L6983 datasheet(21/63 Pages) STMICROELECTRONICS

21 / 63 page

6.5

Switch-over feature

The switch-over maximizes the efficiency at light load that is crucial for low consumption application.

Figure 18. Switch-over

PGOOD

BOOT

VBIAS

VOUT/FB

PGND

E.P.

L6983

VOUT

GND

CBOOT

RPGOOD

RFBH

RFBL

COUT

PGOOD

BOOT

VBIAS

VOUT/FB

PGND

E.P.

L6983

VOUT

GND

CBOOT

RPGOOD

RFBH

RFBL

COUT

Switch -Over

No Switch -Over

In order to minimize the regulator quiescent current sink from the input voltage, the VBIAS pin can be connected

to an external voltage source in the range of 3 V < VBIAS < VIN.

In case the VBIAS pin is connected to the regulated output voltage (VOUT), the total current drawn from the input

voltage is given by the following equation:

IQVIN=IQOPVIN+ 1ηL6983∙VBIASVIN∙IQOPVBIAS

(4)

6.6

Spread spectrum

The spread spectrum is selectable by connecting the RFSW resistor to VCC (please refer to Table 7. FSW

selection). The internal dithering circuit changes the switching frequency in a range of ± 5%.

∆FSW=5%∙Fsw

(5)

The device updates the frequency every clock period by fixed steps:

•

Ramps up in 63 steps from minimum to maximum FSW

•

Ramps down in 63 steps from maximum to minimum FSW

The modulation shape is almost triangular with a frequency of:

FDitℎering=Fsw126

(6)

6.7

Overvoltage protection

The overvoltage protection monitors the FB pin and enables the low-side MOSFET to discharge the output

capacitor if the output voltage is 20% (typ.) over the nominal value.

This is a second level protection and it should never be triggered in normal operating conditions if the system is

properly dimensioned. In other words, the selection of the external power components and the dynamic

performance determined by the compensation network should guarantee an output voltage regulation within the

overvoltage threshold even during the worst-case scenario in term of load transitions.

The protection is reliable and able to operate even during normal load transitions for a system whose dynamic

performance is not in line with the load dynamic request. Consequently, the output voltage regulation would be

affected.

The L6983 device implements a 1.5 A (IVY_SINK refer to Table 6. Electrical characteristics) negative current

limitation to limit the maximum reversed switch current during the overvoltage operation.

L6983

Switch-over feature

DS13116 - Rev 1

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