LM3478MA_15 Datasheet, PDF (22/26 Pages) Texas Instruments – High-Efficiency Low-Side N-Channel Controller for Switching Regulator

LM3478MA
SNVS705B – FEBRUARY 2011 – REVISED FEBRUARY 2013
www.ti.com
(41)
(42)
Maintaining the condition I_L > Δi_L/2 to ensure continuous current conduction yields:
(V_IN - V_Q)(1-D)
L1 >
2I_OUTf_S
(43)
(V_IN - V_Q)D
L2 > _2IOUTfS
(44)
Peak current in the inductor, to ensure the inductor does not saturate:
(45)
(46)
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
(47)
where ESR is the effective series resistance of the output capacitor.
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.
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
(48)
Sepic Capacitor Selection
The selection of the SEPIC capacitor, CS, depends on the RMS current. The RMS current of the SEPIC
capacitor is given by:
(49)
The SEPIC capacitor must be rated for a large ACrms current relative to the output power. This property makes
the SEPIC much better suited to lower power applications where the RMS current through the capacitor is
relatively small (relative to capacitor technology). The voltage rating of the SEPIC capacitor must be greater than
the maximum input voltage. There is an energy balance between CS and L1, which can be used to determine
the value of the capacitor. The basic energy balance equation is:
(50)
where
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