LMZ12003EXT datasheet(16/29 Pages) TI

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LMZ12003EXT

SNVS663H – JUNE 2010 – REVISED AUGUST 2015

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Additionally, consider the minimum OFF-time of 260 ns limits the maximum duty ratio. Larger RON (lower FSW)

must be selected in any application requiring large duty ratio.

8.2.2.6.1

Discontinuous Conduction and Continuous Conduction Modes Selection

Operating frequency in DCM can be calculated as follows:

fSW(DCM) ≊ VO × (VIN – 1) × 6.8 μH × 1.18 × 10

20 × I

O / (VIN – VO) × RON

(12)

In CCM, current flows through the inductor through the entire switching cycle and never falls to zero during the

OFF-time. The switching frequency remains relatively constant with load current and line voltage variations. The

CCM operating frequency can be calculated using Equation 7 above.

Figure 22 is a comparison pair of waveforms of the showing both CCM (upper) and DCM operating modes.

VIN = 12 V, VO = 3.3 V, IO = 3 A /0.4 A 2 μs/div

Figure 22. CCM and DCM Operating Modes

The approximate formula for determining the DCM/CCM boundary is as follows:

IDCB ≊ VO × (VIN – VO) / (2 × 6.8 μH × fSW(CCM) × VIN)

(13)

Figure 23 is a typical waveform showing the boundary condition.

VIN = 12 V, VO = 3.3 V, IO = 0.5 A 2 μs/div

Figure 23. Transition Mode Operation

The inductor internal to the module is 6.8

μH. This value was chosen as a good balance between low and high

input voltage applications. The main parameter affected by the inductor is the amplitude of the inductor ripple

current (ILR). ILR can be calculated with:

ILR P-P = VO × (VIN – VO) / (6.8 µH × fSW × VIN)

where

•

VIN is the maximum input voltage

•

and fSW is determined from Equation 7.

(14)

If the output current IO is determined by assuming that IO = IL, the higher and lower peak of ILR can be

determined. Be aware that the lower peak of ILR must be positive if CCM operation is required.

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