9.1.1 On/Off Control

The ON pin controls the state of the switch. Activating ON continuously holds the switch in the on state as long as there is no fault. An undervoltage lockout or thermal shutdown event will override the ON pin control and turn off the switch. ON is active high and has a low threshold, making it capable of interfacing with low-voltage signals.

9.1.2 Undervoltage Lockout

The undervoltage lockout turns off the switch if the input voltage drops below the undervoltage lockout threshold. With the ON pin active, the input voltage rising above the undervoltage lockout threshold causes a controlled turn-on of the switch, which limits current overshoots.

9.1.3 Reverse Voltage

If the voltage at the V_OUT pin is larger than the V_IN pin, large currents may flow and can cause permanent damage to the device. TPS22941/2/3/4/5 is designed to control current flow only from V_IN to V_OUT.

9.1.4 Input Capacitor

To limit the voltage drop on the input supply caused by transient in-rush currents when the switch turns on into a discharged load capacitor or a short-circuit, a capacitor needs to be placed between V_IN and GND. A 1-μF ceramic capacitor, C_IN, placed close to the pins is usually sufficient. Higher values of C_IN can be used to further reduce the voltage drop.

9.1.5 Output Capacitor

A 0.1-μF capacitor, C_OUT, should be placed between V_OUT and GND. This capacitor will prevent parasitic board inductances from forcing V_OUT below GND when the switch turns off. For the TPS22941/2/3/4/5, the total output capacitance needs to be kept below a maximum value, C_OUT(MAX), to prevent the part from registering an over-current condition and turning-off the switch.

Due to the integrated body diode in the PMOS switch, a C_IN greater than C_OUT is highly recommended. A C_OUT greater than C_IN can cause V_OUT to exceed V_IN when the system supply is removed. This could result in current flow through the body diode from V_OUT to V_IN.

9.2.1 Design Requirements

For this design example, use the following as the input parameters:

9.2.2 Detailed Design Procedure

To begin the design process, the designer needs to know the following:

• VIN voltage
• Load current

9.2.2.2 Maximum Output Capacitance

When designing this device, it is important to ensure the inrush current of the output capacitance does not cause the device to exceed the current limiting time beyond the blanking time. The maximum output capacitance can be determined from Equation 2 :

Equation 2.

where

• C_OUT = output capacitance
• I_LIM(MAX) = maximum current limit
• t_BLANK(MIN) = minimum blanking time
• V_IN = input voltage

9.2.2.3 Power Dissipation

During normal operation as a switch, the power dissipation is small and has little effect on the operating temperature of the part. The parts with the higher current limits will dissipate the most power and that will only be,

Equation 3. P_D = (I_LIM)² × r_ON

If the part goes into current limit the maximum power dissipation will occur when the output is shorted to ground. For TPS22941/2/5, the power dissipation scales by the auto-restart time (t_RESTART) and the overcurrent blanking time (t_BLANK) so that the maximum power dissipated is:

Equation 4.

When using the TPS22943 and TPS22944, a short on the output causes the part to operate in a constant current state, dissipating a worst-case power as calculated above until the thermal shutdown activates. It then cycles in and out of thermal shutdown so long as the ON pin is active and the short is present.

9.2.2.4 Application Curves

VDRV signal forces the device to go into over-current mode.

Figure 20. t_BLANK Response

CL = 0.1 µF

RL = 500 Ω

VIN = 3.3 V

Figure 22. t_ON Response

CIN = 10 µF

COUT = 1 µF

Figure 24. Short-Circuit Response Time (Output Shorted to Ground)

CIN = 10 µF

COUT = 1 µF

Figure 26. Current Limit Response Time

VDRV signal forces the device to go into over-current mode.

Figure 21. t_RESTART Response

CL = 0.1 µF

RL = 500 Ω

VIN = 3.3 V

Figure 23. t_ON Response

CIN = 10 µF

COUT = 1 µF

Figure 25. Short-Circuit Response Time (Switch Powerup to Hard Short)