LTC3633A-3_15 Datasheet, PDF (20/28 Pages) Linear Technology – Dual Channel 3A, 20V Monolithic Synchronous Step-Down Regulator

LTC3633A-2/LTC3633A-3
APPLICATIONS INFORMATION
As an example, consider the case when one of the regu-
lators is used in an application where V_IN = SV_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 145mΩ and the
bottom switch on-resistance is nominally 70mΩ 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
=
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 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 + SV_IN • (I_GATECHG + I_Q)
P_D = (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,
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 = 0.776W • 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 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 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 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)
3633a23 F08
Figure 8. 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 es-
tablished, then the junction temperature rise can be measured
as a function of power loss in the package with corresponding
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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