AMS1117-18 ADMOS [Advanced Monolithic Systems], AMS1117-18 Datasheet - Page 5

• Manufacturer Part Number: AMS1117-18
• Description: 800mA LOW DROPOUT VOLTAGE REGULATOR
• Manufacturer: ADMOS [Advanced Monolithic Systems]

APPLICATION HINTS
Connected as shown , R
*
Kelvin internally, and the ground pin can be used for negative
side sensing.
Thermal Considerations
The AMS1117 series have internal power and thermal limiting
circuitry designed to protect the device under overload conditions.
However maximum junction temperature ratings of 125
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
AMS1117 is 15
can be as low as 30 C/W.
Table 1.
TOP SIDE*
* Tab of device attached to topside copper.
Advanced Monolithic Systems, Inc. 6680B Sierra Lane, Dublin, CA 94568 Phone [phone] Fax [phone]
2500 Sq. mm
1000 Sq. mm
1000 Sq. mm
1000 Sq. mm
In the case of fixed voltage devices the top of R1 is connected
225 Sq. mm
100 Sq. mm
CONNECT R1 TO CASE
CONNECT R2 TO LOAD
V
IN
Figure 3. Connections for Best Load Regulation
COPPER AREA
BACK SIDE
IN
C/W.
AMS1117
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
1000 Sq. mm
ADJ
OUT
P
0
Thermal resistance from tab to ambient
is not multiplied by the divider ratio
LINE RESISTANCE
R1*
R2*
PARASITIC
BOARD AREA
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
2500 Sq. mm
1000 Sq. mm
1000 Sq. mm
R
P
R
C
L
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
should
45 C/W
45 C/W
53 C/W
59 C/W
52 C/W
55 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 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 AMS1117 is equal to:
P
Maximum junction temperature will be equal to:
T
Maximum junction temperature must not exceed 125 C.
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 100
200 ) 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=100
the adjust pin capacitor should be 13 F. At 10kHz only 0.16 F
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
D
J
= T
= ( V
A(MAX)
IN
- V
+ P
OUT
D
(Thermal Resistance (junction-to-ambient))
)( I
OUT
OUT
/ V
)
REF
).
AMS1117
at 120Hz
to