AMS2908
APPLICATION HINTS
Connected as shown , R
P
is not multiplied by the divider ratio
R
P
PARASITIC
LINE RESISTANCE
V
IN
AMS2908
IN
OUT
ADJ
R1*
R
L
R2*
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 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 AMS2908 is equal to:
P
D
= ( V
IN
- V
OUT
)( I
OUT
)
Maximum junction temperature will be equal to:
T
J
= T
A(MAX)
+ P
D
(Thermal Resistance (junction-to-ambient))
Maximum junction temperature must not exceed 125°C.
*
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.
Ripple Rejection
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
).
Thermal Considerations
The AMS2908 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 thermal resistance from the junction to the tab for the
AMS2908 is 15
°C/W.
Thermal resistance from tab to ambient can
be as low as 30°C/W.
Table 1.
COPPER AREA
TOP SIDE*
BACK SIDE
BOARD AREA
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
1000 Sq. mm
2500 Sq. mm
2500 Sq. mm
225 Sq. mm
2500 Sq. mm
2500 Sq. mm
100 Sq. mm
2500 Sq. mm
2500 Sq. mm
1000 Sq. mm
1000 Sq. mm
1000 Sq. mm
1000 Sq. mm
0
1000 Sq. mm
* Tab of device attached to topside copper.
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
45°C/W
45°C/W
53°C/W
59°C/W
52°C/W
55°C/W
Advanced Monolithic Systems, Inc.
www.advanced-monolithic.com
Phone (925) 443-0722
Fax (925) 443-0723
ADMOS [ ADVANCED MONOLITHIC SYSTEMS ]
