TPS2021 datasheet(19/23 Pages) TI

19 / 23 page

TPS2020, TPS2021, TPS2022, TPS2023, TPS2024

POWER-DISTRIBUTION SWITCHES

SLVS175A – DECEMBER 1998 – REVISED NOVEMBER 1999

POST OFFICE BOX 655303

• DALLAS, TEXAS 75265

APPLICATION INFORMATION

GND

OUT

TPS202x

GND

OUT

TPS202x

Rpullup

V+

Rfilter

Rpullup

Cfilter

V+

Figure 39. Typical Circuit for OC Pin and RC Filter for Damping Inrush OC Responses

power dissipation and junction temperature

The low on-resistance on the n-channel MOSFET allows small surface-mount packages, such as SOIC, to pass

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

is good design practice to check power dissipation and junction temperature. The first step is to find rDS(on) at

the input voltage and operating temperature. As an initial estimate, use the highest operating ambient

temperature of interest and read rDS(on) from Figures 33–36. Next, calculate the power dissipation using:

D +

DS(on)

Finally, calculate the junction temperature:

J +

RqJA ) TA

Where:

TA = Ambient Temperature °C

RθJA = Thermal resistance SOIC = 172°C/W, PDIP = 106°C/W

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 an acceptable answer.

thermal protection

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

extended periods of time. The faults force the TPS202x into constant current mode, which causes the voltage

across the high-side switch to increase; under short-circuit conditions, the voltage across the switch is equal

to the input voltage. The increased dissipation causes the junction temperature to rise to high levels. The

protection circuit senses the junction temperature of the switch and shuts it off. Hysteresis is built into the thermal

sense circuit, and after the device has cooled approximately 20 degrees, the switch turns back on. The switch

continues to cycle in this manner until the load fault or input power is removed.

undervoltage lockout (UVLO)

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

falls below approximately 2 V, the power switch will be 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 will also keep the switch from being turned on until the power supply has reached at least 2 V, even if

the switch is enabled. Upon reinsertion, the power switch will be turned on, with a controlled rise time to reduce

EMI and voltage overshoots.

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