Control of a Power Supply
Digital Control of a
Any analogue power supply can be controlled
digitally as long as the designer has access to the
feedback pin. FIG 1 shows a typical buck regulator
based on the LTC3864.
Normally the output voltage is set using two
feedback resistors, R1 and R2 and the chip regulates
the output voltage to keep the junction of these two
resistors at a set voltage. In the case of the
LTC3864 it is 0.8V.
An additional voltage (say, from a DAC) can be used
to pull the feedback pin up and down to thus adjust
the output voltage. A design example is shown below.
To Design a Digitally
Adjustable Power Supply with Vout Ranging from 5V to
We are going to calculate the feedback resistors
needed to adjust the output voltage of the above
power supply from 5V to 15V using a DAC with an
output voltage from 0V to 3v3.
It can be seen that when the DAC voltage is high,
the FB pin will be pulled high, so the chip will
compensate by reducing the output voltage. Likewise
when the DAC voltage is low, the FB pin will be
pulled down, so the output voltage will rise. Thus
the DAC output voltage is in antiphase with the
power supply output voltage.
Summing the currents into the FB node, we can see
that the current flowing down R1 added to the
current flowing from VSET through R3 is equal to the
current flowing in R2, since no current flows into
the FB pin.
VFB = 0.8V.
When the DAC output is 3v3, the output voltage needs
to be 5V, thus
Let R2 = 10k
When the DAC output is 0V, the output voltage needs
to be 15V, thus
Equating the above two equations gives
Putting this back into equation 1 gives
So R3 = 48.576k
So R1 = 147.187k
It is worth putting these values back into the
original two equations to make sure they balance.
Once the calculations are correct, select preferred
resistor values closest to the calculated values. If
R3 is lowered, the DAC voltage will have more
influence over the feedback voltage meaning VDAC
will not have to rise to 3v3 to achieve 5V or go
down to 0V to achieve 15V. The simulations shows the
output voltage starting at 15.0V and decaying to
4.7V since the value for R3 is 47k and not the 48.5k
we calculated. Either make up 48.576k using other
resistors or write the software to ensure the DAC
voltage goes not go up to 3v3.
An LTspice simulation demonstrating this is shown in
based solutions for implementing digital control of
power supplies are provided by Linear Technology.
Please refer to: LTC2974, LTC2977, LTC3883, LTC3880,
The circuit of FIG 2 can be downloaded here:
Digitally Controlled Power Supply
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