REV. 0

–6–

ADM663A/ADM666A

Temperature Proportional Output

ture coefficient of +2.5 mV/

°C. This may be connected to the

tor resulting in a negative temperature coefficient at the output

of the regulator. This is especially useful in multiplexed LCD

displays to compensate for the inherent negative temperature

coefficient of the LCD threshold. At +25

°C the voltage at the

VTC output is typically 0.9 V. The equations for setting both

the output voltage and the tempco are given below. If this func-

R2

R1









+

R2

R3

()

– R2

R3

()

R3

R1

R2

ADM663A

V

TC

V

SET

SENSE

V

OUT2

V

OUT

Figure 8. ADM663A Temperature Proportional Output

APPLICATION HINTS

Input-Output (Dropout Voltage)

A regulator’s minimum input-output differential or dropout

voltage determines the lowest input voltage for a particular out-

put voltage. The ADM663A/ADM666A dropout voltage is 1 V

at its rated output current. For example when used as a fixed

+5 V regulator, the minimum input voltage is +6 V. At lower

be used as the output driver in order to achieve lower dropout

voltages. In this case the dropout voltage depends on the voltage

drop across the internal FET transistor. This may be calculated

by multiplying the FET’s saturation resistance by the output

Ω. Therefore,

the dropout voltage for 5 mA is 100 mV. As the current limit

GND

ADM663A

SENSE

V

OUT2

V

IN

V

SET

SHDN

+5V

OUTPUT

+6V TO +16V

INPUT

V

OUT1

Figure 9. Low Current, Low Dropout Configuration

Thermal Considerations

The ADM663A/ADM666A can supply up to 100 mA load cur-

rent and can operate with input voltages up to 16.5 V, but the

package power dissipation and hence the die temperature must

be kept within the maximum limits. The package power dissi-

pation is calculated from the product of the voltage differential

across the regulator times the current being supplied to the load.

The power dissipation must be kept within the maximum limits

given in the Absolute Maximum Ratings section.

The die temperature is dependent on both the ambient tempera-

ture and on the power being dissipated by the device. The

ADM663A/ADM666A contains an internal thermal limiting cir-

cuit which will shut down the regulator if the internal die tem-

perature exceeds 125

°C. Therefore, care must be taken to

ensure that, under normal operating conditions, the die tem-

perature is kept below the thermal limit.

This may be expressed in terms of power dissipation as follows:

where:

θ

If the device is being operated at the maximum permitted ambi-

ent temperature of 85

°C the maximum power dissipation per-

mitted is:

θ

θ

= 40/

θ

JA

θ

°C/W for the 8-pin DIP (N-8) package

θ

°C/W for the 8-pin SOIC (R-8) package

Therefore, for a maximum ambient temperature of 85

°C

At lower ambient temperatures the maximum permitted power

dissipation increases accordingly up to the maximum limits

specified in the absolute maximum specifications.

The thermal impedance (

θ

air conditions and are reduced considerably where fan assisted

cooling is employed. Other techniques for reducing the thermal

impedance include large contact pads on the printed circuit

board and wide traces. The copper will act as a heat exchanger

thereby reducing the effective thermal impedance.

High Power Dissipation Recommendations

Where excessive power dissipation due to high input-output dif-

ferential voltages and or high current conditions exists, the sim-

plest method of reducing the power requirements on the

regulator is to use a series dropper resistor. In this way the ex-

cess power can be dissipated in the external resistor. As an ex-

ample, consider an input voltage of +12 V and an output

voltage requirement of +5 V @ 100 mA with an ambient tem-

perature of +85

°C. The package power dissipation under these

conditions is 700 mW which exceeds the maximum ratings. By

using a dropper resistor to drop 4 V, the power dissipation re-

quirement for the regulator is reduced to 300 mW which is

within the maximum specifications for the N-8 package at