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TPS2074 Datasheet(PDF) 19 Page - Texas Instruments

Part No. TPS2074
Description  FOUR-PORT USB HUB POWER CONTROLLERS
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Maker  TI1 [Texas Instruments]
Homepage  http://www.ti.com
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TPS2074 Datasheet(HTML) 19 Page - Texas Instruments

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TPS2074, TPS2075
FOUR-PORT USB HUB POWER CONTROLLERS
SLVS288A – SEPTEMBER 2000 – REVISED FEBRUARY 2001
19
POST OFFICE BOX 655303
DALLAS, TEXAS 75265
APPLICATION INFORMATION
power dissipation and junction temperature
The major source of power dissipation for the TPS2074 and TPS2075 comes from the internal voltage regulator
and the N-channel MOSFETs. Checking the power dissipation and junction temperature is always a good
design practice. Begin by determining the rDS(on) of the N-channel MOSFET according to the input voltage and
operating temperature. As an initial estimate, use the highest operating ambient temperature of interest and
read rDS(on) from the graphs shown under the typical characteristics section of this data sheet. Using this value,
the power dissipation per switch can be calculated by:
P
D + rDS(on)
I2
Multiply this number by four to get the total power dissipation coming from the N-channel MOSFETs.
The power dissipation for the internal voltage regulator is calculated using:
P
D + V
I(BP)
–V
O(min)
I
O(OUT)
The total power dissipation for the device becomes:
P
D(total) + P
D(voltage regulator) )
4
P
D(switch)
Finally, calculate the junction temperature:
T
J + PD
RqJA ) TA
Where:
TA = ambient temperature °C
RθJA = Thermal resistance °C /W, equal to inverting of derating factor found on the power dissipation
table in this data sheet.
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
Thermal protection prevents damage to the IC when heavy-overload or short-circuit faults are present for
extended periods. The faults force the TPS2074 and TPS2075 into constant-current mode at first, 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 TPS2074 and TPS2075 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 rises. Once the die
temperature rises to approximately 140
°C, the internal thermal-sense circuitry determines which power switch
is in an overcurrent condition and turns only that power switch off, thus isolating the fault without interrupting
operation of the adjacent power switch. If the die temperature exceeds the first thermal trip point of 140
°C and
reaches 150
°C, the device turns off. The OC output is asserted (active low) when overtemperature or
overcurrent occurs.


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