7 years ago in reply to Mert Kantarcioglu

TI__Genius 11085 points

If the input voltage is 36V, yes, from the formula, you should be able to get 3.3A. But the formula did not consider the loss in the converter. You may get 3A max.
One more thing to be careful when you increase the supply voltage is that the input voltage seen by the TPS54560 equals VIN+|VOUT| = 36+18=54V. This device is rated at 60V max input voltage. So make sure the 36V rail is well regulated and/or voltage limited.

-Yang

0 Mert Kantarcioglu over 7 years ago in reply to YangZhang

Intellectual 390 points

I have find the calculation excel sheet for TPS54560 inverting buck design on its website. Now I have calculated all the values for inverting design and will try on circuit asap. I hope it will work for 3 Amps.

By the way I would like to ask another question. For the +18VDC design with TPS54560, compensation resistance and capacitors values are different in webench simulation and excel sheet given on website. O could not understand why. You can find them below. Which one should we use?

Webench simulation: /cfs-file/__key/communityserver-discussions-components-files/196/0753.EK_2D00_5_5F00_TPS54560_5F00_SS_5F00_SIM.pdf

Excel Sheet: /cfs-file/__key/communityserver-discussions-components-files/196/TPS54360_2D00_361FAMILY_5F00_CALC_5F00_TOOL_5F00_REVE.xls

0 JohnTucker over 7 years ago in reply to Mert Kantarcioglu

TI__Guru**** 191445 points

It is normal for webench to calculate different values. The excel spreadsheet uses a simple formula and Webench uses an iterative algorithm.

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TPS54560: Inverting Power Supply Circuit