AMS2501/AMS2502
APPLICATION HINTS
Protection Diodes
Unlike older regulators, the AMS2501/AMS2502 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
AMS2501/AMS2502 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 V
POWER
pin are not usually
needed. Microsecond surge currents of 10A to 25A can be
handled by the internal diode between the Output pin and V
POWER
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 V
POWER
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
AMS2501/AMS2502 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.
V
CONTROL
+
Control section can run up to 125°C, and 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 0.5W 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 0.5W 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 0.5W.
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.
The power in the drive circuit is equal to:
P
DRIVE
= (V
CONTROL
- V
OUT
)(I
CONTROL
)
where I
CONTROL
is equal to between I
OUT
/100(typ) and
I
OUT
/60(max).
The power in the output transistor is equal to:
D1*
CONTROL
POWER
OUTPUT
D2*
+
V
POWER
+
V
OUT
P
OUTPUT
= (V
POWER
-V
OUT
)(I
OUT
)
The total power is equal to:
AMS2501
SENSE
ADJ
R1
R2
P
TOTAL
= P
DRIVE
+ P
OUTPUT
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
PCB proper mounting is required.
Figure 6. Optional Clamp Diodes Protect Against
Input Crowbar Circuits
If the AMS2501/AMS2502 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.
Thermal Considerations
The AMS2501/AMS2502 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.
Advanced Monolithic Systems, Inc.
6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
