Advanced Monolithic Systems, Inc.

6680B Sierra Lane, Dublin, CA 94568 Phone (925) 556-9090 Fax (925) 556-9140

AMS1117

APPLICATION HINTS

AMS1117

IN

OUT

ADJ

R

L

R1*

R2*

V

IN

R

P

PARASITIC

LINE RESISTANCE

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 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.

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:

Maximum junction temperature will be equal to:

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

Ω to

200

Ω) 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

Ω 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

Table 1.

COPPER AREA

THERMAL RESISTANCE

TOP SIDE*

BACK SIDE

BOARD AREA

(JUNCTION-TO-AMBIENT)

2500 Sq. mm

2500 Sq. mm

2500 Sq. mm

45

°C/W

1000 Sq. mm

2500 Sq. mm

2500 Sq. mm

45

°C/W

225 Sq. mm

2500 Sq. mm

2500 Sq. mm

53

°C/W

100 Sq. mm

2500 Sq. mm

2500 Sq. mm

59

°C/W

1000 Sq. mm

1000 Sq. mm

1000 Sq. mm

52

°C/W

1000 Sq. mm

0

1000 Sq. mm

55

°C/W

* Tab of device attached to topside copper.