
Keywords: DCDC converter, resistive feedback divider network, current DAC, fullscale current, source current, sink current, output voltage margin
Related Parts


Using the DS4412 to Margin the Output Voltage of a DCDC Converter

Abstract: A DS4412 adjustablecurrent DAC is used to adjust the margin of a DCDC converter's output voltage. This article describes how to properly select the resistor values of a DCDC converter's feedback divider network when a DS4412 is employed in the design.
The Adjustable Power Supply
The
DS4412 contains two I²C adjustable current sources capable of sinking and sourcing current. A typical application for these DACs is margining the output voltage of a DCDC converter. (See
Figure 1.)
Figure 1. DCDC converter circuit with adjustablecurrent DACs used to margin the converter's output voltage.
The DS4412 sinks and sources from its OUT pins. Valid fullscale current values range from 0.5mA to 2.0mA. The value of the fullscale current, I
_{FS}, is determined by the size of the resistor connected to the DAC's FS pin of the corresponding OUT pin. The source/sink current generated by the DS4412 is most commonly used to adjust the DCDC converter's feedback voltage divider.
Determining the Relationship Between V_{OUT} and I_{FS}
Choosing the right I
_{FS} depends on how much margin is desired on the DCDC converter's V
_{OUT} pin. To determine this margin, we must discover the relationship between V
_{OUT} and I
_{FS}.
Summing currents into the V
_{FB} node, we find that:
Where:
And
However, since R
_{B} and V
_{FB} are constant, there is no change in I
_{RB}. Thus:
We are looking for the relationship between the margin on V
_{OUT}, ΔV
_{OUT}, and the selected range of I
_{FS}, ΔI
_{FS}. Since we know that the change in the I
_{FS} current equals the change in the current across R
_{A}, we can subtract one set of V
_{OUT} and I
_{RA} values from another to determine the relationship between V
_{OUT} and I
_{FS}.
First, solving Equation 3 to find V
_{OUT}, we find that:
Use Equation 5 to create two equations. For one equation, we chose the maximum margin on V
_{OUT}, V
_{OUTMAX}, and the maximum I
_{RA} current, I
_{RAMAX}. For the other equation, we choose the nominal values for V
_{OUT} and I
_{RA}, V
_{OUTNOM} and I
_{RANOM}. Subtracting the two equations, we get:
Using Equation 4, Equation 6 translates into the relationship:
Equation 7 shows that the relationship between the margin on V
_{OUT} and I
_{FS} is determined by the value of the resistor R
_{A}.
Calculating the Right Resistor Value for the Margin on V_{OUT}
Now that we know the relationship between V
_{OUT} and I
_{FS}, we can select the correct value of R
_{A} and, thus, R
_{B} to generate the desired margin on V
_{OUT}. Since the fullscale current sink/source range of the DS4412 is 0.5mA to 2.0mA, we select 1mA as the I
_{FS} current for the DAC. To set this value, choose R
_{FS} based on the following equation found on page 6 of the DS4412 datasheet:
With V
_{RFS} = 0.607V, we solve Equation 8 and find that R
_{FS} needs to be 4.612kΩ to produce a 1mA fullscale current.
With the DS4412 I
_{FS} selected, we must determine the size of R
_{A} to achieve the desired margin on V
_{OUT}. A 2.0V V
_{OUT} with a 20% margin requires ±0.4V of change. Sinking and sourcing settings of the DS4412 will manage the sign. The change in I
_{FS} equals the I
_{FS} value of 1mA, and the desired change in V
_{OUT} is 0.4V. After substituting for ΔV
_{OUT} and ΔI
_{FS} in Equation 7, we solve for R
_{A} and get R
_{A} = 400Ω.
Determining the Relationship Between R_{A} and R_{B}
The feedback network of the circuit in Figure 1 is a voltagedivider with resistors R
_{A} and R
_{B}. Looking at Figure 1 and assuming I
_{FS} = 0A, we can create a simple voltagedivider equation.
We assume that the desired nominal value for V
_{OUT} is 2.0V and the DCDC converter has a feedback voltage, V
_{FB}, of 0.8V. Substituting the values for V
_{OUT} and V
_{FB}, the relationship between R
_{A} and R
_{B} is determined to be
We use Equation 10 to solve for R
_{B} and get R
_{B} = 267Ω.
Conclusion
The resistivefeedbackdivider network and the currentsinking/sourcing capabilities of the DS4412 DACs control the margin of V
_{OUT} of a DCDC converter. The relationship between the fullscale current, I
_{FS}, to the margin on V
_{OUT} is determined by the value of the resistor R
_{A}. By choosing the correct I
_{FS} value for your application, you can determine the correct resistor values for the feedback divider network, and achieve the desired margin on V
_{OUT}.
Related Parts 
DS4412 
DualChannel, I²C Adjustable Sink/Source Current DAC 
Samples

© Mar 20, 2008, Maxim Integrated Products, Inc.

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