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Selection of Buck Converter with Negative Output

Vijeendra Nadgir
Intellectual 265 points
Other Parts Discussed in Thread: TPS63700, TL497A, LT1054, TPS62150, TPS62175

We need your support

1. To select the part from the list.

2. If there are any other efficient/mathing part that I have missed

 

Requirement :  We need two Buck Converters with Inverted output

Requirement Spec

Requirement 1

Buck & Negative Output Module / Inverting Converters

Requirement Specs

Voltage Source

Battery

Vin

+12V

Vout

(-) 3V or (-)5V

Iout

Min :100 mA

Typ : 300 mA

 

Application Details
  • -ve voltage will be connected to OPA547 Op Amp
  • Op-Amp’s +ve Supply  IS    12V from Battery
  • Op-Amp’s Load           IS      Inductive load of  500 mA
  • (Load +ve is connected to op-amp output & -ve is grounded)

 

Requirement 2

Same as requirement 1 EXCEPT Vin is in the range of 3.3V to 5V

 We went through Power Management parts and shortlisted these parts :

Part # Mnfr Category Iout (Max) Vin (min - Max) Vout (Min - Max) # Pins Package Approx Price
PTNN78000A TI Buck/Boost & Negative Output Module 1.5 7 - 29 -15 to -3 5 S M / T T $8.25
TPS63700  TI Inverting Converters 1 2.7 - 5.5 -15 to -2   SON $0.90
TL497A  TI Inverting Converters 0.5 4.5 - 12 -25 to 30   DIP/SOIC $0.86
LT1054 TI Buck/Boost Charge Pump 0.1 3.5 - 15 -5   DIP/SOIC $1.00

 I have another question :

While discussing on op-amp forum for part # OPA547,  MC 33063 is recommended.

My query regarding this part is :

I am seeing of questions on forum pertaining to issues like part getting heated, drop in output voltage.  So need your suggestion to use this part.

Also I want to understand this calcuation :

Vin - Vout(Vin + |Vout|)  should be less than 40 V

The other Vout that is provided between the lines, what does it mean?

Refering Fig 10 in the data sheet, how to calcuate the source current, (data sheet provided current for -12V output)

On referring different figures on schematic, Vin is quite confusing. For this configuraton (Inverting Output) what is the range of Vin (Min to Max)

General Question :

I cam across two categories

1. Buck converters with positive output, but can also be used for negative output by refering application diagrams provided in the data sheet

2. Inverting converters that are designed for negative output.

What is the difference between these two categories and which category is recommended?

 

 

 

  • 0
    TI__Guru**** 191445 points

    Most buck converters can be configured as inverting buck boost.  The current for the output is derated by a factor of (1-d) for the inverting buck boost relative to the simple buck case.  Since the IC ground is now at -Vout potential rather than ground, Vin -(-Vout) must be less than the input voltage rating of the device.  To get started, you could look at:

    http://www.ti.com/lit/an/slva317b/slva317b.pdf

    and

    http://www.ti.com/lit/an/slyt286/slyt286.pdf

  • 0 in reply to JohnTucker
    Intellectual 265 points

    Thanks John, for basics.

     

    But still my core question is open, do I need it to elaborate my question/explanation?

  • 0 in reply to Vijeendra Nadgir
    TI__Guru*** 145930 points

    Your power requirements are not completely clear.  Since your 12V is from a battery, there is likely quite a bit of variation above and below this 12V that the input source will output.  This range is critical to know to select the right device.  As well, your maximum output current needed is not completely clear.

    I recommend looking at the TPS62175 for your 12V input inverter and the TPS62150 family for your 3.3V or 5V input inverter.  There are app notes on the product page showing how to use these as inverters.

  • 0 in reply to Chris Glaser
    Intellectual 265 points

    Voltage source - Its regulated voltage, used to power up other components on Backhoe loader

    Maximum Output current - As we are in the design stage, we have the option to change the op-amp, op-amp is provided with -ve supply.  so prefrred is 300 mA, but even greater than 100 mA can be used.