AMS317-18 Ver la hoja de datos (PDF) - Advanced Monolithic Systems Inc

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AMS317-18

1A LOW DROPOUT VOLTAGE REGULATOR

Advanced Monolithic Systems Inc 

AMS317

APPLICATION HINTS

Load Regulation

True remote load sensing it is not possible to provide, because the

AMS317 is a three terminal device. The resistance of the wire

connecting the regulator to the load will limit the load regulation.

The data sheet specification for load regulation is measured at the

bottom of the package. Negative side sensing is a true Kelvin

connection, with the bottom of the output divider returned to the

negative side of the load.

The best load regulation is obtained when the top of the resistor

divider R1 is connected directly to the case not to the load. If R1

were connected to the load, the effective resistance between the

regulator and the load would be:

RP x ( R2+R1 ) , RP = Parasitic Line Resistance

R1

Connected as shown , RP is not multiplied by the divider ratio

RP

PARASITIC

AMS317

LINE RESISTANCE

VIN

IN OUT

ADJ

R1*

RL

R2*

*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.

Thermal Considerations

The AMS317 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

AMS317 is 15°C/W; thermal resistance from tab to ambient 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 the TO-252 package, if the maximum allowable value is

found to be ≥92°C/W, no heat sink is needed since the package

alone will dissipate enough heat to satisfy these requirements. If

the calculated value for θJA falls bellow these limits, a heat sink

is required.

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 AMS317 is equal to:

PD = ( VIN - VOUT )( IOUT )

Maximum junction temperature will be equal to:

TJ = TA(MAX) + PD(Thermal Resistance (junction-to-ambient))

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 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 (VOUT / VREF ).

Advanced Monolithic Systems, Inc. www.advanced-monolithic.com Phone (925) 443-0722 Fax (925) 443-0723

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