10

LT3473/LT3473A

3473f

between the OUT pin and ground. A ceramic capacitor with

a value of 1µF is a good choice. The voltage drop (PNP

according to the following formula:

VV

V

V

R

R

V

OUT

INT

CESAT

REF

CESAT

==

+

⎛

⎝

⎜

⎞

⎠

⎟

–•

–

1

2

1

Auxiliary NPN Devices (LT3473A Only)

The LT3473A has two auxiliary NPNs as shown in the

Block Diagram that can provide intermediate outputs less

than OUT. The collectors of the NPNs are connected to the

OUT pin internally. Each NPN can dissipate 100mW safely

and has a minimum beta of 60. A resistor string can be

APPLICATIO S I FOR ATIO

2

4

NE1

3 NB1

OUT

6

NE2

5 NB2

3473 F05

connected to the two bases as shown in Figure 5 to

generate buffered voltage at the emitters. When sourcing

high current at low voltage, keep in mind that the NPNs

will be dissipating a fair amount of power, which must be

supplied by the DC/DC converter.

Thermal Shutdown

The LT3473 has thermal shutdown circuitry that shuts down

the part when the junction temperature reaches approxi-

mately 145°C to protect the part from abnormal operation

with high power dissipation, such as an output short cir-

cuit or excessive power dissipation in the auxiliary NPNs.

The part will turn back on when the junction cools down to

approximately 125°C. If the abnormal condition remains,

the part will turn on and off while maintaining the junction

temperature within the window between 125°C and 145°C.

Board Layout Consideration

As with all switching regulators, careful attention must be

paid to the PCB board layout and component placement.

To maximize efficiency, switch rise and fall times are made

as short as possible. To prevent electromagnetic interfer-

ence (EMI) problems, proper layout of the high frequency

switching path is essential. The voltage signal of the SW

pin has sharp rise and fall edges. Minimize the length and

area of all traces connected to the SW pin and always use

a ground plane under the switching regulator to minimize

interplane coupling. Recommended component place-

ment is shown in Figure 6.

Figure 6. Recommended Component Placement

OUT

1

2

3

4

13

5

6

12

11

10

9

8

7

3473 F06a

OUT

1

2

3

4

9

8

7

6

5

3473 F06b