OUT

IN

GND

FAULT

ILIM

EN

20 kW

100 kW

Input

TPS2557-Q1

0.1 µF

Thermal Pad

R

LOAD

0.22 µF

Output

1 kW

C

LOAD

R

ILIM

R

FAULT

C

RETRY

TPS2556-Q1, TPS2557-Q1

SLVSC97A – MARCH 2014 – REVISED MARCH 2014

www.ti.com

Feature Description (continued)

8.3.3 Thermal Sense

The TPS2556-Q1 and TPS2557-Q1 devices self-protect 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 TPS2556-Q1 and TPS2557-Q1 devices operate in constant-current

mode during an overcurrent 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 overcurrent condition. The first thermal sensor (OTSD) 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 TPS2556-Q1 and TPS2557-Q1 devices also have a second thermal sensor (OTSD2). This thermal sensor

turns off the power switch when the die temperature exceeds 155°C (minimum) regardless of whether the power

switch is in current limit, and turns on the power switch after the device has cooled approximately 20°C. The

TPS2556-Q1 and TPS2557-Q1 devices continue to cycle off and on until the fault is removed.

8.4 Device Functional Modes

8.4.1 Undervoltage Lockout (UVLO)

The undervoltage lockout (UVLO) circuit disables the power switch until the input voltage reaches the UVLO

turnon threshold. Built-in hysteresis prevents unwanted on-and-off cycling due to input voltage droop during

turnon.

8.4.2 Enable (EN OR EN)

The logic enable controls the power switch and device supply current. The supply current is reduced to less than

2

μA when a logic high is present on EN or when a logic low is present on EN. A logic low input on EN or 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.

8.4.3 Auto-Retry Functionality

Some applications require that an overcurrent condition disable the device momentarily during a fault condition

and re-enables it after a preset time. This auto-retry functionality can be implemented with an external resistor

and capacitor. During a fault condition, FAULT pulls EN low. Pulling EN below the turnoff threshold disables the

re-enables when the voltage on EN reaches the turnon threshold. The resistor-capacitor time constant

determines the auto-retry time. The device continues to cycle in this manner until removal of the fault condition.

Figure 12. Auto-Retry Functionality

Some applications require auto-retry functionality and the ability to enable and disable with an external logic

functionality. The resistor-capacitor time constant determines the auto-retry time-out period.

10

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