SC486 datasheet(18/26 Pages) SEMTECH

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SC486

POWER MANAGEMENT

Thermal Considerations

The junction temperature of the device may be calculated

as follows:

•

Where:

A = ambient temperature (°C)

D = power dissipation in (W)

JA = thermal impedance junction to ambient from

absolute maximum ratings (°C/W)

The power dissipation may be calculated as follows,

assuming that VTT spends 50% of its time sourcing

current and 50% sinking:

()

ITT

VTT

VTTIN

VBST

VDDP

VCCA

VDDP

VCCA

•

−

•

Where:

VCCA = chip supply voltage (V)

VCCA = operating current (A)

VDDP = gate drive supply voltage (V)

VDDP = gate drive operating current (A)

g = gate drive voltage, typically 5V (V)

g = FET gate charge, from the FET datasheet (C)

f = switching frequency (Hz)

VBST = boost pin voltage during t

ON (V)

D = duty cycle

VTTIN = input voltage for VTT LDO (V)

ITT = maximum VTT current (A)

Inserting the following values for VBAT

(MIN) condition (since

this is the worst case condition for power dissipation in

the controller) as an example):

A = 85°C

JA = 29°C/W

VCCA = VDDP = 5V

VCCA = 2500µA (data sheet maximum)

VDDP = 150µA (data sheet maximum)

g = 5V

g = 60nC

f = 366kHz

VBAT

(MIN) = 8V

VBST

(MIN) = VBAT(MIN)+VDDP = 13V

1(MIN) = 1.8/8 = 0.225

VDDQ = VTTIN = 1.8V

VTT = 0.9V

ITT = 1.2A

gives us:

()

206

225

366

150

2500

•

−

•

−

and therefore:

120

206

•

As can be seen, the heating effects due to internal power

dissipation are dominated by the VTT LDO, but they can

be managed comfortably by the MLPQ-24 package which

is heatsunk to the ground plane using 4 vias from its

thermal pad.

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