LTC1701
APPLICATIO S I FOR ATIO
THERMAL CONSIDERATIONS
The power handling capability of the device at high ambi-
ent temperatures will be limited by the maximum rated
junction temperature (125°C). It is important to give
careful consideration to all sources of thermal resistance
from junction to ambient. Additional heat sources mounted
nearby must also be considered.
For surface mount devices, heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Copper board stiffeners and plated
through-holes can also be used to spread the heat gener-
ated by power devices.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with one ounce
copper.
Table 1. Measured Thermal Resistance
COPPER AREA
TOPSIDE*
2500mm
2
1000mm
2
225mm
2
100mm
2
50mm
2
BACKSIDE
2500mm
2
2500mm
2
2500mm
2
2500mm
2
2500mm
2
BOARD AREA
2500mm
2
2500mm
2
2500mm
2
2500mm
2
2500mm
2
THERMAL RESISTANCE
θ
JA
125°C/W
125°C/W
130°C/W
135°C/W
150°C/W
*Device is mounted on topside.
Calculating Junction Temperature
In a majority of applications, the LTC1701 does not
dissipate much heat due to its high efficiency. However, in
applications where the switching regulator is running at
high duty cycles or the part is in dropout with the switch
turned on continuously (DC), some thermal analysis is
required. The goal of the thermal analysis is to determine
whether the power dissipated by the regulator exceeds the
maximum junction temperature. The temperature rise is
given by:
T
RISE
= P
D
•
θ
JA
where P
D
is the power dissipated by the regulator and
θ
JA
is the thermal resistance from the junction of the die to the
ambient temperature.
U
The junction temperature is given by:
T
J
= T
RISE
+ T
AMBIENT
As an example, consider the case when the LTC1701 is in
dropout at an input voltage of 3.3V with a load current of
0.5A. The ON resistance of the P-channel switch is ap-
proximately 0.30Ω. Therefore, power dissipated by the
part is:
P
D
= I
2
• R
DS(ON)
= 75mW
The SOT package junction-to-ambient thermal resistance,
θ
JA
, will be in the range of 125°C/W to 150°C/W. There-
fore, the junction temperature of the regulator operating in
a 25°C ambient temperature is approximately:
T
J
= 0.075 • 150 + 25 = 36°C
Remembering that the above junction temperature is
obtained from a R
DS(ON)
at 25°C, we might recalculate the
junction temperature based on a higher R
DS(ON)
since it
increases with temperature. However, we can safely as-
sume that the actual junction temperature will not exceed
the absolute maximum junction temperature of 125°C.
Board Layout Considerations
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1701. These items are also illustrated graphically in
the layout diagram of Figure 4. Check the following in your
layout:
1. Does the capacitor C
IN
connect to the power V
IN
(Pin 5)
and GND (Pin 2) as close as possible? This capacitor
provides the AC current to the internal P-channel MOSFET
and its driver.
2. Is the Schottky diode closely connected between the
ground (Pin 2) and switch output (Pin 1)?
3. Are the C
OUT
, L1 and D1 closely connected? The
Schottky anode should connect directly to the input ca-
pacitor ground.
4. The resistor divider, R1 and R2, must be connected
between the (+) plate of C
OUT
and a ground line terminated
near GND (Pin 2). The feedback signal FB should be routed
away from noisy components and traces, such as the SW
line (Pin 1).
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