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LT3430

sn3430 3430is

THERMAL CALCULATIONS

Power dissipation in the LT3430 chip comes from four

sources: switch DC loss, switch AC loss, boost circuit

current, and input quiescent current. The following formu-

las show how to calculate each of these losses. These

formulas assume continuous mode operation, so they

should not be used for calculating efficiency at light load

currents.

Switch loss:

SW OUT

OUT

EFF

OUT

() (

)

()( )( )

(/ )

Boost current loss:

BOOST

OUT

()

Quiescent current loss:

OUT

()+

()

0 0015

0 003

RSW = Switch resistance (≈ 0.15) hot

tEFF = Effective switch current/voltage overlap time

= (tr + tf + tIr + tIf)

tr = (VIN/1.2)ns

tf = (VIN/1.1)ns

tIr = tIf = (IOUT/0.05)ns

f = Switch frequency

Example: with VIN = 40V, VOUT = 5V and IOUT = 2A:

BOOST

= (

)()()

()()()( )()

= () () =

−

015 2

150 10

1 2 2 40 200 10

008 1 2

1 28

52 36

004

40 0 0015

5 0 003

0 08

•/

•

)

( .

)

Total power dissipation in the IC is given by:

PTOT = PSW + PBOOST + PQ

= 1.28W + 0.04W + 0.08W = 1.4W

Thermal resistance for the LT3430 package is influenced

by the presence of internal or backside planes.

TSSOP (Exposed Pad) Package: With a full plane under the

TSSOP package, thermal resistance will be about 45

°C/W.

To calculate die temperature, use the proper thermal

resistance number for the desired package and add in

worst-case ambient temperature:

TJ = TA + (θJA • PTOT)

When estimating ambient, remember the nearby catch

diode and inductor will also be dissipating power:

DIODE

OUT

LOAD

( )(

–

)(

)

VF = Forward voltage of diode (assume 0.52V at 2A)

DIODE ==

)(

– )( )

052 40 5 2

091

Notice that the catch diode’s forward voltage contributes

a significant loss in the overall system efficiency. A larger,

lower VF diode can improve efficiency by several percent.

PINDUCTOR = (ILOAD)(LDCR)

LDCR = Inductor DC resistance (assume 0.1Ω)

PINDUCTOR (2)(0.1) = 0.2W

Typical thermal resistance of the board is 5

°C/W. Taking

the catch diode and inductor power dissipation into ac-

count and using the example calculations for LT3430

dissipation, the LT3430 die temperature will be estimated

as:

TJ = TA + (θJA • PTOT) + [10 • (PDIODE + PINDUCTOR)]

With the TSSOP package (

θJA = 45°C/W), at an ambient

temperature of 50

°C:

TJ = 50 + (45 • 1.4) + (5 • 1.11) = 119°C

Die temperature can peak for certain combinations of VIN,

VOUT and load current. While higher VIN gives greater

switch AC losses, quiescent and catch diode losses, a

lower VIN may generate greater losses due to switch DC

losses. In general, the maximum and minimum VIN levels

should be checked with maximum typical load current for

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