LTC3633 LINER [Linear Technology], LTC3633 Datasheet - Page 20

• Manufacturer Part Number: LTC3633
• Description: Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator
• Manufacturer: LINER [Linear Technology]

LTC3633
is to determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
As an example, consider the case when one of the regula-
tors is used in an application where V
frequency = 2MHz, V
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 R
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 I
(see Electrical Characteristics). Therefore, the total power
dissipated by a single regulator is:
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,
θ
of the regulator operating in a 70°C ambient temperature
is approximately:
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 R
the junction temperature based on a higher R
it increases with temperature. Redoing the calculation
assuming that R
APPLICATIONS INFORMATION
20
JA
T
P
P
= 0.419W
T
R
, is around 43°C/W. Therefore, the junction temperature
RISE
J
D
D
DS(ON)TOP
= 0.838W • 43°C/W + 70°C = 106°C
= I
= (2A)
= P
OUT
D
2
2
• θ
• R
• (0.089Ω) + (12V) • (4.6mA + 0.65mA)
•
1.8V
SW
JA
12V
SW
DS(ON)
increased 12% at 106°C yields a new
+ V
OUT
+R
IN
= 1.8V. From the R
DS(ON)BOT
• (I
at 70°C, we might recalculate
Q
GATECHG
of one channel is 0.65mA
•
10.2V
IN
12V
+ I
= 12V, I
Q
)
DS(ON)
= 89mΩ
SW
DS(ON)
OUT
is:
graphs
= 2A,
since
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 fl ow.
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.
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 fl ow 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
Figure 7. Temperature Derating Curve for DC1347 Demo Circuit
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
MAXIMUM ALLOWABLE AMBIENT
CH2 LOAD = 0A
CH2 LOAD = 1A
CH2 LOAD = 2A
CH2 LOAD = 3A
25
TEMPERATURE (°C)
50
75
100
3633 F07
125
3633f