LTC3634
22
3634fc
For more information www.linear.com/LTC3034
APPLICATIONS INFORMATION
be 1MHz • 2.3nC = 2.3mA, and the total I
Q
of both chan-
nels is 1.3mA (see the Electrical Characteristics section).
Therefore, the total power dissipated by both regulators is:
P
D
= I
OUT1
( )
2
•R
SW1
+ I
OUT2
2
•R
SW2
+ V
IN
• I
GATECHG
+I
Q
( )
P
D
=(2A)
2
•0.0848Ω+(2A)
2
•0.0799Ω
+12V • 2.3mA •2
( )
+1.3mA
= 0.730W
The QFN 4mm × 5mm 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
= 0.730W • 43°C/W + 70°C = 101°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 exposed pad
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 ob-
tained from a 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 as-
suming that R
SW
increased 12% at 101°C yields a new
junction temperature of 105°C.
Figure 8 is a temperature derating curve based on the
DC1708 demo board (QFN package). It can be used as
a guideline 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 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
Figure 8. Temperature Derating Curve for DC1708 Demo Circuit
temperature directly is to use the internal junction diode
on one of the PGOOD pins 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
voltage on PGOOD and ambient temperature. Once this
slope is established, 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.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of
the LTC3634. Check the following in your layout:
1. Do the input capacitors connect to the V
IN
and PGND
pins as close as possible? These capacitors provide
the AC current to the internal power MOSFETs and their
drivers.
2. The output capacitor, C
OUT
, and inductor L should be
closely connected to minimize loss. The (–) plate of
0
CHANNEL 1 LOAD CURRENT (A)
0.5
1.0
1.5
2.0
3.0
2.5
50
125
3634 F08
0
25 75 100
MAXIMUM ALLOWABLE AMBIENT
TEMPERATURE (°C)
CH2 LOAD = 0A
CH2 LOAD = 1A
CH2 LOAD = 2A
CH2 LOAD = 3A
