TPS2061, TPS2062, TPS2063
TPS2065, TPS2066, TPS2067
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SLVS490B–DECEMBER 2003–REVISED DECEMBER 2004
APPLICATION INFORMATION (continued)
V+
R
pullup
TPS2062
GND
OC1
OUT1
OUT2
OC2
IN
EN1
EN2
Figure 26. Typical Circuit for the OC Pin
POWER DISSIPATION AND JUNCTION TEMPERATURE
The low on-resistance on the N-channel MOSFET allows the small surface-mount packages 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. Begin by determining the rDS(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 rDS(on) from Figure 20. Using this value, the power
dissipation per switch can be calculated by:
•
PD = rDS(on)× I2
Multiply this number by the number of switches being used. This step renders the total power dissipation from
the N-channel MOSFETs.
Finally, calculate the junction temperature:
•
TJ = PD x RΘJA + TA
Where:
•
•
•
TA= Ambient temperature °C
ΘJA = Thermal resistance
PD = Total power dissipation based on number of switches being used.
R
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 of time. The TPS206x implements a thermal sensing to monitor the operating junction
temperature of the power distribution switch. In an overcurrent or short-circuit condition, the junction temperature
rises due to excessive power dissipation. Once the die temperature rises to approximately 140°C due to
overcurrent conditions, the internal thermal sense circuitry turns the power switch off, thus preventing the power
switch from damage. Hysteresis is built into the thermal sense circuit, and after the device has cooled
approximately 10°C, the switch turns back on. The switch continues to cycle in this manner until the load fault or
input power is removed. The OCx open-drain output is asserted (active low) when an overtemperature shutdown
or overcurrent occurs.
UNDERVOLTAGE LOCKOUT (UVLO)
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 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. On reinsertion, the power switch is turned on, with a controlled rise time to reduce EMI and
voltage overshoots.
17
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