11

Applications Information (continued)

SC4540

must withstand high voltage for an extended period of

time.

Soft-Start

The soft-start mode reduces in-rush current by utilizing

the external compensation network. As the error amplifier

slowly charges the COMP node voltage, the duty cycle of

the boost switch ramps from 0% to its final value when in

regulation. The gradual increase of the duty cycle slowly

charges the output capacitor and limits in-rush current

during start up. Soft-start is implemented only when the

power is cycled on the part.

Thermal Shutdown

A thermal shutdown mode is included for protection in

the event the junction temperature exceeds 155°C. In

thermal shutdown, the on-chip power switch is disabled.

Switching and sinking resumes when the temperature

drops by 20°C.

Current Limit

The power switch of the boost converter is protected by

an internal current limit function. The switch is opened

when the current exceeds the maximum switch current

value.

Inductor Selection

The inductor value should be within the range of 22μH to

68μH. The DCR needs to be considered when selecting

the inductor to ensure optimum efficiency. The largest

inductor package that can be accommodated in the circuit

area should be used since the DCR generally decreases

with increasing package size.

The saturation current of the inductor should be much

higher than the peak current of the internal boost switch

to ensure that the inductor never enters saturation during

normal operation of the part. The equation to calculate

the peak inductor current is:

2

I

I

I

L

IN

L(Peak)

where

osc

IN

L

f

L

D

V

I

OUT

IN

V

V

1

D

IN

OUT

OUT

IN

V

I

V

I

D is the duty cycle for continuous operation. Efficiency (η)

can be approximated by using the curves provided in the

Typical Characteristics section. Table 1 lists inductors that

have been proven to work with SC4540.

Table 1 — Recommended Inductors

Part Number

Value

(μH)

DCR

(Ω)

Rated

Current (A)

Toler-

ance

Dimensions

(L x W x H)

(mm)

Coilcraft

LPS6235

56

0.28

1.1

±20%

6.2 x 6.2 x 3.5

Coilcraft

LPS4018-223ML

22

0.360

0.70

±20%

3.9 x 3.9 x 1.7

Capacitor Selection

The input capacitor should be at least 2.2μF. A larger

capacitor will reduce the voltage ripple on the input. The

output capacitor values can range from 0.22μF to 1μF. The

compensation capacitor value should be 47nF. Capacitors

of X5R type material or better can be used for any of the

capacitors. See Table 2 for recommended capacitors.

Table 2 — Recommended Capacitors

Part Number

Value

(μF)

Rated

Voltage (V)

Type

Case Size

Input Capacitor

Murata

GRM219R61E225KA12

2.2

25

X5R

0805

Output Capacitor

Murata

GRM21BR71H105KA12L

1.0

50

X7R

0805

Compensation Capacitor

Taiyo Yuden

EMK105BJ473KV-F

0.047

16

X7R

0402