AMS1505
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 AMS1505 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 AMS1505
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 50A to 100A 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
1000mF to 5000mF 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
AMS1505 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
+
D1*
D2*
The power in the drive circuit is equal to:
CONTROL
POWER OUTPUT
VOUT
VPOWER
PDRIVE = (VCONTROL - VOUT)(ICONTROL
)
+
+
AMS1505
SENSE
where ICONTROL is equal to between IOUT/100(typ) and
IOUT/58(max).
The power in the output transistor is equal to:
ADJ
R1
R2
POUTPUT = (VPOWER -VOUT)(IOUT
)
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 AMS1505 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 AMS1505 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. 6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
