9.3.1 Thermal Sense

The TPS2559-Q1 self protects by using two independent thermal sensing circuits that monitor the operating temperature of the power switch and disable operation if the temperature exceeds recommended operating conditions. The TPS2559-Q1 device operates in constant-current mode during an over-current condition, which increases the voltage drop across power switch. The power dissipation in the package is proportional to the voltage drop across the power switch, which increases the junction temperature during an over-current condition. The first thermal sensor (OTSD1) turns off the power switch when the die temperature exceeds 135°C (min) and the part is in current limit. Hysteresis is built into the thermal sensor, and the switch turns on after the device has cooled approximately 20°C.

The TPS2559-Q1 also has a second ambient thermal sensor (OTSD2). The ambient thermal sensor turns off the power switch when the die temperature exceeds 155°C (min) regardless of whether the power switch is in current limit and will turn on the power switch after the device has cooled approximately 20°C. The TPS2559-Q1 continues to cycle off and on until the fault is removed.

9.3.2 Overcurrent Protection

The TPS2559-Q1 responds to overcurrent conditions by limiting their output current to IOS. When an overload condition is present, the device maintains a constant output current, with the output voltage determined by (I_OS × R_LOAD). Two possible overload conditions can occur.

The first condition is when a short circuit or partial short circuit is present when the device is powered-up or enabled. The output voltage is held near zero potential with respect to ground and the TPS2559-Q1 ramps the output current to I_OS. The TPS2559-Q1 limits the current to I_OS until the overload condition is removed or the device begins to thermal cycle (see Figure 24).

The second condition is when a short circuit, partial short circuit, or transient overload occurs while the device is enabled and powered on. The device responds to the overcurrent condition within time t_IOS (see Figure 15). The response speed and shape will vary with the overload level, input circuit, and rate of application. The current-limit response will vary between simply settling to I_OS, or turnoff and controlled return to IOS. Similar to the previous case, the TPS2559-Q1 limits the current to I_OS until the overload condition is removed or the device begins to thermal cycle.

The TPS2559-Q1 thermal cycles if an overload condition is present long enough to activate thermal limiting in any of the above cases. The device turns off when the junction temperature exceeds 135°C (min) while in current limit. The device remains off until the junction temperature cools 20°C (typ) and then restarts. The TPS2559-Q1 cycles on/off until the overload is removed (see Figure 25).

9.3.3 FAULT Response

The FAULT open-drain output is asserted (active low) during an over-current or over-temperature condition. The TPS2559-Q1 asserts the FAULT signal until the fault condition is removed and the device resumes normal operation. The TPS2559-Q1 is designed to eliminate false FAULT reporting by using an internal delay "deglitch" circuit for over-current (9-ms typ.) conditions without the need for external circuitry. This ensures that FAULT is not accidentally asserted due to normal operation such as starting into a heavy capacitive load. The deglitch circuitry delays entering and leaving current-limit induced fault conditions. The FAULT signal is not deglitched when the MOSFET is disabled due to an over-temperature condition but is deglitched after the device has cooled and begins to turn on. This unidirectional deglitch prevents FAULT oscillation during an over-temperature event.

9.4.1 Operation with V_IN Undervoltage Lockout (UVLO) Control

The undervoltage lockout (UVLO) circuit disables the power switch until the input voltage reaches the UVLO turn-on threshold. Built-in hysteresis prevents unwanted on/off cycling due to input voltage droop during turn on.

9.4.2 Operation with EN Control

The logic enable controls the power switch and device supply current. The supply current is reduced to less than 2-μA when a logic low is present on EN. A logic high input on EN enables the driver, control circuits, and power switch. The enable input is compatible with both TTL and CMOS logic levels.