The TPS4022 device uses a differential current-sense scheme to sense the output current. The sense element can be either the series resistance of the power stage filter inductor or a separate current sense resistor. When using the inductor series resistance as in Figure 8-2, a filter must be used to remove the large AC component of voltage across the inductor and leave only the component of the voltage that appears across the resistance of the inductor. The values of R5 and C4 for the ideal case can be found using Equation 2. The time constant of the R-C filter should be equal to or greater than the time constant of the inductor itself. If the time constants are equal, the voltage appearing across C4 is the current in the inductor multiplied the inductor resistance. The voltage across C4 perfectly reflects the inductor ripple current. Therefore, there is no need to have a shorter R-C time constant.

Extending the R-C filter time constant beyond the inductor time constant lowers the AC ripple component of voltage present at the current sense pins of the device, but allows the correct DC current information to remain intact. This extension also delays slightly the response to an overcurrent event, but reduces noise in the system leading to cleaner overcurrent performance and current reporting data over the PMBus interface.

Equation 2.

where (from Figure 8-2)

• R5 and R_ESR are in Ω
• C4 is in F (suggest 100 nF, 10^-7F)
• L is in H

TPS4022 device is designed to accept a maximum voltage of 60 mV across the current-sense pins. Most inductors have a copper conductor which results in a fairly large temperature coefficient of resistance. Because of this large temperature coefficient, the resistance of the inductor and the current through the inductor should make a peak voltage less than 60 mV when the inductor is at the maximum temperature for the converter. This situation also applies for the external resistor in Figure 8-3. The full-load output current multiplied by the sense resistor value, must be less than 60 mV at the maximum converter operating temperature.

In all cases, C4 should be placed as close to the current sense pins as possible to help avoid problems with noise.

Figure 8-2 Current Sensing Using Inductor Resistance

Figure 8-3 Current Sensing Using Sense Resistor

After choosing the current sensing method, set the current-sense element resistance. This value allows the proper calculation of thresholds for the overcurrent fault and warning, as well as more accurate reporting of the actual output current. The IOUT_CAL_GAIN command is used to set the value of the sense element resistance of the device. IOUT_OC_WARN_LIMIT and IOUT_OC_FAULT_LIMIT set the levels for the overcurrent warning and fault levels respectively. (See the Section 8.5.1 section for more details.)