
Keywords: DS4404, DS4402, power supply, power supplies, margining, power supply margining, power supply tolerance, tolerance, feedback, resistive, divider, vfb, feedback node, vref, reference voltage, DC/DC, DCDC, converter, current, source, sink
Related Parts


How to Add Margining Capability to a DCDC Converter

Abstract: This application note explains how easy it is to add margining capability to a DCDC converter by connecting it to a DS4404 4channel adjustable current DAC (or the DS4402, 2channel version).
This application note shows how to connect the
DS4404, a 4channel adjustable current DAC (or the DS4402, a 2channel version of the DS4404), to a DCDC converter circuit to add margining capability.
The circuit in
Figure 1 illustrates how simple it is to integrate the DS4404 into an existing design. The DS4404 is added to the feedback node (see the dashed line) so that the V
_{OUT} of the DCDC converter can be adjusted. On power up, the DS4404 outputs a current of 0A (appears as high impedance), essentially making the DS4404 transparent until it is written to through I²C.
Figure 1. Connecting a DS4404 to a DCDC converter feedback circuit.
Assume the following for our example (which are independent of the DS4404):
V
_{IN} = 3V to 5.5V
V
_{OUT} = 1.8V (desired nominal output voltage)
V
_{FB} = 0.6V (not to be confused with V
_{REF} of the DS4404)
The value of V
_{FB} is found in the DCDC converter's data sheet. It is important to verify that this voltage is within the OUTx voltage range specified in the DS4404's data sheet (V
_{OUT:SINK} and V
_{OUT:SOURCE}). Finally, it is also important to check the input impedance of the DCDC converter's FB pin; the example here assumes that it is high impedance.
Assume that we are using the DS4404 to add ±20% margining of V
_{OUT}. Therefore, we have:
V
_{OUTMAX} = 2.16V
V
_{OUTNOM} = 1.8V
V
_{OUTMIN} = 1.44V
Begin by determining the necessary relationship between R
_{TOP} and R
_{BOTTOM} which yield nominal V
_{OUT}, V
_{OUTNOM}, when I
_{DS4404} = 0A.
Solving for R
_{TOP}, we get:
So for our example:
The current required to make V
_{OUT} increase to V
_{OUTMAX}, I
_{DS4404}, is derived by summing the currents at the FB node.
This equation can be simplified by solving Equation 1 for R
_{BOTTOM} and then substituting.
Or in terms of margin percentage:
where margin = 0.2 for this example of ±20% margining.
However, before we can use this relationship to calculate R
_{TOP} and R
_{BOTTOM}, the fullscale current, I
_{FS}, must be selected.
According to the DS4404's data sheet, the fullscale current needs to be between 0.5mA and 2.0mA (specified as I
_{OUT:SINK} and I
_{OUT:SOURCE} in the DS4404 data sheet, depending on whether sinking or sourcing current) to assure the accuracy and linearity specifications. Unfortunately, there is no one formula to calculate the ideal fullscale current. Every application will be different. Some items that influence the selection of the fullscale current are the desired number of steps, step size, as well as R
_{TOP} and R
_{BOTTOM} values. Likewise, there may be times when you want a particular register setting to correspond to a particular margin percentage. Either way, selecting the ideal fullscale current for your application will usually take several iterations of arbitrarily selecting a fullscale current (within the range) and then calculating R
_{TOP}, R
_{BOTTOM}, R
_{FS}, and step size. Then, once a fullscale current is determined, you can choose to adjust it or some of the resistor values in order to end up with common resistor values.
Returning to the original example, to calculate R
_{TOP }we will make I
_{FS} = I
_{DS4404}. This will give us 31 steps from V
_{OUTNOM} to V
_{OUTMAX} and 31 steps from V
_{OUTNOM} to V
_{OUTMIN}, which is more than adequate for our example.
Alternatively, we could begin by arbitrarily choosing I
_{FS} to be in the center (1.25mA) of the specified range, and then perform all the calculations. Instead, for illustrative purposes we will perform the calculations for the endpoints of the range.
So for I
_{FS} = I
_{DS4404} = 0.5mA.
Using Eq. 3 and solving for R
_{TOP}:
Then R
_{FS} is calculated using the formula given in the DS4404 data sheet along with V
_{REF}, also found in the DS4404 data sheet:
Finally, for completeness we can determine the DS4404 output current as a function of register setting:
Note: the register setting here does not include the sign bit, which is used to select sink or source. The DS4404 sinks current when the sign bit = 0, thus making V
_{OUT} increase to V
_{OUTMAX}. The DS4404 sources current when the sign bit = 1, thus decreasing V
_{OUT} towards V
_{OUTMIN}.
Likewise for I
_{FS} = I
_{DS4404} = 2.0mA:
Comparing R
_{TOP} and R
_{BOTTOM} of the two cases, one can see that I
_{FS} = 0.5mA is more attractive because the resistances are higher.
A similar article was published September 18, 2008 on the
EDN website.
Related Parts 
DS4402 
Two/FourChannel, I²C Adjustable Current DAC 
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DS4404 
Two/FourChannel, I²C Adjustable Current DAC 
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DS4412 
DualChannel, I²C Adjustable Sink/Source Current DAC 
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DS4424 
Two/FourChannel, I²C, 7Bit Sink/Source Current DAC 
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© Nov 03, 2008, Maxim Integrated Products, Inc.

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APP 4149: Nov 03, 2008
TUTORIAL 4149,
AN4149,
AN 4149,
APP4149,
Appnote4149,
Appnote 4149
