9

Date: 6/20/06 Rev T

SP7685, 1.2A Buck/BoostCharge Pump LED Driver

© 2006 Sipex Corporation

COMPONENT SELECTION

The input and output capacitors should be

located as close to the V

possible to obtain best bypassing, and the

returns should be connected directly to the

P

under the SP7685. The fly capacitor should

be located as close to the C1 and C2 pins as

possible. See typical circuit layout at the end

of this section for details on the recom-

mended layout.

To obtain low output ripple, a value of 10μF

is recommended for C

For output cur-

rents of 500mA to 1.2A, the recommended

C

used. Output currents in Flash of 100mA to

400mA can use a 0.47μF C

4.7μF C

RESISTOR SELECTION

The sense resistor R

the value needed in the Torch mode for the

desired output current by the equation:

R

Mode)

Once the R

for Torch mode, the V

selected for Flash mode using the following

equation:

V

for Flash Mode.

The SP7685 charge pump circuit requires

three capacitors: 10μF input, 10μF output

and 1μF fly capacitor are typically recom-

mended. For the input capacitor, a value of

10μF will help reduce input voltage ripple for

applications sensitive to ripple on the battery

voltage. All the capacitors should be sur-

face mount ceramic for low lead inductance

necessary at the 2.4MHz switching frequency

of the SP7685 and to obtain low ESR, which

improves bypassing on the input and output

and improves output voltage drive by reduc-

ing output resistance. Ceramic capacitors

with X5R or X7R temperature grade are

recommended for most applications. A se-

lection of recommended capacitors is in-

cluded in Table 1 below.

Table 1: Recommended Capacitors

Next, the R

Flash mode using the following equation:

R

(

14uA

For an example of 200mA Torch mode and

700mA Flash mode, the values R

0.22Ω, V

=56KΩ are calculated. The power obtained

in the Flash mode would be:

P

The typical 0603 surface mount resistor is

rated 1/10 Watt continuous power and 1/5

Manufacturer's Website

Part Number

Capacitance/

Voltage

CapacitorSize/

Type/Thickness

ESR

@100kHz

TDK: www.tdk.com

C1005X5R0J105M

1uF/6.3V

0402/X5R/0.5mm

0.03

TDK: www.tdk.com

C1608X5R0J475K

4.7uF/6.3V

0603/X5R/0.9mm

0.02

TDK: www.tdk.com

C2012X5R0J106M

10uF/6.3V

0805/X5R/1.35mm

0.02

Murata: www.murata.com

GRM155R60J105KE19B

1uF/6.3V

0402/X5R/0.55mm

0.03

Murata: www.murata.com

GRM188R60J475KE19

4.7uF/6.3V

0603/X5R/0.9mm

0.02

Murata: www.murata.com

GRM21BR60J106KE19L

10uF/6.3V

0805/X5R/1.35mm

0.02