BL8521 datasheet(6/8 Pages) BELLING

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REV2.0

Page6

BL8521

5.5V, 1.4MHz, 3A Synchronous Buck Converter

inductor core losses generally account for less than

2% of the total loss.

Thermal Considerations

In most applications, the BL8521 does not

dissipate much heat due to its high efficiency. But,

in applications where it is running at high ambient

temperature with low supply voltage and high duty

cycles, such as in dropout, the heat dissipated

may exceed the maximum junction temperature of

the part. If the junction temperature reaches

approximately 150

C, both power switches will be

turned off and the SW node will become high

impedance. To avoid the BL8521 from exceeding

the maximum junction temperature, the user will

need to do some thermal analysis. The goal of the

thermal analysis is to determine whether the

power dissipated exceeds the maximum junction

temperature of the part.

The temperature rise is given by: TR = PD

x θJA

Where PD is the power dissipated by the regulator

and θJA is the thermal resistance from the junction

of the die to the ambient temperature.

The junction temperature, TJ, is given by :

TJ = TA + TR Where TA is the ambient temperature.

As an example, consider the BL8521 in dropout at

an input voltage of 3.3V, a load current of 2A and

an ambient temperature of 70

C. The RDS(ON) of

the P-Channel switch at 70

C is approximately

121m

. Therefore, power dissipated by the part

is :

PD = (ILOAD)2 (RDS(ON)) = (2A)

2 (121m

) = 0.484W

For the DFN3x3 package, the

JA is 110C /W. Thus

the junction temperature of the regulator is :

TJ = 70

C + (0.484W) (110C /W) = 123.24C

Which is below the maximum junction temperature

of 125

C.

Note that at higher supply voltages, the junction

temperature

lower

due

reduced

switch

resistance

(RDS(ON)).

ELECTRICAL PERFOMANCE

(VDD=5V, VOUT=2.5V, TA=25C, unless otherwise specified)

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