TPS7A45XX_0808 Datasheet, PDF (18/25 Pages) Texas Instruments – LOW-NOISE FAST-TRANSIENT-RESPONSE 1.5-A LDO VOLTAGE REGULATORS

TPS7A45xx
SLVS720A – JUNE 2008 – REVISED AUGUST 2008 ........................................................................................................................................................ www.ti.com
Calculating Junction Temperature
Example: Given an output voltage of 3.3 V, an input
voltage range of 4 V to 6 V, an output current range
of 0 mA to 500 mA, and a maximum ambient
temperature of 50°C, what is the maximum junction
temperature?
The power dissipated by the device is equal to:
I_OUT(MAX)(V_IN(MAX) – V_OUT) + I_GND(V_IN(MAX))
where,
I_OUT(MAX) = 500 mA
V_IN(MAX) = 6 V
I_GND at (I_OUT = 500 mA, V_IN = 6 V) = 10 mA
So,
P = 500 mA × (6 V – 3.3 V) + 10 mA × 6 V = 1.41 W
Using a KTT package, the thermal resistance is in the
range of 23°C/W to 33°C/W, depending on the
copper area. So the junction temperature rise above
ambient is approximately equal to:
1.41 W × 28°C/W = 39.5°C
The maximum junction temperature is then be equal
to the maximum junction-temperature rise above
ambient plus the maximum ambient temperature or:
T_JMAX = 50°C + 39.5°C = 89.5°C
Protection Features
The TPS7A45xx regulators incorporate several
protection features which make them ideal for use in
battery-powered circuits. In addition to the normal
protection features associated with monolithic
regulators, such as current limiting and thermal
limiting, the devices are protected against reverse
input voltages, reverse output voltages and reverse
voltages from output to input.
Current limit protection and thermal overload
protection are intended to protect the device against
current overload conditions at the output of the
device. For normal operation, the junction
temperature should not exceed 125°C.
The input of the device withstands reverse voltages
of 20 V. Current flow into the device is limited to less
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.
The output of the TPS7A45xx can be pulled below
ground without damaging the device. If the input is
left open circuit or grounded, the output can be pulled
below ground by 20 V. For fixed voltage versions, the
output acts like a large resistor, typically 5 kΩ or
higher, limiting current flow to typically less than 600
µA. For adjustable versions, the output acts like an
open circuit; no current flows out of the pin. If the
input is powered by a voltage source, the output
sources the short-circuit current of the device and
protects itself by thermal limiting. In this case,
grounding the SHDN pin turns off the device and
stops the output from sourcing the short-circuit
current.
The ADJ pin of the adjustable device can be pulled
above or below ground by as much as 7 V without
damaging the device. If the input is left open circuit or
grounded, the ADJ pin acts like an open circuit when
pulled below ground and like a large resistor (typically
5 kΩ) in series with a diode when pulled above
ground.
In situations where the ADJ pin is connected to a
resistor divider that would pull the ADJ pin above its
7-V clamp voltage if the output is pulled high, the ADJ
pin input current must be limited to less than 5 mA.
For example, a resistor divider is used to provide a
regulated 1.5-V output from the 1.21-V reference
when the output is forced to 20 V. The top resistor of
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
divided by the 5-mA maximum current into the ADJ
pin yields a minimum top resistor value of 2.6 kΩ.
In circuits where a backup battery is required, several
different input/output conditions can occur. The output
voltage may be held up while the input is either pulled
to ground, pulled to some intermediate voltage, or is
left open circuit.
When the IN pin of the TPS7A45xx is forced below
the OUT pin or the OUT pin is pulled above the IN
pin, input current typically drops to less than 2 µA.
This can happen if the input of the device is
connected to a discharged (low voltage) battery and
the output is held up by either a backup battery or a
second regulator circuit. The state of the SHDN pin
has no effect on the reverse output current when the
output is pulled above the input.
18
Submit Documentation Feedback
Product Folder Link(s): TPS7A45xx
Copyright © 2008, Texas Instruments Incorporated