MIC4680 datasheet(14/24 Pages) MICROCHIP

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MIC4680

DS20006623A-page 14



2021 Microchip Technology Inc. and its subsidiaries

5.4

Checking the Maximum Junction

Temperature

For this example, with an output power (POUT) of 5W,

(5V output at 1A maximum with VIN = 12V) and 65°C

maximum ambient temperature, what is the maximum

junction temperature?

Referring to Figure 2-11, read the efficiency (η) for 1A

output current at VIN = 12V or perform you own

measurement.

η = 79%

The efficiency is used to determine how much of the

output power (POUT) is dissipated in the regulator

circuit (PD).

EQUATION 5-4:

Calculate the worst-case junction temperature:

EQUATION 5-5:

Calculating the maximum junction temperature given a

maximum ambient temperature of 65°C:

EQUATION 5-6:

This value is less than the allowable maximum

operating junction temperature of 125°C as listed in the

Operating Ratings ‡. Typical thermal shutdown is

160°C and is listed in the Electrical Characteristics

table.

5.5

Increasing the Maximum Output

Current

The maximum output current at high input voltages can

be increased for a given board layout. The additional

three components shown in Figure 5-3 will reduce the

overall loss in the MIC4680 by about 20% at high VIN

and high IOUT.

Even higher output current can be achieved by using

the MIC4680 to switch an external FET. See Figure 5-7

for a 5A supply with current limiting.

FIGURE 5-3:

Increasing Maximum Output

Current at High Input Voltages.

5.6

Layout Considerations

Layout is very important when designing any switching

regulator. Rapidly changing switching currents through

the printed circuit board traces and stray inductance

can generate voltage transients which can cause

problems.

To minimize stray inductance and ground loops, keep

trace lengths, indicated by the heavy lines in

Figure 5-4, as short as possible. For example, keep D1

close to Pin 3 and Pins 5 through 8, keep L1 away from

sensitive node FB, and keep CIN close to Pin 2 and

Pins 5 though 8. See Thermal Considerations for

ground plane layout.

The feedback pin should be kept as far way from the

switching elements (usually L1 and D1) as possible.

FIGURE 5-4:

Critical Traces for Layout.

POUT



------------- POUT

–

0.79

---------- 5W

–

1.33W

TJ PD IC

 

JC



TC TA

–





TA MAX





Where:

TJ = Junction temperature.

PD(IC) = The MIC4680 power dissipation.

θJC = Junction-to-case thermal resistance (approx.

20°C/W for the MIC4680).

TC = Pin temperature measurement taken at the

entry point of pin 6 or 7 into the plastic package at the

ambient temperature at which TC is measured.

TA = Ambient temperature at which TC is measured.

TA(MAX) = Maximum ambient operating temperature

for the specific design.

TJ 1.064 20C/W



45C 25C

–





65C

TJ 106.3C

GND

SHDN

1N4148

2.2nF

MIC4680YM

5678

68μH

GND

SHDN

OUT

VOUT

MIC4680YM

GND

CIN

+4V to +34V

Power

SOIC-8

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