9.5.1 Programming the Current-Limit Threshold

The over-current threshold is user programmable through an external resistor. The TPS25221 uses an internal regulation loop to provide a regulated voltage on the ILIM pin. The current-limit threshold is proportional to the current sourced out of ILIM. The recommended 1% resistor range for R_ILIM is 20 kΩ ≤ R_ILIM ≤ 210 kΩ to ensure stability of the internal regulation loop. Many applications require that the minimum current limit is above a certain current level or that the maximum current limit is below a certain current level, so it is important to consider the tolerance of the over-current threshold when selecting a value for R_ILIM. The following equations and Figure 24 can be used to calculate the resulting over-current threshold for a given external resistor value (R_ILIM). Figure 24 includes current-limit tolerance due to variations caused by temperature and process. However, the equations do not account for tolerance due to external resistor variation, so it is important to account for this tolerance when selecting R_ILIM. The traces routing the R_ILIM resistor to the TPS25221 must be as short as possible to reduce parasitic effects on the current-limit accuracy.

R_ILIM can be selected to provide a current-limit threshold that occurs: 1) above a minimum load current or 2) below a maximum load current.

To design above a minimum current-limit threshold, find the intersection of R_ILIM and the maximum desired load current on the I_OS(min) curve and choose a value of R_ILIM below this value. Programming the current limit above a minimum threshold is important to ensure start-up into full load or heavy capacitive loads. The resulting maximum current-limit threshold is the intersection of the selected value of R_ILIM and the I_OS(max) curve.

To design below a maximum current-limit threshold, find the intersection of R_ILIM and the maximum desired load current on the I_OS(max) curve and choose a value of R_ILIM above this value. Programming the current limit below a maximum threshold is important to avoid current limiting upstream power supplies, causing the input voltage bus to droop. The resulting minimum current-limit threshold is the intersection of the selected value of R_ILIM and the I_OS(min) curve.

Current-Limit Threshold Equation (I_OS):

Equation 1.

where

Figure 24. Current-Limit Threshold vs Current-Limit Resistor (R_ILIM)