10
LTC1265/LTC1265-3.3/LTC1265-5
APPLICATIONS INFORMATION
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output voltage can potentially float above the maximum
allowable tolerance. To prevent this from occuring, a
resistor must be connected between V
OUT
and ground
with a value low enough to sink the maximum possible
leakage current.
THERMAL CONSIDERATIONS
In a majority of applications, the LTC1265 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), the user will need to do some
thermal analysis. The goal of the thermal analysis is to
determine whether the power dissipated by the regulator
exceeds the maximum junction temperature of the part.
The temperature rise is given by:
T
R
= P(θ
JA
)
where P 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 simply given by:
T
J
= T
R
+ T
A
As an example, consider the LTC1265 is in dropout at an
input voltage of 4V with a load current of 0.5A. From the
Typical Performance Characteristics graph of Switch Re-
sistance, the ON resistance of the P-channel is 0.55Ω.
Therefore power dissipated by the part is:
P = I
2
(R
DSON
) = 0.1375W
For the SO package, the θ
JA
is 110°C/W.
Therefore the junction temperature of the regulator when
it is operating in ambient temperature of 25°C is:
T
J
= 0.1375(110) + 25 = 40.1°C
Remembering that the above junction temperature is
obtained from a R
DSON
at 25°C, we need to recalculate the
junction temperature based on a higher R
DSON
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.
Now consider the case of a 1A regulator with V
IN
= 4V and
T
A
= 65°C. Starting with the same 0.55Ω assumption for
R
DSON
, the T
J
calculation will yield 125°C. But from the
graph, this will increase the R
DSON
to 0.76Ω, which when
used in the above calculation yields an actual T
J
> 148°C.
Therefore the LTC1265 would be unsuitable for a 4V input,
1A output regulator operating at T
A
= 65°C.
Board Layout Checklist
When laying out the printed circuit board, the following
checklist should be used to ensure proper operation of the
LTC1265. These items are also illustrated graphically in
the layout diagram of Figure 6. Check the following in your
layout:
1. Are the signal and power grounds segregated? The
LTC1265 signal ground (Pin 11) must return to the (–)
plate of C
OUT
. The power ground (Pin 12) returns to the
anode of the Schottky diode, and the (–) plate of C
IN
,
whose leads should be as short as possible.
2. Does the (+) plate of the C
IN
connect to the power V
IN
(Pins 1,13) as close as possible? This capacitor pro-
vides the AC current to the internal P-channel MOSFET
and its driver.
3. Is the input decoupling capacitor (0.1µF) connected
closely between power V
IN
(Pins 1,13) and power
ground (Pin 12)? This capacitor carries the high fre-
quency peak currents.
4. Is the Schottky diode closely connected between the
power ground (Pin 12) and switch (Pin 14)?
5. Does the LTC1265 SENSE
–
(Pin 7) connect to a point
close to R
SENSE
and the (+) plate of C
OUT
? In adjustable
applications, the resistive divider, R1 and R2, must be
connected between the (+) plate of C
OUT
and signal
ground.
6. Are the SENSE
–
and SENSE
+
leads routed together with
minimum PC trace spacing? The 1000pF capacitor
between Pins 7 and 8 should be as close as possible to
the LTC1265.
7. Is SHDN (Pin 10) actively pulled to ground during
normal operation? The SHDN pin is high impedance
and must not be allowed to float.
