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GND
IN
IN
EN
OUT
OC
OUT
OUT
TPS202x
GND
IN
IN
EN
OUT
OC
OUT
OUT
TPS202x
R
pullup
V+
R
filter
R
pullup
C
filter
V+
POWER DISSIPATION AND JUNCTION TEMPERATURE
THERMAL PROTECTION
UNDERVOLTAGE LOCKOUT (UVLO)
TPS2020 , , TPS2021
TPS2022 , TPS2023 , TPS2024
SLVS175C – DECEMBER 1998 – REVISED SEPTEMBER 2007
Figure 39. Typical Circuit for OC Pin and RC Filter for Damping Inrush OC Responses
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 r
DS(on)
at the
input voltage and operating temperature. As an initial estimate, use the highest operating ambient temperature of
interest and read r
DS(on)
from Figures 33 – 36. Next, calculate the power dissipation using:
P
D
= r
DS(on)
× I
2
Finally, calculate the junction temperature:
T
J
= P
D
× R
θ JA
+ T
A
where:
T
A
= Ambient temperature ° C
R
θ JA
= 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 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.
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 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. Upon reinsertion, the power switch is turned on, with a controlled rise time to reduce EMI and voltage
overshoots.
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