L6730D datasheet(34/50 Pages) STMICROELECTRONICS

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Application details

L6730C - L6730D

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6.5

Two quadrant or one quadrant operation mode (L6730C)

After the soft-start phase the L6730C 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 25). 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 28.). 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 28. 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

L6730D Arkusz danych(PDF) 34 Page - STMicroelectronics