Datasheet

TPS2044, TPS2054
QUAD POWER-DISTRIBUTION SWITCHES
SLVS174B – JULY 1998 – REVISED FEBRUARY 1999
17
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
OC
response
The OC open-drain output is asserted (active low) when an overcurrent or overtemperature condition is
encountered. The output will remain asserted until the overcurrent or overtemperature 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. An RC filter of 500 µs (see
Figure 30) can be connected to the OC
pin to reduce false overcurrent reporting. Using low-ESR electrolytic
capacitors on the output lowers the inrush current flow through the device during hot-plug events by providing
a low impedance energy source, thereby reducing erroneous overcurrent reporting.
GND1
IN1
EN1
EN2
OC1
OC2
OUT1
OUT2
TPS2044
GND1
IN1
EN1
EN2
OC1
OC2
OUT1
OUT2
TPS2044
R
pullup
V+
R
filter
R
pullup
C
filter
To USB
Controller
V+
OC3
OC4
OUT3
OUT4
GND2
IN2
EN3
EN4
GND2
IN2
EN3
EN4
OC3
OC4
OUT3
OUT4
Figure 30. Typical Circuit for OC Pin and RC Filter for Damping Inrush OC Responses
power dissipation and junction temperature
The low on-resistance on the n-channel MOSFET allows small surface-mount packages, such as SOIC, 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. The first step is to find r
DS(on)
at
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 21. Next, calculate the power dissipation using:
P
D
r
DS ( on )
I
2
Finally, calculate the junction temperature:
T
J
P
D
R
JA
T
A
Where:
T
A
= Ambient Temperature °C
R
θJA
= Thermal resistance SOIC = 172°C/W
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.