Advanced Monolithic Systems, Inc.

www.advanced-monolithic.com

Phone (925) 443-0722

Fax (925) 443-0723

AMS2908

APPLICATION HINTS

AMS2908

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 AMS2908 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

AMS2908 is 15

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 AMS2908 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.