TC55_05 datasheet(10/18 Pages) MICROCHIP

10 / 18 page

TC55

DS21435F-page 10

5.0

THERMAL CONSIDERATIONS

5.1

Power Dissipation

The amount of power dissipated internal to the low

dropout linear regulator is the sum of the power dissi-

pation within the linear pass device (P-Channel MOS-

FET) and the quiescent current required to bias the

internal reference and error amplifier. The internal lin-

ear pass device power dissipation is calculated by mul-

tiplying the voltage across the linear device by the

current through the device.

EQUATION

The internal power dissipation, as a result of the bias

current for the LDO internal reference and error

amplifier, is calculated by multiplying the ground or

quiescent current by the input voltage.

EQUATION

The total internal power dissipation is the sum of PD

(Pass Device) and PD (Bias).

EQUATION

For the TC55, the internal quiescent bias current is so

low (1 µA typical) that the PD (Bias) term of the power

dissipation equation can be ignored. The maximum

power dissipation can be estimated by using the

maximum input voltage and the minimum output

voltage to obtain a maximum voltage differential

between input and output. The next step would be to

multiply the maximum voltage differential by the

maximum output current.

EQUATION

Given:

VIN

= 3.3V to 4.1V

VOUT = 3.0 V ± 2%

IOUT = 1 mA to 100 mA

TAMAX = 55°C

PMAX = (4.1V – (3.0V x 0.98)) x 100 mA

PMAX = 116.0 milliwatts

To determine the junction temperature of the device, the

thermal resistance from junction-to-ambient must be

known. The 3-pin SOT-23 thermal resistance from junc-

tion-to-air (RθJA) is estimated to be approximately

359°C/W. The SOT-89 RθJA is estimated to be approxi-

mately 110°C/W when mounted on 1 square inch of

copper. The TO-92 RθJA is estimated to be 131.9°C/W.

The RθJA will vary with physical layout, airflow and other

application-specific conditions.

The device junction temperature is determined by

calculating

the

junction

temperature

rise

above

ambient, then adding the rise to the ambient

temperature.

EQUATION

PD (Pass Device) = (VIN – VOUT) x IOUT

PD (Bias) = VIN x IGND

PTOTAL = PD (Pass Device) + PD (Bias)

PD = (VINMAX – VOUTMIN) x IOUTMAX

Junction Temperature

TJ = PDMAX x RθJA + TA

TJ = 116.0 milliwatts x 359°C/W + 55°C

TJ = 96.6°C

SOT-23 Example:

SOT-89 Example:

TJ = 116.0 milliwatts x 110°C/W + 55°C

TJ = 67.8°C

TO-92 Example:

TJ = 116.0 milliwatts x 131.9°C/W + 55°C

TJ = 70.3°C

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