AMS1503
APPLICATION HINTS
Thermal resistance specification for both the Control Section and
the Power Transistor are given in the electrical characteristics.
The thermal resistance of the Control section is given as
0.65°C/W and junction temperature of the Control section can run
up to 125°C. The thermal resistance of the Power section is given
as 2.7°C/W and junction temperature of the Power section can run
up to 150°C. Due to the thermal gradients between the power
transistor and the control circuitry there is a significant difference
in thermal resistance between the Control and Power sections.
Virtually all the power dissipated by the device is dissipated in the
power transistor. The temperature rise in the power transistor will
be greater than the temperature rise in the Control section making
the thermal resistance lower in the Control section. At power
levels below 12W the temperature gradient will be less than 25°C
and the maximum ambient temperature will be determined by the
junction temperature of the Control section. This is due to the
lower maximum junction temperature in the Control section. At
power levels above 12W the temperature gradient will be greater
than 25°C and the maximum ambient temperature will be
determined by the Power section. In both cases the junction
temperature is determined by the total power dissipated in the
device. For most low dropout applications the power dissipation
will be less than 12W.
Protection Diodes
Unlike older regulators, the AMS1503 family does not need any
protection diodes between the adjustment pin and the output and
from the output to the input to prevent die over-stress. Internal
resistors are limiting the internal current paths on the AMS1503
adjustment pin, therefore even with bypass capacitors on the
adjust pin no protection diode is needed to ensure device safety
under short-circuit conditions. The Adjust pin can be driven on a
transient basis ±7V with respect to the output without any device
degradation.
Diodes between the Output pin and VPOWER pin are not usually
needed. Microsecond surge currents of 25A to50A can be handled
by the internal diode between the Output pin and VPOWER pin of
the device. In normal operations it is difficult to get those values
of surge currents even with the use of large output capacitances. If
high value output capacitors are used, such as 1000µF to 5000µF
and the VPOWER pin is instantaneously shorted to ground, damage
can occur. A diode from output to input is recommended, when a
crowbar circuit at the input of the AMS1503 is used (Figure 6).
Normal power supply cycling or even plugging and unplugging in
the system will not generate current large enough to do any
damage.
The power in the device is made up of two components: the power
in the output transistor and the power in the drive circuit. The
power in the control circuit is negligible.
VCONTROL
The power in the drive circuit is equal to:
+
D1*
CONTROL
POWER OUTPUT
D2*
P
DRIVE = (VCONTROL - VOUT)(ICONTROL
where ICONTROL is equal to between IOUT/100(typ) and
OUT/58(max).
The power in the output transistor is equal to:
OUTPUT = (VPOWER -VOUT)(IOUT
)
VOUT
VPOWER
+
+
AMS1503
SENSE
I
ADJ
R1
R2
P
)
The total power is equal to:
PTOTAL = PDRIVE + POUTPUT
Figure 6. Optional Clamp Diodes Protect Against
Input Crowbar Circuits
Junction-to-case thermal resistance is specified from the IC
junction to the bottom of the case directly below the die. This is
the lowest resistance path for the heat flow. In order to ensure the
best possible thermal flow from this area of the package to the
heat sink proper mounting is required. Thermal compound at the
If the AMS1503 is connected as a single supply device with the
control and power input pins shorted together the internal diode
between the output and the power input pin will protect the control
input pin. As with any IC regulator, none the protection circuitry
will be functional and the internal transistors will break down if
the maximum input to output voltage differential is exceeded.
case-to-heat sink interface is recommended.
A thermally
conductive spacer can be used, if the case of the device must be
electrically isolated, but its added contribution to thermal
resistance has to be considered.
Thermal Considerations
The AMS1503 series have internal power and thermal limiting
circuitry designed to protect the device under overload conditions.
However maximum junction temperature ratings should not be
exceeded under continuous normal load conditions. Careful
consideration must be given to all sources of thermal resistance
from junction to ambient, including junction-to-case, case-to-heat
sink interface and heat sink resistance itself.
Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723
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