Datasheet

 
 
SLVS333 − AUGUST 2001
17
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APPLICATION INFORMATION
overcurrent (continued)
In the third condition, the load has been gradually increased beyond the recommended operating current. The
current is permitted to rise until the current-limit threshold is reached or until the thermal limit of the device is
exceeded. The TPS2145 and TPS2147 are capable of delivering current up to the current-limit threshold without
damaging the device. Once the threshold has been reached, the device switches into its constant-current mode.
OC response
The OCx open-drain output is asserted (active low) when an overcurrent condition is encountered. The output
will remain asserted until the overcurrent condition is removed. Connecting a heavy capacitive load to an
enabled device can cause momentary false overcurrent reporting from the inrush current flowing through the
device, charging the downstream capacitor. The TPS2145 and TPS2147 are designed to reduce false
overcurrent reporting. An internal overcurrent transient filter eliminates the need for external components to
remove unwanted pulses. Using low-ESR electrolytic capacitors on OUTx lowers the inrush current flow through
the device during hot-plug events by providing a low-impedance energy source, also reducing erroneous
overcurrent reporting.
power dissipation and junction temperature
The major source of power dissipation for the TPS2145 and TPS2147 comes from the internal voltage regulator
and the N-channel MOSFETs. Checking the power dissipation and junction temperature is always a good
design practice and it starts with 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 in the Typical Characteristics section of this data sheet. Using
this value, the power dissipation per switch can be calculated using:
P
D
+ r
DS(on)
I
2
Multiply this number by two 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
+
ǒ
V
I
–V
O(min)
Ǔ
I
O
The total power dissipation for the device becomes:
P
D(total)
+ P
D(voltage regulator)
)
ǒ
2 P
D(switch)
Ǔ
Finally, calculate the junction temperature:
T
J
+ P
D
R
qJA
) T
A
Where:
T
A
= Ambient Temperature °C
R
θJA
= Thermal resistance °C/W, equal to inverting the 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.
(3)
(4)
(5)
(6)