
Keywords: DC/DC Converter, resistive feedback divider network, current DAC, fullscale current, source current, sink current, output voltage margin
Related Parts


Using the DS4402/DS4404 to Margin the Output Voltage of a DCDC Converter

Abstract: A DS4402 or DS4404 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 in a DCDC converter's feedback divider network when a DS4402 or DS4404 is employed in the design.
The Adjustable Power Supply
The
DS4402/DS4404 DACs contain two/four 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 DS4402/DS4404 sink and source current from their 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 DS4402/DS4404 is most commonly used to adjust the DCDC converter's feedback voltagedivider.
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 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 choose the maximum margin on V
_{OUT}, V
_{OUTMAX}, and the maximum I
_{RA}, 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 DS4402/DS4404 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 (Equation 1 in the DS4402/DS4404 data sheet):
With V
_{REF} = 1.23V, we solve Equation 8 and find that R
_{FS} needs to be 9.53kΩ to produce a 1mA fullscale current.
With the DS4402/DS4404 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 DS4402/DS4404 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 that I
_{FS} = 0A, we create a simple voltagedivider equation:
We assume that the desired nominal value for V
_{OUT} is 2.0V and that 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:
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 DS4402/DS4404 DACs control the margin of V
_{OUT} on 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 
DS4402 
Two/FourChannel, I²C Adjustable Current DAC 
Samples

DS4404 
Two/FourChannel, I²C Adjustable Current DAC 
Samples

© Jun 18, 2007, Maxim Integrated Products, Inc.

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APP 4055: Jun 18, 2007
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AN4055,
AN 4055,
APP4055,
Appnote4055,
Appnote 4055
