LM3557 Datasheet, PDF (11/19 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

LM3557
www.ti.com
SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013
I_Peak
| I_L
(avg)
+
'i_L
2
(12)
I_Peak
|
[(1
[I_OUT]
- D) * Eff]
+
[V_IN * D]
[2 * L * Fs]
where
• V_IN is the input voltage
• V_OUT is the output voltage
• Eff is the efficiency of the LM3557
• Fs is the switching frequency
• I_OUT is the white LED current/load current
• L is the inductance magnitude/inductor value
• D is the duty cycle for CCM operation
• Δi_L is the inductor ripple current
• I_L(avg) is the average inductor current
(13)
The cycle-by-cycle peak inductor current for DCM operation can be computed with:
I_Peak
|
[V_IN * D]
[L * Fs]
where
• V_IN is the input voltage
• V_OUT is the output voltage
• Eff is the efficiency of the LM3557
• Fs is the switching frequency
• I_OUT is the white LED current/load current
• L is the inductance magnitude/inductor value
• D is the duty cycle for CCM operation
• Δi_L is the inductor ripple current
• I_L(avg) is the average inductor current
(14)
Some recommended inductor manufacturers are:
Coilcraft [www.coilcraft.com]
Coiltronics [www.cooperet.com]
TDK [www.tdk.com]
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 aforementioned factors of your application. Before selecting a capacitor 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
requirement is going to be achieved for a particular application.
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