Abstract

This application note explains how easy it is to add margining capability to a DC-DC converter by connecting it to a DS4404 4-channel adjustable current DAC (or the DS4402, 2-channel version).

This application note shows how to connect the DS4404, a 4-channel adjustable current DAC (or the DS4402, a 2-channel version of the DS4404), to a DC-DC 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 DC-DC 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.

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 DC-DC 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 DC-DC 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 full-scale current, I_FS, must be selected.

According to the DS4404's data sheet, the full-scale 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 full-scale current. Every application will be different. Some items that influence the selection of the full-scale 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 full-scale current for your application will usually take several iterations of arbitrarily selecting a full-scale current (within the range) and then calculating R_TOP, R_BOTTOM, R_FS, and step size. Then, once a full-scale 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.