An SVS monitors a critical system voltage and generates a reset if this voltage is too low. Likewise, an LBI pin monitors a voltage (typically a battery) and drives the low battery output (LBO) pin low when the battery has dropped below the set voltage. A PFI pin monitors a system voltage level and drives a power fail output (PFO) if the PFI voltage gets too low. These three pin types are simply a comparator and a reference voltage that monitor a voltage to ensure proper operation of a processor (SVS), to alert the user that the batteries must be replaced or recharged (LBI), or to send a signal to the host that some system voltage is too low and action needs to be taken (PFI). In each case, all of the voltages monitored are critical to ensure the proper operation of the entire system.

Ideally, a comparator would have infinite input impedance that produces no current at the inputs. In practice, however, a real comparator has a measurable input impedance and some degree of leakage current. These effects impact the accuracy of the trip point set by the resistive divider at the inputs, because this leakage current cannot be exactly determined and varies from device to device. When selecting the resistances, there are two extremes to consider: infinite or very low resistance. With infinite resistance, the trip point is dominated by the leakage current, which usually varies and causes a great loss in accuracy. At a very low resistance, however, amps of current are drawn through the divider, which is also unacceptable. ICs that use a resistive divider at a comparator input must have an accurate trip point and yet not consume a significant amount of current.

As a starting point for making the decision about the tradeoff of accuracy versus current consumption, a good rule of thumb is to have the current through the divider be 100 times larger than the leakage current. However, a given application may require more accuracy or require less current at the cost of reduced accuracy. In this report, an example divider circuit is analyzed using the low quiescent current, programmable-delay TPS3808G01 SVS, although the equations are applicable to any IC or circuit that uses a voltage divider at the comparator input.

As Figure 2-1 illustrates, the SENSE pin input of the TPS3808G01 is compared to a 0.405 V internal reference (V_REF). A voltage divider is used to scale down the monitored voltage (V_I) to the level of the SENSE pin. The voltage divider ratio is selected based on the desired trip point of V_I at which the SVS should generate a reset. This trip point is the threshold voltage, V_IT. An accurate trip point is necessary to prevent the system from resetting too early or too late.

Figure 2-1 Example Resistor Divider into the SENSE Pin Comparator (TPS3808G01 Adjustable Version)

When selecting the resistors to use, R₂ should be chosen first; then solve for R₁ to achieve the desired V_IT. Equation 1 shows the calculation for R₁, given a value of R₂, while Equation 2 calculates the actual value of V_IT based on the selected values for R₁ and R₂.

Equation 1.

Equation 2.

However, as a result of the leakage current (I_S), the voltage at the SENSE pin (V_S) is not what is expected at the desired V_IT. The actual V_S can be found using Equation 3. The actual input threshold voltage varies because of the leakage current, and can be calculated with Equation 4. The resulting accuracy of the divider can be found using Equation 5.

Equation 3.

Equation 4.

Equation 5.

Including the effect of the leakage current, the current drawn by the divider, I_R1, is simply the current that passes through the top resistor in the divider. This value can be found using Equation 6. The maximum current into the divider occurs when I_S is positive (flowing into the pin; refer to Figure 2-1). Equation 6 shows that current drawn from the divider varies almost linearly with the input voltage. When the leakage current is very small and/or the resistors in the divider are small, Equation 6 simplifies to Ohm's law.

Equation 6.

By rearranging Equation 5 and solving for R₂, we can derive Equation 7. This formula can be used to design a voltage divider to meet a desired accuracy requirement. Note that a negative accuracy is equivalent to using a negative leakage current, and produces the same resistance (R₂) that exists when both accuracy and leakage are positive. To state it differently, if the leakage current is negative (that is, flowing out of the pin), the threshold voltage is lower than expected, which equates to negative accuracy.

Equation 7.

By rearranging Equation 4 and solving for R₂, we can derive Equation 8. With this formula, we can now design a voltage divider to achieve a desired current, I_R1.

Equation 8.

Table 3-1 lists the system requirements that necessitate the use of an SVS with 1% accuracy threshold voltage. When the supply voltage falls below 10% of its nominal value of 3.3 V to 2.97 V, the processor should reset. V_REF and I_S can be found directly from the device data sheet.

First, calculate R₂ to its nearest standard value, using Equation 9.

Equation 9.

With R₂ known, calculate R₁ to the nearest standard value, using Equation 10.

Equation 10.

As an effect of using standard-value resistors, the expected threshold voltage can be found by using
Equation 11.

Equation 11.

Then, using Equation 6, the input current is found. I_S is evaluated as a positive value to find the maximum amount of current through the divider.

Equation 12.

One can also observe that this current is roughly 100 times the leakage, which validates the rule of thumb noted earlier.

Finally, using Equation 5 and Equation 6, Figure 3-1 shows the variation in worst-case accuracy and input current to the divider with R₂ ranging from 1 kΩ to 1 MΩ. Note the accuracy is centered on the new calculated V_IT: 2.974 V.

Figure 3-1 Accuracy and Current Variations through the Divider vs. Resistance of R₂

Figure 3-1 also clearly illustrates the linear relationship of accuracy and exponential relationship of current to R₂ (note the logarithmic axes). This correlation demonstrates the diminishing returns for both an increase and a decrease in the resistance R₂. At the lower end, the current increases significantly with a small increase in accuracy. On the high end, the accuracy decreases significantly for small decreases in current. The accuracy from the leakage is nearly always better than what is shown in the graph because the amount of leakage is not always at the worst-case conditions. The current, however, does not vary significantly because the leakage current is virtually always a fraction of the current into the divider.