Table 1. Design Parameters

10.2.1.2.1 Step by Step Design Produce

To begin the design process a few parameters must be decided upon. The designer needs to know the following:

• Normal Input Operation Voltage
• Output transient voltage
• Minimum Current Limit
• Maximum Current Limit

10.2.1.2.2 Input and Output Capacitance

Input and output capacitance improves the performance of the device; the actual capacitance should be optimized for the particular application. For all applications, a ceramic bypass capacitor with a value of 0.1 µF or greater is recommended between the IN and GND pins. This capacitor should be placed as close to the device as possible for local noise decoupling.

When V_IN ramp up exceeds 7.6 V, V_OUT follows V_IN until the TPS25200-Q1 device turns off the internal MOSFET after t_{(OVLO_off_delay)}. Because t_{(OVLO_off_delay)} largely depends on the V_IN ramp rate, V_OUT receives some peak voltage. Increasing the output capacitance can lower the output peak voltage as shown in Figure 16.

Figure 16. V_OUT Peak Voltage vs C_OUT
(V_IN Step From 5 V to 15 V With 1-V/us Ramp-Up Rate)

10.2.1.2.3 Programming the Current-Limit Threshold

The overcurrent threshold is user programmable through an external resistor. The TPS25200-Q1 device 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 the ILIM pin. The recommended 1% resistor range for R_ILIM is 36 kΩ ≤ R_ILIM ≤ 1100 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. Therefore, considering the tolerance of the overcurrent threshold is important when selecting a value for R_ILIM. The following equations approximate the resulting overcurrent threshold for a given external resistor value, R_ILIM. See the Electrical Characteristics table for specific current-limit settings. The traces routing the R_ILIM resistor to the TPS25200-Q1 device should 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 either above a minimum load current or below a maximum load current.

To design above a minimum current-limit threshold, find the intersection of R_ILIM and the minimum desired load current on the I_OS(min) curve. Select 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.

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. Select a value of R_ILIM above this value. Programming the current limit below a maximum threshold is important to avoid current limiting the upstream power supplies which causes the input voltage bus to droop.

Use Equation 1, Equation 2, and Equation 3, to calculate the minimum, nominal, and maximum current-limit thresholds for I_OS (respectively). For each equation, 36 kΩ ≤ R_ILIM ≤ 1100 kΩ.

Equation 1.

Equation 2.

Equation 3.

36 kΩ ≤ RILIM ≤ 150 KΩ

Figure 17. Current-Limit Threshold vs R_ILIM I

150 kΩ ≤ RILIM ≤ 1100 KΩ

Figure 18. Current-Limit Threshold vs R_ILIM II

10.2.1.2.4 Design Above a Minimum Current Limit

Some applications require that current limiting does not occur below a certain threshold. For this example, assume that 2.1 A must be delivered to the load so that the minimum desired current-limit threshold is 2100 mA. Use Equation 1 and Figure 17 to select a value for R_ILIM, with I_OSmin = 2100 mA, as shown in Equation 4.

Equation 4.

Select the closest 1% resistor less than the calculated value: R_ILIM = 42.2 kΩ. This value sets the minimum current-limit threshold at 2130 mA as shown in Equation 5.

Equation 5.

Use Equation 3, Figure 17, and the previously calculated value for R_ILIM to calculate the maximum resulting current-limit threshold as shown in Equation 6.

Equation 6.

The resulting current-limit threshold minimum is 2130 mA and maximum is 2479 mA with R_ILIM = 42.2kΩ ± 1%.

10.2.1.2.5 Design Below a Maximum Current Limit

Some applications require that current limiting must occur below a certain threshold. For this example, assume that 2.9 A must be delivered to the load so that the minimum desired current-limit threshold is 2900 mA. Use Equation 3 and Figure 18 to select R_ILIM.

Equation 7.

Select the closest 1% resistor greater than the calculated value: R_ILIM = 36 kΩ. This value sets the maximum current-limit threshold at 2894 mA as shown in Equation 8.

Equation 8.

Use Equation 1, Figure 18, and the previously calculated value for R_ILIM to calculate the minimum resulting current-limit threshold as shown in Equation 9.

Equation 9.

The resulting minimum current-limit threshold minimum is 2592 mA and maximum is 2894 mA with R_ILIM = 36 kΩ ± 1%.

10.2.1.2.6 Power Dissipation and Junction Temperature

The low on-resistance of the internal N-channel MOSFET allows small surface-mount packages to pass large currents. Estimating the power dissipation and junction temperature is good design practice. The following analysis provides an approximation for calculating the junction temperature based on the power dissipation of the package.

Begin by determining the r_DS(on) value of the N-channel MOSFET relative to the input voltage (V_IN) and operating temperature. As an initial estimate, use the highest operating ambient temperature of interest and read r_DS(on) from Figure 4 in the Typical Characteristics section. When V_IN is lower than V_(OVC), the TPS25200-Q1 device is an traditional power switch. Using this value, calculate the power dissipation with Equation 10.

When V_IN exceeds V_(OVC), but is lower than V_(OVLO), the TPS25200-Q1 clamp output is fixed to V_(OVC). Use Equation 11 to calculate the power dissipation.

This step calculates the total power dissipation of the N-channel MOSFET.

Finally, calculate the junction temperature using Equation 12.

Compare the calculated junction temperature with the initial estimate. If these two values are not within a few degrees, repeat the calculation using the refined r_DS(on) value from the previous calculation as the new estimate. Two or three iterations are generally sufficient to achieve the desired result. The final junction temperature is highly dependent on thermal resistance R_θJA, and the thermal resistance is highly dependent on the individual package and board layout.