LM3557 Datasheet, PDF (9/19 Pages) National Semiconductor (TI) – Step-Up Converter for White LED Applications

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Vout = ^{[ R4 + R3 ] x VOvp}
[ R4 ]
where
•
•
•
•
V_OVP is the OVP voltage threshold
V_OUT is the maximum output voltage (<35V)
R3 is a resistor
R4 is a resistor
tON
LM3557
SNVS338B – NOVEMBER 2004 – REVISED FEBRUARY 2013
(3)
I_L (avg)
'i_L
Time
T_S
Figure 13. Inductor Current Waveform
CONTINUOUS AND DISCONTINUOUS MODES OF OPERATION
Since the LM3557 is a constant frequency pulse-width-modulated step-up regulator, care must be taken to make
sure the maximum duty cycle specification is not violated. The duty cycle equation depends on which mode of
operation the LM3557 is in. The two operational modes of the LM3557 are continuous conduction mode (CCM)
and discontinuous conduction mode (DCM). Continuous conduction mode refers to the mode of operation where
during the switching cycle, the inductor’s current never goes to and stays at zero for any significant amount of
time during the switching cycle. Discontinuous conduction mode refers to the mode of operation where during the
switching cycle, the inductor’s current goes to and stays at zero for a significant amount of time during the
switching cycle. Figure 13 illustrates the threshold between CCM and DCM operation. In Figure 13, the inductor
current is right on the CCM/DCM operational threshold. Using this as a reference, a factor can be introduced to
calculate when a particular application is in CCM or DCM operation. R is a CCM/DCM factor we can use to
compute which mode of operation a particular application is in. If R is ≥ 1, then the application is operating in
CCM. Conversely, if R is < 1, the application is operating in DCM. The R factor inequalities are a result of the
components that make up the R factor. From Figure 13, the R factor is equal to the average inductor current,
I_L(avg), divided by half the inductor ripple current, Δi_L. Using Figure 13, the following equation can be used to
compute R factor:
2 x I_L (avg)
R=
'i_L
(4)
[I_OUT]
I_L (avg) = _{[(1-D) x Eff]}
(5)
[V_IN x D]
'i_L = _{[L x Fs]}
(6)
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