LM3668_15 Datasheet, PDF (20/30 Pages) Texas Instruments – 1-A, High-Efficiency Dual-Mode Single-Inductor Buck-Boost DC-DC Converter

LM3668
SNVS449O – JUNE 2007 – REVISED APRIL 2015
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Typical Application (continued)
9.2.2 Detailed Design Procedure
9.2.2.1 Inductor Selection
There are two main considerations when choosing an inductor: the inductor should not saturate, and the inductor
current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current
rating specifications are followed by different manufacturers so attention must be given to details. Saturation
current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of
application should be requested from the manufacturer. Shielded inductors radiate less noise and should be
preferred.
In the case of the LM3668, there are two modes (Buck & Boost) of operation that must be consider when
selecting an inductor with appropriate saturation current. The saturation current should be greater than the sum
of the maximum load current and the worst case average to peak inductor current. Equation 1 shows the buck
mode operation for worst case conditions and the second equation for boost condition.
I_SAT > I_OUTMAX + IRIPPLE
For Buck
(V_IN - V_OUT) V_OUT
Where I_RIPPLE =
(2 x L x f)
x
V_IN
I_OUTMAX
I_SAT >
'¶
+ I_RIPPLE
For Boost
(V_OUT - V_IN)
Where I_RIPPLE = _{(2 x L x f)}
x
V_IN
V_OUT
(V_OUT - V_IN)
Where D = _(VOUT) & '¶= (1-D)
where
• I_RIPPLE: Peak inductor current
• I_OUTMAX: Maximum load current
• V_IN: Maximum input voltage in application
• L : Min inductor value including worst case tolerances (30% drop can be considered)
• f : Minimum switching frequency
• V_OUT: Output voltage
• D: Duty Cycle for CCM Operation
• V_OUT : Output voltage
• V_IN: Input voltage
Example using above equations:
• V_IN = 2.8 V to 4 V
• V_OUT = 3.3 V
• I_OUT = 500 mA
• L = 2.2 µH
• F = 2 MHz
• Buck: I_SAT = 567 mA
• Boost: I_SAT = 638 mA
(1)
As a result, the inductor should be selected according to the highest of the two I_SAT values.
A more conservative and recommended approach is to choose an inductor that has a saturation current rating
greater than the maximum current limit of 2.05 A.
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