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GND

IN

EN1

EN2

OC1

OC2

OUT1

OUT2

TPS2042B

V+

POWER DISSIPATION AND JUNCTION TEMPERATURE

THERMAL PROTECTION

UNDERVOLTAGE LOCKOUT (UVLO)

TPS2041B,, TPS2042B

TPS2043B, TPS2044B, TPS2051B

TPS2052B, TPS2053B, TPS2054B

SLVS514E – APRIL 2004 – REVISED JANUARY 2006

APPLICATION INFORMATION (continued)

Figure 29. Typical Circuit for the OC Pin (Example, TPS2042B)

The low on-resistance on the N-channel MOSFET allows the small surface-mount packages to pass large

currents. The thermal resistances of these packages are high compared to those of power packages; it is good

N-channel MOSFET relative to the input voltage and operating temperature. As an initial estimate, use the

dissipation per switch can be calculated by:

•

P

2

Multiply this number by the number of switches being used. This step renders the total power dissipation from

the N-channel MOSFETs.

Finally, calculate the junction temperature:

•

Where:

•

•

•

Compare the calculated junction temperature with the initial estimate. If they do not agree within a few degrees,

repeat the calculation, using the calculated value as the new estimate. Two or three iterations are generally

sufficient to get a reasonable answer.

Thermal protection prevents damage to the IC when heavy-overload or short-circuit faults are present for

extended periods of time. The TPS204xB/TPS205xB implements a thermal sensing to monitor the operating

junction temperature of the power distribution switch. In an overcurrent or short-circuit condition, the junction

temperature rises due to excessive power dissipation. Once the die temperature rises to approximately 140°C

due to overcurrent conditions, the internal thermal sense circuitry turns the power switch off, thus preventing the

power switch from damage. Hysteresis is built into the thermal sense circuit, and after the device has cooled

approximately 10°C, the switch turns back on. The switch continues to cycle in this manner until the load fault or

input power is removed. The OCx open-drain output is asserted (active low) when an overtemperature shutdown

or overcurrent occurs.

An undervoltage lockout ensures that the power switch is in the off state at power up. Whenever the input

voltage falls below approximately 2 V, the power switch is quickly turned off. This facilitates the design of

hot-insertion systems where it is not possible to turn off the power switch before input power is removed. The

UVLO also keeps the switch from being turned on until the power supply has reached at least 2 V, even if the

switch is enabled. On reinsertion, the power switch is turned on, with a controlled rise time to reduce EMI and

voltage overshoots.

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