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Searching for DC-DC step down solution

Other Parts Discussed in Thread: LM5010, LM5085, LM5008A

Hi there,

I am searching for a DC-DC stepdown converter with up to at least 80V maximum input voltage, 200-300mA maximum output current and a programmable output voltage. The output will be adjusted to a voltage between 7V and 12V. The input voltage will range between 7 and 70V. An important feature is that when the input voltage drops below the output voltage, the output must track the input (of course minus a certain drop out voltage). So the converter is not allowed to shutdown in this mode.

Is this possible with an LM5010?, LM250xx or any other TI chip?

Thank you!

  •  

    Daniel,

    I not aware of any regulator/controller that will meet all of your requirements, but there are several COT parts you should look at, the LM5008A and the LM5085.  The LM5008A will operate from 6V to 95V, but when VIN is 7V, the LM5008A can't achieve a 100% duty cycle so Vout will fall out of regulation.

    Another option is the LM5085 which is a PFET Controller.  The advantage of this part is it can operate at 100% duty cycle, so with VIN=7V, Vout will be very close to 7V.  The LM5085 is limited to a maximum VIN of 75V maximum and as VIN increases the efficiency will be reduced.

    Terry Allinder

  • Hi Terry,

    Thank you for your reply. When we would implement the LM5008A, what would happen to Vout if Vin drops below 7V? Obviously, when Vout was originally set to 12V it has already dropped below 7V due to the 'drop out' voltage of the regulator. But at which input voltage will Vout immediately drop to zero because of loss of bias?

    We can allow a situation where Vin has dropped down to 6.5V-7V, while Vout is approximately 5V. Only at a Vin below 6V, Vout is allowed to drop to zero.

    What is important as well, is that the regulator is capable to start up at a Vin of 7V.

    Thank you for your support.

    Daniel...

  •  

    Daniel,

    The LM5008A will turn-off and the output will drop to Zero when the VCC voltage is 5.1 V typical.  When VIN is in the range of 4V-8.1V typical VCC will track VIN.  When VIN>8.2V the LM5008A internal VCC regulator kicks in and provides a regulated 7V VCC, refer to Figure 4 in the data sheet.

    If you refer to the data sheet Figure 10 it shows that when VCC reaches the VCCUVLO threshold (5.3V typical) that the Buck Switch is enabled.  If you have an external bias voltage, >8.5V  but less than 14V, you can Diode OR that into the VCC pin to keep the LM5008A running at low VIN.

    Terry

  • Hi Terry,

    We are now in a stage that we need to make some decisions regarding the DC-DC solution in the above mentioned project. Besides starting to evaluate the low input voltage performance of the LM5008A solution, we would also like to setup a test with the LM5085 PFET controller.

    Looking at the calculation example in the LM5085 datasheet on page 15, I have difficulties in understanding how TI has selected the Vishay Si7465 PMOSFET for this application. This MOSFET has a rated continuous drain current of 3.2A max, while the maximum output current of the converter is 5A. Page 16 shows that the peak current under short circuit conditions can reach 11A. Isn't the Si7465 MOSFET too 'light' for this application?

    Translating this to our application (7-70VDC input, 12V output, 300mA maximum load current), what should be a good choice for the PFET? I think we should at least choose a -100V device to get some voltage headroom. But I am a bit lost in the maximum drain current needs for our application. I have checked the Vishay SUD09P10-195 , but maybe this DPAK device is a bit too much for our application. Small footprint and low BOM costs are very important.

    I am looking forward hearing your thoughts.

    Thank you.

    Daniel...

  • Daniel,

    The calculations for the CL on page 15 are worst case, but I agree the Si7465 maybe a little light.  For your application I would pick a FET with a 100V rating.  The nominal VGS provided to the P- FET is 7.7V with VIN >8.5V.  In your application VIN is 7V, so make sure the FET you pick will be fully turned on with 7.0V.  As far as the current rating I would pick a FET with a low Rdson and  Qg, vs cost.  Both Rdson and Qg affect the power losses.

    The SUD09P10-195 will certainly work, but you could get away with the higher Rdson FET.

    Terry

  • Thanks for your help Terry!