AMS317
APPLICATION HINTS
Load Regulation
True remote load sensing it is not possible to provide, because the
AMS317 is a three terminal device. The resistance of the wire
connecting the regulator to the load will limit the load regulation.
The data sheet specification for load regulation is measured at the
bottom of the package. Negative side sensing is a true Kelvin
connection, with the bottom of the output divider returned to the
negative side of the load.
The best load regulation is obtained when the top of the resistor
divider R1 is connected directly to the case not to the load. If R1
were connected to the load, the effective resistance between the
regulator and the load would be:
R
P
x ( R2+R1 ) ,
R1
R
P
= Parasitic Line Resistance
The thermal resistance from the junction to the tab for the
AMS317 is 15
°C/W;
t
hermal resistance from tab to ambient can
be as low as 30°C/W. The total thermal resistance from junction
to ambient can be as low as 45°C/W. This requires a reasonable
sized PC board with at least on layer of copper to spread the heat
across the board and couple it into the surrounding air.
Experiments have shown that the heat spreading copper layer
does not need to be electrically connected to the tab of the device.
The PC material can be very effective at transmitting heat
between the pad area, attached to the pad of the device, and a
ground plane layer either inside or on the opposite side of the
board. Although the actual thermal resistance of the PC material
is high, the Length/Area ratio of the thermal resistance between
layers is small. The data in Table 1, was taken using 1/16” FR-4
board with 1 oz. copper foil, and it can be used as a rough
guideline for estimating thermal resistance.
For the TO-252 package, if the maximum allowable value is
found to be
≥92°C/W,
no heat sink is needed since the package
alone will dissipate enough heat to satisfy these requirements. If
the calculated value for
θ
JA
falls bellow these limits, a heat sink
is required.
For each application the thermal resistance will be affected by
thermal interactions with other components on the board. To
determine the actual value some experimentation will be
necessary.
The power dissipation of the AMS317 is equal to:
P
D
= ( V
IN
- V
OUT
)( I
OUT
)
Maximum junction temperature will be equal to:
R2*
Connected as shown , R
P
is not multiplied by the divider ratio
R
P
PARASITIC
LINE RESISTANCE
V
IN
AMS317
IN
OUT
ADJ
R1*
R
L
T
J
= T
A(MAX)
+ P
D
(Thermal Resistance (junction-to-ambient))
Maximum junction temperature must not exceed 125°C.
Ripple Rejection
*
CONNECT R1 TO CASE
CONNECT R2 TO LOAD
Figure 3. Connections for Best Load Regulation
In the case of fixed voltage devices the top of R1 is connected
Kelvin internally, and the ground pin can be used for negative
side sensing.
Thermal Considerations
The AMS317 series have internal power and thermal limiting
circuitry designed to protect the device under overload conditions.
However maximum junction temperature ratings of 125
°C
should
not be exceeded under continuous normal load conditions.
Careful consideration must be given to all sources of thermal
resistance from junction to ambient. For the surface mount
package SOT-223 additional heat sources mounted near the
device must be considered. The heat dissipation capability of the
PC board and its copper traces is used as a heat sink for the
device.
The ripple rejection values are measured with the adjustment pin
bypassed. The impedance of the adjust pin capacitor at the ripple
frequency should be less than the value of R1 (normally 100Ω to
200Ω) for a proper bypassing and ripple rejection approaching
the values shown. The size of the required adjust pin capacitor is
a function of the input ripple frequency. If R1=100Ω at 120Hz
the adjust pin capacitor should be
>13µF.
At 10kHz only 0.16µF
is needed.
The ripple rejection will be a function of output voltage, in
circuits without an adjust pin bypass capacitor. The output ripple
will increase directly as a ratio of the output voltage to the
reference voltage (V
OUT
/ V
REF
).
Advanced Monolithic Systems, Inc.
6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
