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LTC3633A-2/LTC3633A-3

3633a23f

APPLICATIONS INFORMATION

As an example, consider the case when one of the regu-

lators is used in an application where VIN = SVIN = 12V,

IOUT=2A,frequency=2MHz,VOUT=1.8V.FromtheRDS(ON)

graphs in the Typical Performance Characteristics section,

the top switch on-resistance is nominally 145mΩ and the

bottom switch on-resistance is nominally 70mΩ at 70°C

ambient. The equivalent power MOSFET resistance RSW is:

RDS(ON)TOP •

1.8V

12V

+RDS(ON)BOT •

10.2V

12V

= 81.3mΩ

From the previous section’s discussion on gate drive, we

estimate the total gate drive current through the LDO to be

2MHz • 2.3nC = 4.6mA, and IQ of one channel is 0.65mA

(see Electrical Characteristics). Therefore, the total power

dissipated by a single regulator is:

PD = IOUT2 • RSW + SVIN • (IGATECHG + IQ)

PD = (2A)2 • (0.0813Ω) + (12V) • (4.6mA + 0.65mA)

= 0.388W

Running two regulators under the same conditions would

result in a power dissipation of 0.776W. The QFN 5mm

× 4mm package junction-to-ambient thermal resistance,

θJA, is around 43°C/W. Therefore, the junction temperature

of the regulator operating in a 70°C ambient temperature

is approximately:

TJ = 0.776W • 43°C/W + 70°C = 103°C

which is below the maximum junction temperature of

125°C. With higher ambient temperatures, a heat sink

or cooling fan should be considered to drop the junc-

tion-to-ambient thermal resistance. Alternatively, the

TSSOP package may be a better choice for high power

applications, since it has better thermal properties than

the QFN package.

Remembering that the above junction temperature is

obtained from an RDS(ON) at 70°C, we might recalculate

the junction temperature based on a higher RDS(ON) since

it increases with temperature. Redoing the calculation

assuming that RSW increased 12% at 103°C yields a new

junction temperature of 107°C. If the application calls for

a higher ambient temperature and/or higher load currents,

care should be taken to reduce the temperature rise of the

part by using a heat sink or air ﬂow.

Figure 8. Temperature Derating Curve for DC1347 Demo Circuit

Figure 8 is a temperature derating curve based on the

DC1347 demo board (QFN package). It can be used to

estimate the maximum allowable ambient temperature

for given DC load currents in order to avoid exceeding

the maximum operating junction temperature of 125°C.

0.5

1.0

1.5

2.0

3.0

2.5

125

3633a23 F08

3.5

100

MAXIMUM ALLOWABLE AMBIENT

TEMPERATURE (°C)

CH2 LOAD = 0A

CH2 LOAD = 1A

CH2 LOAD = 2A

CH2 LOAD = 3A

Junction Temperature Measurement

The junction-to-ambient thermal resistance will vary de-

pending on the size and amount of heat sinking copper

on the PCB board where the part is mounted, as well as

the amount of air ﬂow on the device. In order to properly

evaluate this thermal resistance, the junction temperature

needs to be measured. A clever way to measure the junction

temperature directly is to use the internal junction diode

on one of the pins (PGOOD) to measure its diode voltage

change based on ambient temperature change.

First remove any external passive component on the PGOOD

pin, then pull out 100μA from the PGOOD pin to turn on its

internal junction diode and bias the PGOOD pin to a negative

voltage. With no output current load, measure the PGOOD

voltage at an ambient temperature of 25°C, 75°C and 125°C

to establish a slope relationship between the delta voltage on

PGOOD and delta ambient temperature. Once this slope is es-

tablished,thenthejunction temperaturerisecanbemeasured

asafunctionofpowerlossinthepackagewithcorresponding

output load current. Although making this measurement with

this method does violate absolute maximum voltage ratings

on the PGOOD pin, the applied power is so low that there

should be no signiﬁcant risk of damaging the device.

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