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APPLICATION INFORMATION
POWER-SUPPLY CONSIDERATIONS
OVERCURRENT
POWER DISSIPATION AND JUNCTION TEMPERATURE
P
D
+ r
DS(on)
I
2
(1)
T
J
+ P
D
R
qJA
) T
A
(2)
TPS2010A, TPS2011A
TPS2012A, TPS2013A
SLVS189C – DECEMBER 1998 – REVISED SEPTEMBER 2007
Figure 35. Typical Application
A 0.01- μ F to 0.1- μ F ceramic bypass capacitor between IN and GND, close to the device, is recommended.
Placing a high-value electrolytic capacitor on the output and input pins is recommended when the output load is
heavy. This precaution reduces power supply transients that may cause ringing on the input. Additionally,
bypassing the output with a 0.01- μ F to 0.1- μ F ceramic capacitor improves the immunity of the device to
short-circuit transients.
A sense FET checks for overcurrent conditions. Unlike current-sense resistors, sense FETs do not increase the
series resistance of the current path. When an overcurrent condition is detected, the device maintains a constant
output current and reduces the output voltage accordingly. Complete shutdown occurs only if the fault is present
long enough to activate thermal limiting.
Three possible overload conditions can occur. In the first condition, the output has been shorted before the
device is enabled or before V
I(IN)
has been applied (see Figure 6 ). The TPS201xA senses the short and
immediately switches into a constant-current output.
In the second condition, the excessive load occurs while the device is enabled. At the instant the excessive load
occurs, very high currents may flow for a short time before the current-limit circuit can react (see
Figure 12 – Figure 19 ). After the current-limit circuit has tripped (reached the overcurrent trip threshold) the device
switches into constant-current mode.
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 (see Figures Figure 7 7 – Figure 10 ). The TPS201xA is 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.
The low on-resistance on the n-channel MOSFET allows small surface-mount packages, such as SOIC, to pass
large currents. The thermal resistance 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 SLVS1892074Figure 30 – Figure 33 . Next, calculate the power dissipation using:
Finally, calculate the junction temperature:
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