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LT3430/LT3430-1

34301fa

THERMAL CALCULATIONS

Power dissipation in the LT3430/LT3430-1 chip comes

from four sources: switch DC loss, switch AC loss, boost

circuit current, and input quiescent current. The follow-

ing formulas 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

() (

)

()( )( )

(/ )

(Note: Switching losses are less for the LT3430-1 oper-

ating at only 100kHz)

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/0.2)ns

f = Switch frequency

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

BOOST

= (

)() ()

()()()( )()

= () () =

−

015 2

90 10

1 2 2 40 200 10

008 072

52 36

004

40 0 0015

5 0 003

0 08

•/

•

)

Total power dissipation in the IC is given by:

PTOT = PSW + PBOOST + PQ

= 0.8W + 0.04W + 0.08W = 0.92W

Thermal resistance for the LT3430/LT3430-1 package is in-

ﬂuenced 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

PINDUCTOR = (ILOAD)

2(RIND)

RIND = Inductor DC resistance (assume 0.1Ω)

PINDUCTOR (2)

2(0.1) = 0.4W

Only a portion of the temperature rise in the external

inductor and diode is coupled to the junction of the

LT3430. Based on empirical measurements, the thermal

effect on the LT3430 junction temperature due to power

dissipation in the external inductor and catch diode can

be calculated as:

∆TJ(LT3430) ≈ (PDIODE + PINDUCTOR)(5°C/W)

Using the example calculations for LT3430 dissipation, the

LT3430 die temperature will be estimated as:

TJ = TA + (θJA • PTOT) + [5 • (PDIODE + PINDUCTOR)]

With the TSSOP package (θJA = 45°C/W), at an ambient

temperature of 50°C:

TJ = 50 + (45 • 0.92) + (5 • 1.31) = 98°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

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