LM3557 Datasheet, PDF (9/11 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

Application Information (Continued)
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D: Duty Cycle for DCM Operation.
I_Peak: Peak Inductor Current.
Some recommended inductor manufacturers are as follows:
Coilcraft [www.coilcraft.com]
Coiltronics [www.cooperet.com]
TDK [www.tdk.com]
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t_ON: Internal Power FET On-Time.
T_S: Switching Period of Operation.
D: Duty Cycle for DCM Operation.
V_OUT: Output Voltage.
V_IN: Input Voltage.
I_OUT: White LED Current/Load Current.
Fs: Switching Frequency.
Eff: Efficiency of the LM3557.
L: Inductor Value/Inductance Magnitude.
INDUCTOR SELECTION
In order to maintain inductance, an inductor used with the
LM3557 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 (continuous
current mode) and DCM (discontinuous current mode).
The cycle-by-cycle peak inductor current for CCM operation
can be computed with:
CAPACITOR SELECTION
Multilayer ceramic capacitors are the best choice for use
with the LM3557. 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 LM3557.
The reduction in input voltage ripple and noise helps ensure
the LM3557’s proper operation, and reduces the effect of the
LM3557 on other devices sharing the same supply voltage.
To ensure low input voltage ripple, the input capacitor must
have an extremely low ESR. As a result of the low input
voltage ripple requirement multilayer ceramic capacitors are
the best choice. A minimum capacitance of 2.0 µF is required
for normal operation, consult the capacitor manufacturer’s
data curves to verify whether the minimum capacitance re-
quirement is going to be achieved for a particular application.
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V_IN: Input Voltage.
Eff: Efficiency of the LM3557.
Fs: Switching Frequency.
I_OUT: White LED Current/Load Current.
L:
Inductance Magnitude/Inductor Value.
D:
Duty Cycle for CCM Operation.
I_Peak: Peak Inductor Current.
∆i_L: Inductor Ripple Current.
I_L(avg): Average Inductor Current.
The cycle-by-cycle peak inductor current for DCM operation
can be computed with:
OUTPUT CAPACITOR SELECTION
The output capacitor serves as an energy reservoir for the
white LED load when the internal power FET switch (Figure
2: N1) is ON or conducting current. The requirements for the
output capacitor must include worst case operation such as
when the load opens up and the LM3557 operates in over-
voltage protection (OVP) mode operation. A minimum ca-
pacitance of 0.5 µF is required to ensure normal operation.
Consult the capacitor manufacturer’s data curves to verify
whether the minimum capacitance requirement is going to
be achieved for a particular application.
Some recommended capacitor manufacturers are as fol-
lows:
TDK
[www.tdk.com]
Murata
[www.murata.com]
Vishay
[www.vishay.com]
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V_IN: Input Voltage.
Fs: Switching Frequency.
L: Inductance Magnitude/Inductor Value.
DIODE SELECTION
To maintain high efficiency it is recommended that the aver-
age current rating (I_F or I_O) of the selected diode should be
larger than the peak inductor current (I_Lpeak). To maintain
diode integrity the peak repetitive forward current (I_FRM)
must be greater than or equal to the peak inductor current
(I_Lpeak). Diodes with low forward voltage ratings (V_F) and low
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