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
IN
and V
OUT
pins as
possible to obtain best bypassing, and the
returns should be connected directly to the
P
GND
pin or to the thermal pad ground located
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
OUT
. For output cur-
rents of 500mA to 1.2A, the recommended
C
FC
fly capacitor value of 1μF should be
used. Output currents in Flash of 100mA to
400mA can use a 0.47μF C
FC
but a minimum
4.7μF C
OUT
is still needed.
RESISTOR SELECTION
The sense resistor R
SENSE
is determined by
the value needed in the Torch mode for the
desired output current by the equation:
R
SENSE
= V
FB
/ I
OUT
where V
FB
= 50mV (Torch
Mode)
Once the R
SENSE
resistor has been selected
for Torch mode, the V
FB
voltage can be
selected for Flash mode using the following
equation:
V
FB
= I
OUT
*
R
SENSE
(Flash Mode) where I
OUT
is
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
SET
resistor can be selected for
Flash mode using the following equation:
R
SET
=
(
VFB
)
*
5 Ω (Flash Mode)
14uA
For an example of 200mA Torch mode and
700mA Flash mode, the values R
SENSE
=
0.22Ω, V
FB
= 155mV (Flash Mode), and R
SET
=56KΩ are calculated. The power obtained
in the Flash mode would be:
P
FLASH
= V
FB
*
I
OUT
= 155mV
*
700mA = 109mW.
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
