LM3488 Datasheet, PDF (26/37 Pages) National Semiconductor (TI) – High Efficiency Low-Side N-Channel Controller for Switching Regulators

LM3488, LM3488-Q1
SNVS089N – JULY 2000 – REVISED DECEMBER 2014
Peak to peak ripple current, to calculate core loss if necessary:
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maintains the condition I_L > Δi_L/2 to ensure constant current mode.
(V_IN - V_Q)(1-D)
L1 >
2I_OUTf_S
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(V_IN - V_Q)D
L2 > _2IOUTfS
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Peak current in the inductor, to ensure the inductor does not saturate:
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I_L1PK must be lower than the maximum current rating set by the current sense resistor.
The value of L1 can be increased above the minimum recommended to reduce input ripple and output ripple.
However, once D_IL1 is less than 20% of I_L1AVE, the benefit to output ripple is minimal.
By increasing the value of L2 above the minimum recommended, Δ_IL2 can be reduced, which in turn will reduce
the output ripple voltage:
( ) ^VOUT ⁼
IOUT I_L2
1-D ⁺ 2
ESR
where
• ESR is the effective series resistance of the output capacitor.
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If L1 and L2 are wound on the same core, then L1 = L2 = L. All the equations above will hold true if the
inductance is replaced by 2L. A good choice for transformer with equal turns is Coiltronics CTX series Octopack.
8.2.2.2.4 Sense Resistor Selection
The peak current through the switch, I_SW(PEAK) can be adjusted using the current sense resistor, R_SEN, to provide
a certain output current. Resistor R_SEN can be selected using the formula:
V_SENSE - D(V_SL + V_SL)
R_SEN =
I_SWPEAK
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8.2.2.2.5 SEPIC Capacitor Selection
The selection of SEPIC capacitor, CS, depends on the rms current. The rms current of the SEPIC capacitor is
given by:
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