# LT8228: Additional Circuitry for Programmable Output Voltage?

Hello everybody,

I want to be able to dynamically program the output voltage ofthe LT8228 from a MCU, across a large range from say 10V to 60V. For a Buck only design.

I was thinking I could create an op amp gain stage between the voltage divider (RFB2A, RFB2A) and the feedback pin FB2. Assuming no significant delay is introduced to the feedback path, is there any reason why it wouldn’t work?

If not like this, can anybody recommend any approaches?

Related, what is a sensible current range through the voltage divider?

The data sheet states “The bottom resistors RFB2B and RFB1B are selected to be 1.21kΩ for 1mA bias current in the resistor dividers.”

• Too low current would introduce noise into FB2.
• Too high current would impact efficiency. If the voltage divider current is much less than load current?

Thanks

just clarified what IC
[edited by: Chendy at 5:24 PM (GMT -4) on 27 Oct 2020]
• Hello Chendy,

My name is Chris Gass. I am the factory application engineer supporting this part.

There are two alternatives to make an adjustable output voltage:

1. Use a digipot in place of the bottom resistor.

2. Use a DAC. If you use a voltage DAC, then place a resistor between the DAC output and the FB pin. The current flowing through the resistor will add or subtract from the top resistor changing the output voltage. The reference is 1.2 V on the LT8228. Therefore, if the DAC voltage was lower than 1.2 V, then the output voltage would go higher. If the the DAC voltage is higher than 1.2 V, then the output voltage will be lower.

Generally speaking, 1 mA through the bottom resistor is robust. This current is then shared between the top resistor and the resistor in series with the DAC.

You can play with this in circuit and see the effect. I think you can safely go down to 100 uA.

Chris

• Hello Chendy,

My name is Chris Gass. I am the factory application engineer supporting this part.

There are two alternatives to make an adjustable output voltage:

1. Use a digipot in place of the bottom resistor.

2. Use a DAC. If you use a voltage DAC, then place a resistor between the DAC output and the FB pin. The current flowing through the resistor will add or subtract from the top resistor changing the output voltage. The reference is 1.2 V on the LT8228. Therefore, if the DAC voltage was lower than 1.2 V, then the output voltage would go higher. If the the DAC voltage is higher than 1.2 V, then the output voltage will be lower.

Generally speaking, 1 mA through the bottom resistor is robust. This current is then shared between the top resistor and the resistor in series with the DAC.

You can play with this in circuit and see the effect. I think you can safely go down to 100 uA.

Chris