L6730B datasheet(37/52 Pages) STMICROELECTRONICS

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L6730 - L6730B

Application details

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6.5

Two quadrant or one quadrant operation mode (L6730)

After the soft-start phase the L6730 can work in source only (one quadrant operation mode)

or in sink/source (two quadrant operation mode), depending on the setting of the

multifunction pin (see Chapter 5.10 on page 27). The choice of one or two quadrant

operation mode is related to the application. One quadrant operation mode permits to have

a higher efficiency at light load, because the converter works in discontinuous mode (see

Figure 31). Nevertheless in some cases, in order to maintain a constant switching

frequency, it’s preferable to work in two quadrants, even at light load. In this way the

reduction of the switching frequency due to the pulse skipping is avoided. To parallel two or

more modules is requested the one quadrant operation in order not to have current sinking

between different converters. Finally the two quadrant operation allows faster recovers after

negative load transient. For example, let’s consider that the load current falls down from IOUT

to 0A with a slew rate sufficiently greater than L/VOUT (where L is the inductor value). Even

considering that the converter reacts instantaneously setting to 0% the duty-cycle, the

energy ½*L*IOUT

2 stored in the inductor will be transferred to the output capacitors,

increasing the output voltage. If the converter can sink current this overvoltage can be faster

eliminated.

Figure 31.

Efficiency in discontinuous-current-mode and continuous-current-mode

EFFICIENCY: DCM vs. CCM

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

OUTPUT CURRENT (A)

EFFICIENCY DCM

EFFICIENCY CCM

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