LTC1265/LTC1265-3.3/LTC1265-5
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APPLICATIONS INFORMATION
output voltage can potentially float above the maximum
allowable tolerance. To prevent this from occuring, a
resistor must be connected between VOUT and ground
with a value low enough to sink the maximum possible
leakage current.
Now consider the case of a 1A regulator with VIN = 4V and
TA = 65°C. Starting with the same 0.55Ω assumption for
RDSON, the TJ calculation will yield 125°C. But from the
graph, this will increase the RDSON to 0.76Ω, which when
used in the above calculation yields an actual TJ > 148°C.
ThereforetheLTC1265wouldbeunsuitablefora4Vinput,
1A output regulator operating at TA = 65°C.
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
turnedoncontinuously(DC),theuserwillneedtodosome
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:
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 COUT. The power ground (Pin 12) returns to the
anode of the Schottky diode, and the (–) plate of CIN,
whose leads should be as short as possible.
TR = P(θJA)
where P is the power dissipated by the regulator and θJA
is the thermal resistance from the junction of the die to the
ambient temperature.
2. Does the (+) plate of the CIN connect to the power VIN
(Pins 1,13) as close as possible? This capacitor pro-
vides the AC current to the internal P-channel MOSFET
and its driver.
The junction temperature is simply given by:
TJ = TR + TA
3. Is the input decoupling capacitor (0.1µF) connected
closely between power VIN (Pins 1,13) and power
ground (Pin 12)? This capacitor carries the high fre-
quency peak currents.
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:
4. Is the Schottky diode closely connected between the
power ground (Pin 12) and switch (Pin 14)?
P = I2(RDSON) = 0.1375W
5. Does the LTC1265 SENSE– (Pin 7) connect to a point
close to RSENSE and the (+) plate of COUT? In adjustable
applications, the resistive divider, R1 and R2, must be
connected between the (+) plate of COUT and signal
ground.
For the SO package, the θJA is 110°C/W.
Therefore the junction temperature of the regulator when
it is operating in ambient temperature of 25°C is:
TJ = 0.1375(110) + 25 = 40.1°C
6. AretheSENSE– andSENSE+ leadsroutedtogetherwith
minimum PC trace spacing? The 1000pF capacitor
between Pins 7 and 8 should be as close as possible to
the LTC1265.
Remembering that the above junction temperature is
obtained from a RDSON at 25°C, we need to recalculate the
junction temperature based on a higher RDSON 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.
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.
10
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