I found this interesting IC on Linear Technologies site, but I don't understand if it, or its development board, can both boost input voltage to output and buck output voltage back to input. http://www.linear.com/product/LT8705#demoboards Board: http://www.linear.com/demo/DC1924A

I want to use it (or whatelse) to connect a 500F/2.7V supercapacitor to my 60V electric scooter, which do not have regenerative breaking, hence I must use the battery to recharge it. I just need a 5 seconds boost every some minutes.

  • \$\begingroup\$ Sounds like "Forced Continuous Mode" is what you're looking for. \$\endgroup\$ Aug 3, 2013 at 16:54
  • \$\begingroup\$ Sounds like you'd misunderstood the terms boot and buck which relate to transistor/inductor topology, not the capability of the circuit to feed a stored load charge back into the source. \$\endgroup\$
    – Jay M
    Aug 4, 2013 at 12:20

3 Answers 3


I don't understand if it, or its development board, can both boost input voltage to output and buck output voltage back to input.

LT8705 is the IC and using LT8705 is the evaluation board/development board is DC1924. It can work as both boost and buck converter and can generate output voltage above, equal to or below the input voltage.

Power Control Switch

Datsheet shows a simplified diagram of how the four power switches are connected to the inductor, VIN, VOUT and ground. Figure also shows the regions of operation for the LT8705 as a function of VOUT -VIN or switch duty cycle DC. The power switches are properly controlled so the transfer between modes is continuous.

modes of operation


TI has some buck converters which can, as you mentioned, buck down in forward path and boost back in the reverse path. TPS54320 is one of them. However, what you really seem to need is a buck-boost converter as you would want to use all the energy stored in the supercap by draining the voltage all the way close to 0. For that, you should be able to reconfigure most run of the mill buck converters in the way this app note tells us:


I am not sure if the IC discussed in the app note is capable of bidirectional power, but if you take care of the modified control loop, you should be able to use TPS54320 for what you want. The same set of switches, control loop should automatically regulate the rail of the motor drive supply. And also provide a shunt energy buffering circuit.


"The LT®8705 is a high performance buck-boost switching regulator controller that operates from input voltages above, below or equal to the output voltage."

A buck boost concept is not that much different than regenerative braking. in both cases a full H bridge is used. In the buck-boost charger, the voltage can be higher, lower or equal and transfer current to the battery.

By using the desired charge profile (fast,slow,float) it will supply the required higher average output voltage than the open circuit battery voltage. It then controls the average voltage and current by sensing both and regulating with the correct Mosfet switches according to the profile selected (fast, slow) to change the PWM duty cycle.

Now the re-generative brake does a similar thing except the rear brake would be supplemented with dumping the motor energy rectified and regulated back into the battery. There are limits to battery current, just like fast accelerating and fast charging, age the battery faster, you might be able to replace the use of the rear meachanic brake with an electronic break, but also perhaps not without limits.

But let's think about the energy you want to use.

Ecap=1/2 C*V ^2 = 1/2 500F * 2.7^2 = 1822 Wh * efficiency of conversion. Ebattery charger= 60?V * 5?A * 3?h = 900 Wh

So both methods would be very useful to incorporate Ecap and re-gen charging, but losses can be significant.


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