Datasheet

8
LTC3490
3490fa
The output ripple voltage has two primary components.
The first is due to the value of the capacitor and is given by:
VR
IV
CV f
CAP
LPK IN
OUT
=
•
••
The second is due to the capacitor ESR:
VR
ESR
= I
LPK
• R
ESR
The LED current ripple and peak pulsed current are calcu-
lated by:
IR
VR VR
RR
II
IR
LED
CAP ESR
SENSE LED
PPFC OUT
LE
=
+
=+
•
DD
2
where:
R
SENSE
= Internal Sense Resistor = 0.1Ω
R
LED
= Dynamic Impedance of the LED
Low ESR capacitors should be used to minimize output
ripple. Ceramic X5R or X7R type capacitors are recom-
mended. See Table 2 for a list of component suppliers.
Table 2. Capacitor Information
CAPACITOR PART NUMBER DESCRIPTION
TDK C2012X5R0J475K 4.7µF, 6.3V, X5R in 0805
AVX 1210ZC475MAT 4.7µF, 10V, X7R in 1210
Taiyo Yuden CELMK316BJ475ML 4.7µF, 10V, X7R in 1206
Input Capacitor Selection
Most battery-powered applications do not need an input
capacitor. In supply-powered applications or battery ap-
plications with long leads to the battery, a low ESR 3.3µF
capacitor reduces switching noise and peak currents.
Design Example
The example will use a Lumileds DS25 white LED. The key
specifications are:
V
F
(at I
F
= 350mA) = 3.4 ±0.6V
R
LED
= 1Ω
Peak Pulsed Forward Current = 0.5A
Component values will be calculated for 1 or 2 NiMH cells
and assumes the end-of-charge voltage to be 0.9V per cell.
The operating frequency is assumed to be 1MHz, the
worst-case low frequency. The allowed inductor ripple
current is 0.31A. Table 3 shows a summary of the key
parameters.
Table 3. Summary of Key Parameters
PARAMETER 1-CELL 2-CELL UNITS
L
MIN
2.2 3.2 µH
Choose L 3.3 3.3 µH
I
IN
1.56 0.78 A
I
LPK
1.93 0.96 A
Choose C 4.7 4.7 µF
Cap ESR 5 5 mΩ
VR
CAP
0.09 0.09 V
VR
ESR
0.01 0.005 V
IR
LED
0.10 0.09 A
I
PPFC
0.40 0.39 A
where:
I
LPK
is the peak inductor current
VR
CAP
is the ripple voltage due to the output capacitor
value
VR
ESR
is the ripple voltage due to the output capacitor
ESR
IR
LED
is the LED current ripple
I
PPFC
is the LED peak pulsed forward current
PC Board Layout Checklist
Keep the inductor and output capacitor as close to the IC
as possible. Make traces as short and wide as is feasible.
Parasitic resistance and inductance reduce efficiency and
increase ripple.
Keep resistance in the battery connections as low as
possible. In single cell applications, only 0.1Ω in the
battery connections will have a dramatic effect in effi-
ciency and battery life. I
2
R losses can exceed 100mW and
the converter operates lower on the efficiency curve.
APPLICATIO S I FOR ATIO
WUUU