L6730 datasheet(36/52 Pages) STMICROELECTRONICS

36 / 52 page

Application details

L6730 - L6730B

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●

Compensation network design:

–

Put the gain R5/R3 in order to obtain the desired converter bandwidth

–

Place

ωZ1 before the output filter resonance ωLC;

–

Place

ωZ2 at the output filter resonance ωLC;

–

Place

ωP1 at the output capacitor ESR zero ωESR;

–

Place

ωP2 at one half of the switching frequency;

–

Check the loop gain considering the error amplifier open loop gain.

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 30.). 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 29. Asymptotic Bode plot of Converter's open loop gain

Vosc

Vin

⋅

∆

⋅

(18)

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