LT3461/LT3461A

5

3461af

Inrush Current

The LT3461 has a built-in Schottky diode. When supply

from input through the inductor and the Schottky diode to

charge the output capacitor. The maximum nonrepetitive

surge current the Schottky diode in the LT3461 can

sustain is 1.5A. The selection of inductor and capacitor

value should ensure the peak of the inrush current to be

below 1.5A. In addition, turn-on of the LT3461 should be

delayed until the inrush current is less than the maximum

current limit. The peak inrush current can be calculated as

follows:

I

V

L

C

L

C

P

IN

=

−

−

⎛

⎝

⎜

⎜

⎜

⎜

⎞

⎠

⎟

⎟

⎟

⎟

–.

–

• exp

06

12

1

π

where L is the inductance, r is the resistance of the

inductor and C is the output capacitance.

Table 3 gives inrush peak currents for some component

selections.

Table 3. Inrush Peak Current

L (µH)

C (µF)

5

4.7

1

1.1

5

10

1

0.9

Thermal Considerations

Significant power dissipation can occur on the LT3461

and LT3461A, particularly at high input voltage. Device

load, voltage drops in the power path components, and

switching losses are the major contributors. It is impor-

tant to measure device power dissipation in an application

to ensure that the LT3461 does not exceed the absolute

maximum operating junction temperature of 125°C over

the operating ambient temperature range. Generally, for

supply voltages below 5V the integrated current limit

function provides adequate protection for nonfault condi-

tions. For supply voltages above 5V, Figures 3a and 3b

show the recommended operating region of the LT3461

and LT3461A, respectively. These graphs are based

on 250mW on-chip dissipation. Improvement of these

APPLICATIO S I FOR ATIO

numbers can be expected if the LT3461 is supplied from a

separate low voltage rail.

Figure 3a. LT3461 Operating Region

Figure 3b. LT3461A Operating Region

6

80

120

38

3461 F01a

40

0

14

22

30

160

6

80

120

38

3461 F01b

40

0

14

22

30

160

Switching Frequency

The key difference between the LT3461 and LT3461A is the

faster switching frequency of the LT3461A. At 3MHz, the

LT3461A switches at twice the rate of the LT3461. The

higher switching frequency of the LT3461A allows physi-

cally smaller inductors and capacitors to be used in a given

application, but with a slight decrease in efficiency and

maximum output current when compared to the LT3461.

Generally if efficiency and maximum output current are

crucial, or a high output voltage is being generated, the

LT3461 should be used. If application size and cost are

more important, the LT3461A will be the better choice.