VOUT(NOM)

0 V

0 mA

VOUT

VFOLDBACK

ICL

ISC

IRATED

IOUT

Brickwall

Foldback

16

TPS746

SBVS337A – APRIL 2018 – REVISED DECEMBER 2018

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Copyright © 2018, Texas Instruments Incorporated

Feature Description (continued)

7.3.3 Foldback Current Limit

The device has an internal current limit circuit that protects the regulator during transient high-load current faults

or shorting events. The current limit is a hybrid brickwall-foldback scheme. The current limit transitions from a

brickwall scheme to a foldback scheme at the foldback voltage (VFOLDBACK). In a high-load current fault with the

output voltage above VFOLDBACK, the brickwall scheme limits the output current to the current limit (ICL). When the

voltage drops below VFOLDBACK, a foldback current limit activates that scales back the current as the output

voltage approaches GND. When the output is shorted, the device supplies a typical current called the short-

circuit current limit (ISC). ICL and ISC are listed in the Electrical Characteristics table.

For this device, VFOLDBACK = 0.4 V × VOUT(NOM).

The output voltage is not regulated when the device is in current limit. When a current limit event occurs, the

device begins to heat up because of the increase in power dissipation. When the device is in brickwall current

limit, the pass transistor dissipates power [(VIN – VOUT) × ICL]. When the device output is shorted and the output

is below VFOLDBACK, the pass transistor dissipates power [(VIN – VOUT) × ISC]. If thermal shutdown is triggered, the

device turns off. After the device cools down, the internal thermal shutdown circuit turns the device back on. If

the output current fault condition continues, the device cycles between current limit and thermal shutdown. For

more information on current limits, see the Know Your Limits application report.

Figure 45 shows a diagram of the foldback current limit.

Figure 45. Foldback Current Limit

7.3.4 Thermal Shutdown

Thermal shutdown protection disables the output when the junction temperature rises to approximately 170°C.

Disabling the device eliminates the power dissipated by the device, allowing the device to cool. When the

junction temperature cools to approximately 155°C, the output circuitry is again enabled. Depending on power

dissipation, thermal resistance, and ambient temperature, the thermal protection circuit may cycle on and off.

This cycling limits regulator dissipation, protecting the LDO from damage as a result of overheating.

Activating the thermal shutdown feature usually indicates excessive power dissipation as a result of the product

of the (VIN – VOUT) voltage and the load current. For reliable operation, limit junction temperature to 125°C

maximum. To estimate the margin of safety in a complete design, increase the ambient temperature until the

thermal protection is triggered; use worst-case loads and signal conditions.