Datasheet

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GND
IN
IN
EN
OUT
OC
OUT
OUT
TPS2045A
R
pullup
V+
POWER DISSIPATION AND JUNCTION TEMPERATURE
P
D
+ r
DS(on)
I
2
T
J
+ P
D
R
qJA
) T
A
THERMAL PROTECTION
UNDERVOLTAGE LOCKOUT (UVLO)
TPS2045A , , TPS2046A
TPS2047A , TPS2048A , TPS2055A
TPS2056A , TPS2057A , TPS2058A
SLVS251C – SEPTEMBER 2000 – REVISED JANUARY 2008
Figure 25. Typical Circuit for OC Pin (Example, TPS2045A)
The low on-resistance on the n-channel MOSFET allows small surface-mount packages, such as SOIC, to pass
large currents. The thermal resistance of these packages is high compared to those of power packages; it is
good design practice to check power dissipation and junction temperature. Begin by determining the r
DS(on)
of the
N-channel MOSFET relative to the input voltage and operating temperature. As an initial estimate, use the
highest operating ambient temperature of interest and read r
DS(on)
from Figure 18. Using this value, the power
dissipation per switch can be calculated by:
Depending on which device is being used, multiply this number by the number of switches being used. This step
will render the total power dissipation from the N-channel MOSFETs.
Finally, calculate the junction temperature:
Where: T
A
= Ambient temperature ° C R
Θ JA
= Thermal resistance SOIC = 172 ° C/W (for 8 pin), 111 ° C/W (for 16
pin) P
D
= Total power dissipation based on number of switches being used.
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 faults force the TPS204xA and TPS205xA 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.
The TPS204xA and TPS205xA implement a dual thermal trip to allow fully independent operation of the power
distribution switches. In an overcurrent or short-circuit condition the junction temperature will rise. Once the die
temperature rises to approximately 140 ° C, the internal thermal sense circuitry checks which power switch is in an
overcurrent condition and turns that power switch off, thus isolating the fault without interrupting operation of the
adjacent power switch. Should the die temperature exceed the first thermal trip point of 140 ° C and reach 160 ° C,
both switches turn off. The OC open-drain output is asserted (active low) when overtemperature 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 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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Product Folder Link(s): TPS2045A TPS2046A TPS2047A TPS2048A TPS2055A TPS2056A TPS2057A TPS2058A