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LT3437

3437fc

APPLICATIO S I FOR ATIO

THERMAL CALCULATIONS

Power dissipation in the LT3437 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, and 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:

PQ = VIN (500µA) + VOUT (800µA)

RSW = switch resistance (≈1 when hot )

tEFF = effective switch current/voltage overlap time

(tr + tf + tIR + tIF)

tr = (VIN/0.6)ns

tf = (VIN/2)ns

tIR = tIF = (IOUT/0.05)ns

f = switch frequency

Example: with VIN = 40V, VOUT = 5V and IOUT = 250mA:

BOOST

= ( )(

)()

()

()()( )(

)

= () () =

()+ () =

1 0 25

92 12 0 25 40 200 3

0 008 0 092

0 1

50 25 30

0 005

40 0 0005

5 0 0008

0 024

Total power dissipation is:

PTOT = 0.1 + 0.065 + 0.024 = 0.13W

Thermal resistance for the LT3437 package is influenced

by the presence of internal or backside planes. With a full

plane under the package, thermal resistance will be about

°C for the FE and DD packages. No plane will increase

resistance to about 150

°C/W.Tocalculatedietemperature,

use the proper thermal resistance number for the desired

package and add in worst-case ambient temperature:

TJ = TA + QJA (PTOT)

With the DD package (QJA = 45°C/W) at an ambient

temperature of 70

°C:

TJ = 70 + 45(0.1) = 74.5°C

HIGH TEMPERATURE OPERATION

Extreme care must be taken when designing LT3437

applications to operate at high ambient temperatures. The

LT3437 is designed to work at elevated temperatures but

erratic operation can occur due to external components.

Each passive component should be checked for absolute

value and voltage ratings to ensure loop stability at tem-

perature. Boost and Catch diode leakages, as well as

increased series resistance, will adversely affect efficiency

and low quiescent current operation. Junction tempera-

ture increase in the diodes due to self heating (leakage)

and power dissipation should be measured to ensure their

maximum temperature specifications are not violated.

Input Voltage vs Operating Frequency Considerations

The absolute maximum input supply voltage for the LT3437

is specified at 80V. This is based solely on internal semi-

conductor junction breakdown effects. Due to internal

power dissipation, the actual maximum VIN achievable in

a particular application may be less than this.

A detailed theoretical basis for estimating internal power

loss is given in the section Thermal Considerations. Note

that AC switching loss is proportional to both operating

frequency and output current. The majority of AC switch-

ing loss is also proportional to the square of input voltage.

For example, while the combination of VIN = 40V, VOUT =

5V at 700mA and fOSC = 200kHz may be easily achievable,

simultaneously raising VIN to 80V and fOSC to 700kHz is not

possible. Nevertheless, input voltage transients up to 80V

can usually be accommodated, assuming the resulting

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