LT3470 - Negative output regulation by MOSFET

[Krtek]

Dear,
I need help with my design. I used LT3470 in next connection:
SCH is attach.
I need regulate negative output voltage from LT3470 by GATE of MOSFET. Voltage on GATE is 0 to -11V reference to GND.
For 0V, MOSFET is open and Output votage is minimum by FB resistor.
For -11V, MOSFET is closed and Output voltage is maximum.
Is it regular connection and idea?

[Andree Henkel]

What you planning is not going to work.
LT3470 is a positive buck type Regulator - it can only generate positive output voltages.

look for an IC that is explicitly marked as being able of generating a negative voltage from positive input voltage
look for:
*Inverting
*SEPIC
*Cûk
type controller IC

[Jay_Diddy_B]

Krtek,

This circuit will work. By grounding the normal output and taking the output from the ground connection it will work as an inverting regulator.

The LT3470 will have the input plus the output across its supply pins.
The maximum current output will be lower than in the normal configuration.

LTspice model



This is with a fixed output of around 11V.


Adjustable Output

Q1 has been added. It is in common base. The collector is current is equal to emitter current - base current. The emitter current is controlled by V2 and R3.
Note that V2 is referenced to ground.

 3470 inverting output control.asc (2.04 kB - downloaded 15 times.)" alt="" class="bbc_img" />

In the simulation the voltage V2 is being stepped.

I have attached the LTspice model

Regards,

Jay_Diddy_B

 3470 inverting output control.asc (2.04 kB - downloaded 15 times.)

[Krtek]

Hi Jay_Diddy_B,
I fully understand fixed negative output. I don't understand Adjustable output. How can control FB votage by collector current? But It works in simulation.

This connection isn't for me, becouse i have negative control votage on senese resistor. It is from -11V to 0V.

[Jay_Diddy_B]

Hi Jay_Diddy_B,
I fully understand fixed negative output. I don't understand Adjustable output. How can control FB votage by collector current? But It works in simulation.

This connection isn't for me, becouse i have negative control votage on senese resistor. It is from -11V to 0V.

The voltage from the FB to the GND pin is equal to 1.25V when the circuit is in regulation. The current in R1 = 1.25/20k = 62.5 uA.

Without Q1, the same 62.5uA flows in R2. The voltage across R2 = 62.5 uA x 150k = 9.375V

The output voltage equals 9.375V + 1.25V = 10.625V

Q1 is connected in common base. Ic = Ie - Ib, Ib is small and we can consider Ic = Ie.

Ie = (V2 - 0.65V) / R3

The current Ic reduces the current in R2 to maintain equilibrium.

Vout = 1.25 + 150k ( 62.5uA - ((V2 -0.65)/R3))


What is the desired output voltage?
What is the control signal that you have?
Is the control signal referenced to ground or Vout?


Jay_Diddy_B

[Krtek]

Thanks for explainig of output regulation.

I can't public my final design on poblic forum, becouse it is company secret. I am design Howland current pump with Dynamic Power Control.
Output voltage is -15V to -3V. Control signal is -11V to 0V referenced to ground, from sense resistor of current loop.

This is solution only for negative current output. I am resolving this topic in positive part of design too.

[Krtek]

I was inspired by next connection for output power control.

[Jay_Diddy_B]

Krtek,

This is a negative version of the schematic that you posted:





Regards,

Jay_Diddy_B

 3470 inverting tracking current sink.asc (3.21 kB - downloaded 8 times.)

[Andree Henkel]

the magic of simulation...

look at the "ground" symbol below V1, the input Voltage for U1 is provided by V1
so you have to connect this to GND Pin of U1 for U1 to work

now look at "ground" symbol at upper side of output of your circuit

so you have two subnets merged into one via the ground symbol.

I´m really curious, how this is going to work in real life.

[Jay_Diddy_B]

Andree,

This technique of generating a negative voltage with a buck regulator has been documented by several semiconductor manufacturers. Here is a link to an Analog Device document:


https://www.analog.com/en/resources/design-notes/making-a-voltage-inverter-from-a-buck-stepdown-dcdc-converter.html

And from TI:




I have used it several times during my career.

Jay_Diddy_B