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LTC1626

Thermal Considerations

In a majority of applications, the LTC1626 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), 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:

is the thermal resistance from the junction of the die to the

ambient temperature.

The junction temperature is given by:

As an example, consider the case when the LTC1626 is in

dropout at an input voltage of 3V with a load current of

0.5A. From the Typical Performance Characteristics graph

of Switch Resistance, the ON resistance of the P-channel

switch is 0.45

Ω. Therefore, power dissipated by the

part is:

The SO package junction-to-ambient thermal resistance

regulator when it is operating in a 25

°C ambient tempera-

ture is:

Remembering that the above junction temperature is

it increases with temperature. However, we can safely

assume 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

LTC1626. 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 separated? The

LTC1626 signal ground (Pin 11) must return to the

to the anode of the Schottky diode and the (–) plate

1, 13) as close as possible? This capacitor provides the

AC current to the internal P-channel MOSFET and its

driver.

Figure 6. LTC1626 Layout Diagram (See Board Layout Checklist)

+

1

2

13

1626 F06

BOLD LINES INDICATE

HIGH CURRENT PATHS

D1

0.1

µF

L

R1

1k

1000pF

3900pF

R2

SW

LTC1626

PGND

SGND

SHDN

SHUTDOWN

LBO

LBI

3

4

5

6

7

14

12

11

10

9

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