Dual Supply Boost/Inverting Conver

[polfort]

Hi there.

Looking at page 16 of this datasheet: http://cds.linear.com/docs/Datasheet/3471fb.pdf
I am wondering if I could lower the Voltage at pins 10 & 6 using some diodes in series.

Why I want to do so: (Just looking at one part, namely the positive voltage)
The internal switching transistors of the LT3471 are capable of switching a maximum Voltage of 42V, however I want to produce a output voltage of +45V. This would result in exceeding that max. voltage about 5V + the forward voltage of D1.

What do You think about it?

The second issue is the following:
The max. current through the switching transistors is: 1.3A.
I already simulated that circuit with LTspice and realised, that the max. current flowing into pin 10 is about 2A. Not continously of course and only during the first 400us. The peaks are 126ns wide and occur with a frequency of 125kHz (8us spacing between them)

The switch current limit is specified as 2.6A at min. duty cycle and 2A at max. duty cycle.
I am quite not sure if that is the current through the switching transistors.. Hmm, it should..

Hope You could follow..

 

[Hero999]

Hi there.

Looking at page 16 of this datasheet: http://cds.linear.com/docs/Datasheet/3471fb.pdf
I am wondering if I could lower the Voltage at pins 10 & 6 using some diodes in series.

Why would you want to do that?

The output voltage can be changed by altering the potential divider rations as describe on the datasheet.

Why I want to do so: (Just looking at one part, namely the positive voltage)
The internal switching transistors of the LT3471 are capable of switching a maximum Voltage of 42V, however I want to produce a output voltage of +45V. This would result in exceeding that max. voltage about 5V + the forward voltage of D1.

What do You think about it?

The second issue is the following:
The max. current through the switching transistors is: 1.3A.
I already simulated that circuit with LTspice and realised, that the max. current flowing into pin 10 is about 2A. Not continously of course and only during the first 400us. The peaks are 126ns wide and occur with a frequency of 125kHz (8us spacing between them)

The switch current limit is specified as 2.6A at min. duty cycle and 2A at max. duty cycle.
I am quite not sure if that is the current through the switching transistors.. Hmm, it should..

Hope You could follow..

If you want to boost the output voltage and current ratings then you could try boosting the internal transisors by adding external transistors or MOSFETs but I think you're better off looking at LT's website for a more powerful IC or one which is specifically designed to drive external MOSFETS or BJTs.

 

[polfort]

Of course I know about setting the feedback ratio using the voltage divider, this is a fundametal of the LT3471.
Anyway thank You for that advise.

In the meantime I have found another solution for limiting the switching current.
Actually I should have thought about it earlier..
By using a RC combination at the Pins Vshdn I am able to limit the current to a value about 1.2A.
This is a kind of soft start. For more details I refer to the datasheet.

regards

ps: the reason I didnt want to add external switching transistors is, that I already have one LT3471 in my design. Thus I didnt want to use two different IC