Datasheet

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POWER DISSIPATION AND JUNCTION TEMPERATURE
THERMAL PROTECTION
UNDERVOLTAGE LOCKOUT (UVLO)
TPS2070
TPS2071
SLVS287B – SEPTEMBER 2000 – REVISED SEPTEMBER 2007
The major source of power dissipation for the TPS2070 and TPS2071 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 r
DS(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 r
DS(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
= r
DS(on)
× I
2
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
= P
D
× R
θ JA
+ T
A
where:
T
A
= ambient temperature in ° C
R
θ JA
= Thermal resistance in ° 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 prevents damage to the IC when heavy-overload or short-circuit faults are present for
extended periods. The faults force the TPS2070 and TPS2071 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 TPS2070 and TPS2071 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.
An undervoltage lockout ensures that the device (LDO and switches) is in the off state at power up. The UVLO
also keeps the device from being turned on until the power supply has reached the start threshold (see
undervoltage lockout table), even if the switches are enabled. The UVLO activates whenever the input voltage
falls below the stop threshold as defined in the undervoltage lockout table. This facilitates the design of
hot-insertion systems, where it is not possible to turn off the power switches before input power is removed.
Upon reinsertion, the power switches are turned on with a controlled rise time to reduce EMI and voltage
overshoots.
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