10
LTC1701/LTC1701B
SW
V
FB
V
IN
I
TH
/RUN
GND
LTC1701
D1
L1
R2
R1
C
OUT
1701 F04
V
OUT
V
IN
1
2
3
5
4
C
IN
C
C
R
C
R
S
BOLD LINES INDICATE HIGH CURRENT PATHS
++
Figure 4. LTC1701 Layout Diagram (See Board Layout Checklist)
APPLICATIO S I FOR ATIO
WUU
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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 THERMAL RESISTANCE
TOPSIDE* BACKSIDE BOARD AREA θ
JA
2500mm
2
2500mm
2
2500mm
2
125°C/W
1000mm
2
2500mm
2
2500mm
2
125°C/W
225mm
2
2500mm
2
2500mm
2
130°C/W
100mm
2
2500mm
2
2500mm
2
135°C/W
50mm
2
2500mm
2
2500mm
2
150°C/W
*Device is mounted on topside.
Calculating Junction Temperature
In a majority of applications, the LTC1701 does not dissi-
pate 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.
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 approxi-
mately 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. Therefore,
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 ob-
tained 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 capacitor 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).
5. Keep sensitive components away from the SW pin. The
input capacitor C
IN
, the compensation capacitor C
C
and all
the resistors R1, R2, R
C
and R
S
should be routed away from
the SW trace and the components L1 and D1.
