TC59_13 datasheet(4/20 Pages) MICROCHIP

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TC59

DS21438C-page 4

 2001-2012 Microchip Technology Inc.

4.0

THERMAL CONSIDERATIONS

4.1

Power Dissipation

The amount of power dissipated internal to the low drop

out linear regulator is the sum of the power dissipation

within the linear pass device (P-Channel MOSFET),

and the quiescent current required to bias the internal

reference and error amplifier. The internal linear pass

device power dissipation is calculated multiplying the

voltage across the linear device times the current

through the device. The input and output voltages are

negative for the TC59. The power dissipation is

calculated using the absolute value of the voltage

difference between the input and output voltage.

TABLE 4-1:

MAXIMUM POWER

DISSIPATION

EQUATION 4-1:

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 times the input voltage.

EQUATION 4-2:

The total internal power dissipation is the sum of

Equation 4-1 and Equation 4-2.

EQUATION 4-3:

For the TC59, the internal quiescent bias current is so

low (3

A typical), the P

D (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 and multiplying the maximum

voltage differential by the maximum output current.

EQUATION 4-4:

For example, given the following conditions:

VIN

= -7.0V ±5%

VOUT

= -5.0V ±2%

IOUT

= 1mA to 40mA

TAMBIENT (MAX) = 55°C

PMAX

= (7V X (1.05) – (5.0V X 0.98)) X 40mA

PMAX

= 98.0 milli-Watts

To determine the junction temperature of the device,

the thermal resistance from junction to air must be

known. The SOT-23-3 RJA is estimated to be

approximately 359°C/W when mounted on a 4-layer

board. The RJA 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 4-5:

JUNCTION

TEMPERATURE

(SOT-23 EXAMPLE)

Package Type

Maximum Power

Dissipation

SOT-23-3

150mW

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

PD (Bias) = VIN X IGND

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

PMAX = (VIN (MAX) – VOUT (MIN)) X IOUT (MAX)

TJUNCTION = PD (MAX) X RJA + TAMBIENT

TJUNCTION = 98.0 milli-Watts X 359°C/W + 55°C

TJUNCTION = 90.2°C

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