Datasheet

TPS7A45xx
www.ti.com SLVS720D –JUNE 2008–REVISED AUGUST 2011
Calculating Junction Temperature The output of the TPS7A45xx can be pulled below
ground without damaging the device. If the input is
Example: Given an output voltage of 3.3 V, an input
left open circuit or grounded, the output can be pulled
voltage range of 4 V to 6 V, an output current range
below ground by 20 V. For fixed voltage versions, the
of 0 mA to 500 mA, and a maximum ambient
output acts like a large resistor, typically 5 kΩ or
temperature of 50°C, what is the maximum junction
higher, limiting current flow to typically less than 600
temperature?
μA. For adjustable versions, the output acts like an
open circuit; no current flows out of the pin. If the
The power dissipated by the device is equal to:
input is powered by a voltage source, the output
I
OUT(MAX)
(V
IN(MAX)
– V
OUT
) + I
GND
(V
IN(MAX)
)
sources the short-circuit current of the device and
protects itself by thermal limiting. In this case,
where,
grounding the SHDN pin turns off the device and
I
OUT(MAX)
= 500 mA
stops the output from sourcing the short-circuit
current.
V
IN(MAX)
= 6 V
The ADJ pin of the adjustable device can be pulled
I
GND
at (I
OUT
= 500 mA, V
IN
= 6 V) = 10 mA
above or below ground by as much as 7 V without
So,
damaging the device. If the input is left open circuit or
grounded, the ADJ pin acts like an open circuit when
P = 500 mA × (6 V – 3.3 V) + 10 mA × 6 V = 1.41 W
pulled below ground and like a large resistor (typically
Using a KTT package, the thermal resistance is in the
5 kΩ) in series with a diode when pulled above
range of 23°C/W to 33°C/W, depending on the
ground.
copper area. So the junction temperature rise above
In situations where the ADJ pin is connected to a
ambient is approximately equal to:
resistor divider that would pull the ADJ pin above its
1.41 W × 28°C/W = 39.5°C
7-V clamp voltage if the output is pulled high, the ADJ
pin input current must be limited to less than 5 mA.
The maximum junction temperature is then be equal
For example, a resistor divider is used to provide a
to the maximum junction-temperature rise above
regulated 1.5-V output from the 1.21-V reference
ambient plus the maximum ambient temperature or:
when the output is forced to 20 V. The top resistor of
T
JMAX
= 50°C + 39.5°C = 89.5°C the resistor divider must be chosen to limit the current
into the ADJ pin to less than 5 mA when the ADJ pin
is at 7 V. The 13-V difference between OUT and ADJ
Protection Features
divided by the 5-mA maximum current into the ADJ
The TPS7A45xx regulators incorporate several
pin yields a minimum top resistor value of 2.6 kΩ.
protection features which make them ideal for use in
In circuits where a backup battery is required, several
battery-powered circuits. In addition to the normal
different input/output conditions can occur. The output
protection features associated with monolithic
voltage may be held up while the input is either pulled
regulators, such as current limiting and thermal
to ground, pulled to some intermediate voltage, or is
limiting, the devices are protected against reverse
left open circuit.
input voltages, reverse output voltages and reverse
voltages from output to input.
When the IN pin of the TPS7A45xx is forced below
the OUT pin or the OUT pin is pulled above the IN
Current limit protection and thermal overload
pin, input current typically drops to less than 2 μA.
protection are intended to protect the device against
This can happen if the input of the device is
current overload conditions at the output of the
connected to a discharged (low voltage) battery and
device. For normal operation, the junction
the output is held up by either a backup battery or a
temperature should not exceed 125°C.
second regulator circuit. The state of the SHDN pin
The input of the device withstands reverse voltages
has no effect on the reverse output current when the
of 20 V. Current flow into the device is limited to less
output is pulled above the input.
than 1 mA (typically less than 100 μA), and no
negative voltage appears at the output. The device
protects both itself and the load. This provides
protection against batteries that can be plugged in
backward.
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