LTC1265 LINER [Linear Technology], LTC1265 Datasheet - Page 9

• Manufacturer Part Number: LTC1265
• Description: 1.2A, High Efficiency Step-Down DC/DC Converter
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
The output, Pin 3, is an N-channel open drain that goes low
when the battery voltage is below the threshold set by R3
and R4. In shutdown, the comparator is disabled and Pin
3 is in a high impedance state.
LTC1265 ADJUSTABLE APPLICATIONS
The LTC1265 develops a 1.25V reference voltage between
the feedback (Pin 9) terminal and signal ground (see
Figure 4). By selecting resistor R1, a constant current is
caused to flow through R1 and R2 to set overall output
voltage. The regulated output voltage is determined by:
For most applications a 30k resistor is suggested for R1.
To prevent stray pickup, a 100pF capacitor is suggested
across R1 located close to the LTC1265.
V
OUT
V
IN
= 1.25 1 +
Figure 4. LTC1265 Adjustable Configuration
Figure 3. Low-Battery Comparator
R3
R4
4
11
LTC1265
SGND
U
R2
R1
–
+
1.25V REFERENCE
V
FB
INFORMATION
U
9
W
100pF
LTC1265 F04
LTC1265
LTC1265 F03
V
OUT
R2
R1
3
U
LTC1265/LTC1265-3.3/LTC1265-5
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:
where P is the power dissipated by the regulator and
is the thermal resistance from the junction of the die to the
ambient temperature.
The junction temperature is simply given by:
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 0.55 .
Therefore power dissipated by the part is:
For the SO package, the
Therefore the junction temperature of the regulator when
it is operating in ambient temperature of 25 C is:
Remembering that the above junction temperature is
obtained from a R
junction temperature based on a higher R
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
T
R
graph, this will increase the R
used in the above calculation yields an actual T
Therefore the LTC1265 would be unsuitable for a 4V input,
1A output regulator operating at T
A
DSON
T
T
P = I
T
= 65 C. Starting with the same 0.55 assumption for
R
J
J
= T
= 0.1375(110) + 25 = 40.1 C
= P(
, the T
2
(R
R
+ T
DSON
JA
J
)
A
calculation will yield 125 C. But from the
) = 0.1375W
DSON
at 25 C, we need to recalculate the
JA
is 110 C/W.
DSON
A
to 0.76 , which when
= 65 C.
DSON
IN
J
> 148 C.
= 4V and
since it
9
JA