MCP1804_13 datasheet(22/36 Pages) MICROCHIP

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MCP1804

DS20002200D-page 22

 2009-2013 Microchip Technology Inc.

6.3

Voltage Regulator

Internal power dissipation, junction temperature rise,

junction temperature and maximum power dissipation

are calculated in the following example. The power

dissipation resulting from ground current is small

enough to be neglected.

6.3.1

POWER DISSIPATION EXAMPLE

6.3.1.1

Device Junction Temperature Rise

The internal junction temperature rise is a function of

internal power dissipation and the thermal resistance

from junction to ambient for the application. The thermal

resistance from junction to ambient (R

JA) is derived

from an EIA/JEDEC standard for measuring thermal

resistance for small surface mount packages. The EIA/

JEDEC specification is JESD51-7, “High Effective

Thermal Conductivity Test Board for Leaded Surface

Mount Packages”. The standard describes the test

method and board specifications for measuring the

thermal resistance from junction to ambient. The actual

thermal resistance for a particular application can vary

depending on many factors, such as copper area and

thickness. Refer to AN792, “A Method to Determine

How Much Power a SOT23 Can Dissipate in an

Application” (DS00792), for more information regarding

this subject.

6.3.1.2

Junction Temperature Estimate

To estimate the internal junction temperature, the

calculated temperature rise is added to the ambient or

offset temperature. For this example, the worst-case

junction temperature is estimated below.

Maximum Package Power Dissipation at +25°C

Ambient Temperature (minimum PCB footprint)

6.4

Voltage Reference

The MCP1804 can be used not only as a regulator, but

also as a low quiescent current voltage reference. In

many microcontroller applications, the initial accuracy

of the reference can be calibrated using production test

equipment or by using a ratio measurement. When the

initial accuracy is calibrated, the thermal stability and

line regulation tolerance are the only errors introduced

by the MCP1804 LDO. The low-cost, low quiescent

current and small ceramic output capacitor are all

advantages when using the MCP1804 as a voltage

reference.

FIGURE 6-2:

Using the MCP1804 as a

Voltage Reference.

6.5

Pulsed Load Applications

For some applications, there are pulsed load current

events that may exceed the specified 150 mA

maximum specification of the MCP1804. The internal

current limit of the MCP1804 will prevent high peak

load demands from causing non-recoverable damage.

The 150 mA rating is a maximum average continuous

rating. As long as the average current does not exceed

150 mA or the maximum power dissipation of the

packaged device, pulsed higher load currents can be

applied to the MCP1804

. The typical current limit for

the MCP1804 is 200 mA (TA =+25°C).

Package:

Package Type = SOT-23

Input Voltage:

VIN = 3.8V to 4.6V

LDO Output Voltages and Currents:

VOUT =1.8V

IOUT =50 mA

Maximum Ambient Temperature:

TA(MAX) =+40°C

Internal Power Dissipation:

Internal Power dissipation is the product of the LDO

output current times the voltage across the LDO

(VIN to VOUT).

PLDO(MAX) =(VIN(MAX) -VOUT(MIN))x IOUT(MAX)

PLDO(MAX) = (4.6V - (0.98 x 1.8V)) x 50 mA

PLDO(MAX) = 141.8 milli-Watts

TJ(RISE) =PTOTAL xRJA

TJ(RISE) = 141.8 milli-Watts x 256.0°C/Watt

TJ(RISE) =36.3°C

TJ =TJ(RISE) +TA(MAX)

TJ = 76.3°C

SOT-23 (256°C/Watt = R

JA):

PD(MAX) = (125°C - 25°C) / 256°C/W

PD(MAX) = 390 milli-Watts

SOT-89 (180°C/Watt = R

JA):

PD(MAX) = (125°C - 25°C) / 180°C/W

PD(MAX) = 555 milli-Watts

PICmicro®

GND

VIN

CIN

1µF

COUT

1µF

Bridge Sensor

VOUT

VREF

AD0

AD1

Ratio Metric Reference

50 µA Bias

Microcontroller

MCP1804

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