LTC3633 datasheet(20/28 Pages) LINER

20 / 28 page

LTC3633

3633f

APPLICATIONS INFORMATION

is to determine whether the power dissipated exceeds the

maximum junction temperature of the part. The tempera-

ture rise is given by:

TRISE = PD • θJA

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

tors is used in an application where VIN = 12V, IOUT = 2A,

frequency = 2MHz, VOUT = 1.8V. From the RDS(ON) graphs

in the Typical Performance Characteristics section, the top

switch on-resistance is nominally 140mΩ and the bottom

switch on-resistance is nominally 80mΩ at 70°C ambient.

The equivalent power MOSFET resistance RSW is:

DS(ON)TOP •

1.8V

12V

+RDS(ON)BOT •

10.2V

12V

= 89mΩ

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 + VIN • (IGATECHG + IQ)

PD = (2A)2 • (0.089Ω) + (12V) • (4.6mA + 0.65mA)

= 0.419W

Running two regulators under the same conditions would

result in a power dissipation of 0.838W. 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 = 0.838W • 43°C/W + 70°C = 106°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 106°C yields a new

junction temperature of 109°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 7 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.

Figure 7. Temperature Derating Curve for DC1347 Demo Circuit

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 established, then the

junction temperature rise can be measured as a function

0.5

1.0

1.5

2.0

3.0

2.5

125

3633 F07

3.5

100

MAXIMUM ALLOWABLE AMBIENT

TEMPERATURE (°C)

CH2 LOAD = 0A

CH2 LOAD = 1A

CH2 LOAD = 2A

CH2 LOAD = 3A

LTC3633 Arkusz danych(PDF) 20 Page - Linear Technology

LTC3633 Arkusz danych(HTML) 20 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Arkusz danych Pobierz

Link URL