ltc3490 Linear Technology Corporation, ltc3490 Datasheet - Page 8

• Manufacturer Part Number: ltc3490
• Description: Single Cell 350ma Led Driver
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LTC3490
The output ripple voltage has two primary components.
The first is due to the value of the capacitor and is given by:
The second is due to the capacitor ESR:
The LED current ripple and peak pulsed current are calcu-
lated by:
where:
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
TDK C2012X5R0J475K
AVX 1210ZC475MAT
Taiyo Yuden CELMK316BJ475ML
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:
8
VR
R
R
V
R
Peak Pulsed Forward Current = 0.5A
IR
I
VR
PPFC
F
SENSE
LED
LED
LED
(at I
ESR
CAP
= Dynamic Impedance of the LED
= 1Ω
=
F
= I
=
= Internal Sense Resistor = 0.1Ω
= 350mA) = 3.4 ±0.6V
=
I
OUT
LPK
VR
R
C V
I
SENSE
LPK
•
CAP
• R
+
OUT
IR
U
•
ESR
•
V
LE
+
2
VR
IN
•
R
D D
f
LED
ESR
U
DESCRIPTION
4.7µF, 6.3V, X5R in 0805
4.7µF, 10V, X7R in 1210
4.7µF, 10V, X7R in 1206
W
U
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
L
Choose L
I
I
Choose C
Cap ESR
VR
VR
IR
I
where:
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
the converter operates lower on the efficiency curve.
IN
LPK
PPFC
MIN
LED
CAP
ESR
I
VR
value
VR
ESR
IR
I
LPK
PPFC
LED
CAP
ESR
is the peak inductor current
is the LED peak pulsed forward current
is the LED current ripple
is the ripple voltage due to the output capacitor
is the ripple voltage due to the output capacitor
1-CELL
1.56
1.93
0.09
0.01
0.10
0.40
2.2
3.3
4.7
5
2
R losses can exceed 100mW and
2-CELL
0.005
0.78
0.96
0.09
0.09
0.39
3.2
3.3
4.7
5
UNITS
mΩ
µH
µH
µF
A
A
V
V
A
A
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