SPECIFICATION
BM1117A
Connected as shown above , RP is not
multiplied by the divider ratio
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 using1/16” FR-4 board
with 1 oz. copper foil, and it can be used as a
rough guideline for estimating thermal
resistance.
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 BM1117A 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 BM1117A is
15°C/W. Thermal resistance from tab to
ambient can be as low as 30°C/W.
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 BM1117A is equal
to:
PD = ( VIN - VOUT )( IOUT
)
Maximum junction temperature will be equal
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.
to:
TJ = TA(MAX) + PD
PD :Thermal esistance (junction-to-ambient)
Maximum junction temperature must not
exceed 125°C.
SOT223 Table 1.
COPPER AREA
THERMAL RESISTANCE
BOARD AREA
(JUNCTION-TO-AMBIENT)
TOP SIDE*
2500 Sq. mm
1000 Sq. mm
225 Sq. mm
100 Sq. mm
1000 Sq. mm
1000 Sq. mm
BACK SIDE
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
1000 Sq. mm
0
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
1000 Sq. mm
1000 Sq. mm
45°C/W
45°C/W
53°C/W
59°C/W
52°C/W
55°C/W
BOOKLY MICRO ELECTRONICS LIMITED CORP.