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LT1766/LT1766-5

1766fc

Thermal resistance for the LT1766 packages is inﬂuenced

by the presence of internal or backside planes.

SSOP (GN16) package: With a full plane under the GN16

package, thermal resistance will be about 85°C/W.

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 0.63V at 1A)

DIODE

)(

– )( )

063 40 5 1

055

PINDUCTOR = (ILOAD)2 (RL)

RL = Inductor DC resistance (assume 0.1Ω)

PINDUCTOR (1)2 (0.1) = 0.1W

Only a portion of the temperature rise in the external inductor

and diode is coupled to the junction of the LT1766. Based

on empirical measurements the thermal effect on LT1766

junctiontemperatureduetopowerdissipationintheexternal

inductor and catch diode can be calculated as:

ΔTJ(LT1766) ≈ (PDIODE + PINDUCTOR)(10°C/W)

Using the example calculations for LT1766 dissipation, the

LT1766 die temperature will be estimated as:

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

With the GN16 package (

θJA = 85°C/W), at an ambient

temperature of 60°C:

TJ = 60 + (85 • 0.53) + (10 • 0.65) = 112°C

With the TSSOP package (

θJA = 45°C/W), at an ambient

temperature of 60°C:

TJ = 60 + (45 • 0.53) + (10 • 0.65) = 90°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 calculation of the LT1766 die temperature. If a more

accurate die temperature is required, a measurement of

the SYNC pin resistance (to GND) can be used. The SYNC

pin resistance can be measured by forcing a voltage no

greater than 0.5V at the pin and monitoring the pin cur-

rent over temperature in an oven. This should be done

with minimal device power (low VIN and no switching

(VC = 0V)) in order to calibrate SYNC pin resistance with

ambient (oven) temperature.

Note: Some of the internal power dissipation in the IC,

due to BOOST pin voltage, can be transferred outside

of the IC to reduce junction temperature, by increasing

the voltage drop in the path of the boost diode D2 (see

Figure 9). This reduction of junction temperature inside

the IC will allow higher ambient temperature operation for

a given set of conditions. BOOST pin circuitry dissipates

power given by:

DISS BOOST

OUT

()

•(

) •

Typically VC2 (the boost voltage across the capacitor C2)

equals Vout. This is because diodes D1 and D2 can be

considered almost equal, where:

VC2 = VOUT – VFD2 – (–VFD1) = VOUT

Hence the equation used for boost circuitry power dissi-

pation given in the previous Thermal Calculations section

is stated as:

DISS BOOST

OUT

()

•(

)•

Here it can be seen that boost power dissipation increases

as the square of VOUT. It is possible, however, to reduce VC2

below VOUT to save power dissipation by increasing the

voltage drop in the path of D2. Care should be taken that

VC2 does not fall below the minimum 3.3V boost voltage

required for full saturation of the internal power switch.

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