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LTC1265C

1.2A, High Efficiency Step-Down DC/DC Converter

Linear Technology 

LTC1265/LTC1265-3.3/LTC1265-5

APPLICATIONS INFORMATION

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.

VIN

R4

4

–

LTC1265

3

+

R3

1.25V REFERENCE

LTC1265 F03

Figure 3. Low-Battery Comparator

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:

) VOUT = 1.25

1 + R2

R1

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.

9

LTC1265 VFB

SGND

11

VOUT

R2

100pF

R1

LTC1265 F04

Figure 4. LTC1265 Adjustable Configuration

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:

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.

The junction temperature is simply given by:

TJ = TR + TA

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 = I2(RDSON) = 0.1375W

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

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.

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.

Therefore the LTC1265 would be unsuitable for a 4V input,

1A output regulator operating at TA = 65°C.

9

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