9

LT3473/LT3473A

3473f

Inrush Current

The LT3473 has an integrated Schottky power diode.

while the output capacitor is discharged, the voltage

flowing from the input through the inductor and the

internal Schottky diode to charge the output capacitor at

the CAP pin. The maximum current the LT3473’s Schottky

can sustain is 2A. The selection of inductor and capacitor

values should ensure that the peak inrush current is less

than 2A. Peak inrush current can be calculated as follows:

I

V

L

r

L

LC

r

L

P

IN

=

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

=

+

=

–.

•

• exp –

• arctan

• sin arctan

.

•

•

–

•

06

15

2

1

4

2

ω

α

ω

ω

α

ω

α

α

ω

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

inductor and C is the output capacitance. For a low DCR

inductor, which is usually the case for this application, the

peak inrush current can be simplified as follows:

I

V

L

P

IN

=

⎛

⎝

⎜

⎞

⎠

⎟

–.

•

• exp –

•

06

2

ω

α

ω

π

at the CAP pin generate larger inrush current. Table 3 gives

inrush peak currents for some component selections. An

inductor with low saturation current could generate very

large inrush current. For this case, inrush current should

be measured to ensure safe operation. Note that inrush

current is not a concern if the input voltage rises slowly.

Table 3. Inrush Peak Current

R (Ω)L (µH)

C (µF)

5

0.05

6.8

0.47

0.86

10

0.05

6.8

0.47

1.83

3.6

0.05

6.8

0.47

0.58

3.6

0.05

4.7

0.47

0.67

Setting the Output Voltages

The LT3473 has both an internal 1.25V reference and an

external reference input. This allows the user to select

between using the built-in reference and supplying an

external reference voltage. The voltage at the CTRL pin can

be adjusted while the device is operating to alter the output

voltage for purposes such as display dimming or contrast

adjustment. To use the internal 1.25V reference, the CTRL

pin must be held higher than 1.5V. When the CTRL pin is

held between 0V and 1.2V, the LT3473 will regulate the

output such that the FB pin voltage is equal to the CTRL pin

voltage.

The CAP pin should be used as the feedback node. To set

the output voltage, select the values of R1 and R2 accord-

ing to the following equation.

VV

R

R

INT

REF

=+

⎛

⎝

⎜

⎞

⎠

⎟

•1

2

1

To maintain output voltage accuracy, 1% resistors are

recommended.

Soft-Start

The SHDN pin also functions as soft-start. Use an RC filter

at the SHDN pin to limit the start-up current. The small bias

current of the SHDN pin allows using a small capacitor for

a large RC time constant.

APPLICATIO S I FOR ATIO

Figure 4. Soft-Start Circuitry

SHDN

100nF

ON/OFF

3473 F04

20k

LT3473

Output Disconnect Considerations

The LT3473 has an output disconnect PNP that isolates

the load from the input during shutdown. The drive circuit

maintains the PNP at the edge of saturation, adaptively

according to the load, thus yielding the best compromise

loss. To remain stable, it requires a bypass capacitor

connected between the OUT pin and the CAP pin or