LM3503SQX-44 National Semiconductor Corporation, LM3503SQX-44 Datasheet - Page 16

• Manufacturer Part Number: LM3503SQX-44
• Description: Dual-display Constant Current Led Driver With Analog Brightness Control
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Information
Fs:
I
L:
D:
∆i
I
For CCM operation, the duty cycle can be computed with:
D:
V
V
For DCM operation, the duty cycle can be computed with:
D:
V
V
I
Fs:
L:
INDUCTOR SELECTION
In order to maintain inductance, an inductor used with the
LM3503 should have a saturation current rating larger than
the peak inductor current of the particular application. Induc-
tors with low DCR values contribute decreased power losses
and increased efficiency. The peak inductor current can be
computed for both modes of operation: CCM and DCM.
The cycle-by-cycle peak inductor current for CCM operation
can be computed with:
V
Eff:
Fs:
OUT
L
OUT
(avg): Average Inductor Current.
OUT
IN
OUT
IN
IN
L
:
:
:
:
:
: White LED Current/Load Current.
: Output Voltage.
: Output Voltage.
Duty Cycle for CCM Operation.
Input Voltage.
Duty Cycle for DCM Operation.
Input Voltage.
Switching Frequency.
Inductor Value/Inductance Magnitude.
Switching Frequency.
White LED Current/Load Current.
Inductance Magnitude/Inductor Value.
Duty Cycle for CCM operation.
Inductor Ripple Current.
Input Voltage.
Efficiency of the LM3503.
Switching Frequency.
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16
I
L:
D:
I
∆i
I
The cycle-by-cycle peak inductor current for DCM operation
can be computed with:
V
Fs:
L:
D:
I
The minimum inductance magnitude/inductor value for the
LM3503 can be calculated using the following, which is only
valid when the duty cycle is
D:
D’:
R
Fs:
V
L:
This equation gives the value required to prevent subhar-
monic oscillations. The result of this equation and the induc-
tor ripple currents should be accounted for when choosing
an inductor value.
Some recommended Inductor manufactures included but
are not limited to:
CAPACITOR SELECTION
Multilayer ceramic capacitors are the best choice for use
with the LM3503. Multilayer ceramic capacitors have the
lowest equivalent series resistance (ESR). Applied voltage
or DC bias, temperature, dielectric material type (X7R, X5R,
Y5V, etc), and manufacturer component tolerance have an
affect on the true or effective capacitance of a ceramic
capacitor. Be aware of how your application will affect a
particular ceramic capacitor by analyzing the aforemen-
tioned factors of your application. Before selecting a capaci-
tor always consult the capacitor manufacturer’s data curves
to verify the effective or true capacitance of the capacitor in
your application.
INPUT CAPACITOR SELECTION
The input capacitor serves as an energy reservoir for the
inductor. In addition to acting as an energy reservoir for the
inductor the input capacitor is necessary for the reduction in
input voltage ripple and noise experienced by the LM3503.
The reduction in input voltage ripple and noise helps ensure
OUT
PEAK
L
PEAK
Coilcraft
IN
IN
(avg): Average Inductor Current.
DS(ON)
L
:
:
:
:
:
: Peak Inductor Current.
Input Voltage.
Switching Frequency.
Inductance Magnitude/Inductor Value.
Duty Cycle for DCM Operation.
: NMOS Power Switch ON Resistance.
White LED Current/Load Current.
Inductance Magnitude/Inductor Value.
Duty Cycle for CCM Operation.
Peak Inductor Current.
Inductor Ripple Current.
Duty Cycle.
1-D.
Switching Frequency.
Input Voltage.
Inductance Magnitude/Inductor Value.
DO1608C-223
DT1608C-223
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