LM3557 Datasheet, PDF (10/19 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

LM3557
SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013
[2 x I_OUT x L x Fs x (V_OUT)²]
R=
[(V_IN)² x Eff x (V_OUT - V_IN)]
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
For CCM operation, the duty cycle can be computed with:
t_ON
D = _TS
[V_OUT - V_IN]
D = _[VOUT]
where
• t_ON is the internal power FET on-time
• T_S is the switching period of operation
• D is the duty cycle for CCM operation
• V_IN is the input voltage
• V_OUT is the output voltage
For DCM operation, the duty cycle can be computed with:
t_ON
D = _TS
[2 x I_OUT x L x (V_OUT - V_IN) x Fs]
D=
[(V_IN)² x Eff]
where
•
•
•
•
•
•
•
•
•
t_ON is the internal power FET on-time
T_S is the switching period of operation
D is the duty cycle for CCM operation
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
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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. Inductors 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:
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