SP6691EKSLASHTR datasheet(8/15 Pages) SIPEX

8 / 15 page

Jun26-07 Rev D

Micro Power Boost Regulator Series White LED Driver

Inductor Selection

For SP6691, the internal switch will be turned

off only after the inductor current reaches the

typical dc current limit (ILIM=450mA). How-

ever, there is typically propagation delay of

200nS between the time when the current limit

is reached and when the switch is actually turned

off. During this 200nS delay, the peak inductor

current will increase, exceeding the current limit

by a small amount. The peak inductor current

can be estimated by:

IPK = ILIM +

VIN(MAX) • 200nS

The larger the input voltage and the lower the

inductor value, the greater the peak current.

In selecting an inductor, the saturation current

specified for the inductor needs to be greater

than the SP6691 peak current to avoid saturating

the inductor, which would result in a loss in

efficiency and could damage the inductor.

Choosing an inductor with low DCR decreases

power losses and increase efficiency.

Refer to Table 1 for some suggested low ESR

inductors.

Table 1. Suggested Low ESR inductor

Diode Selection

A schottky diode with a low forward drop and

fast switching speed is ideally used here to

achieve high efficiency. In selecting a Schottky

diode, the current rating of the schottky diode

should be larger than the peak inductor current.

Moreover, the reverse breakdown voltage of the

schottky diode should be larger than the output

voltage.

Capacitor Selection

Ceramic capacitors are recommended for their

inherently low ESR, which will help produce

low peak to peak output ripple, and reduce high

frequency spikes.

For the typical application, 4.7µF input capaci-

tor and 2.2µF output capacitor are sufficient.

The input and output ripple could be further

reduced by increasing the value of the input and

output capacitors. Place all the capacitors as

close to the SP6691 as possible for layout. For

use as a voltage source, to reduce the output

ripple, a small feedforward (47pF) across the

top feedback resistor can be used to provide

sufficient overdrive for the error comparator,

thus reduce the output ripple.

Refer to Table 2 for some suggested low ESR

capacitors.

Table 2. Suggested Low ESR Capacitor

LED Current Program

In the white LEDs application, the SP6691 is

generally programmed as a current source. The

bias resistor Rb, as shown in the typical applica-

tion circuit is used to set the operating current of

the white LED using the equation:

Rb =

VFB

where VFB is the feedback pin voltage (1.22V),

IF is the operating current of the White LEDs.

In order to achieve accurate LED current, 1%

APPLICATION INFORMATION

MANUF.

PART NUMBER

CAP

SIZE

/VOLTAGE /TYPE

MURATA

GRM32RR71E

2.2µF

1210

770-436-1300

225KC01B

/25V

/X5R

MURATA

GRM31CR61A

4.7µF

1206

770-436-1300

475KA01B

/10V

/X5R

TDK

C3225X7R1E

2.2µF

1210

847-803-6100

225M

/25V

/X7R

TDK

C3216X5R1A

4.7µF

1206

847-803-6100

475K

/10V

/X5R

MANUF.

PART NUMBER

DCR

Current

( )

Rating

(mA)

MURATA

LQH32CN100K11

0.3

450

770-436-1300

(10µH)

TDK

NLC453232T-100K

0.55

500

847-803-6100

(10µH)

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