LTC3633_15 Datasheet, PDF (20/28 Pages) Linear Technology – Dual Channel 3A, 15V Monolithic Synchronous Step-Down Regulator

LTC3633
APPLICATIONS INFORMATION
is to determine whether the power dissipated exceeds the
maximum junction temperature of the part. The tempera-
ture rise is given by:
T_RISE = P_D • θ_JA
As an example, consider the case when one of the regula-
tors is used in an application where V_IN = 12V, I_OUT = 2A,
frequency = 2MHz, V_OUT = 1.8V. From the R_DS(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 R_SW is:
R_DS(ON)TOP
•
1.8V
12V
+R_DS(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 I_Q of one channel is 0.65mA
(see Electrical Characteristics). Therefore, the total power
dissipated by a single regulator is:
P_D = I_OUT2 • R_SW + V_IN • (I_GATECHG + I_Q)
P_D = (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,
θ_JA, is around 43°C/W. Therefore, the junction temperature
of the regulator operating in a 70°C ambient temperature
is approximately:
T_J = 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 junction-
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_DS(ON) at 70°C, we might recalculate
the junction temperature based on a higher R_DS(ON) since
it increases with temperature. Redoing the calculation
assuming that R_SW 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.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
CH2 LOAD = 0A
CH2 LOAD = 1A
CH2 LOAD = 2A
CH2 LOAD = 3A
25
50
75 100 125
MAXIMUM ALLOWABLE AMBIENT
TEMPERATURE (°C)
3633 F07
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 junc-
tion 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
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