MCP1701A datasheet(11/22 Pages) MICROCHIP

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DS21991C-page 11

MCP1701A

5.0

THERMAL CONSIDERATIONS

5.1

Power Dissipation

The amount of power dissipated internal to the LDO

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 as shown in

Equation 5-1.

EQUATION 5-1:

The internal power dissipation, as a result of the bias

current for the LDO internal reference and error

amplifier, is calculated as shown in Equation 5-2.

EQUATION 5-2:

The total internal power dissipation is the sum of PD

(pass device) and PD (bias).

EQUATION 5-3:

For the MCP1701A, the internal quiescent bias current

is so low (2 µA, typ.) 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 5-4:

To determine the junction temperature of the device, the

thermal resistance from junction-to-ambient must be

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

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

335°C/W. The SOT-89 RθJA is estimated to be

approximately 52°C/W when mounted on 1 square inch

of copper. 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 5-5: JUNCTION TEMPERATURE

– SOT-23A EXAMPLE:

EQUATION 5-6: JUNCTION TEMPERATURE

– SOT-89 EXAMPLE:

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

PD (Bias) = VIN x IGND

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

PD = (VINMAX – VOUTMIN) x IOUTMAX

Given:

VIN = 3.3V to 4.1V

VOUT =3.0V ± 2%

IOUT = 1 mA to 100 mA

TAMAX = 55°C

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

PDMAX RθJA TA

116.0 milliwatts 335°C/W

55°C

93.9°C

116.0 milliwatts 52°C/W

55°C

61°C

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