L6983I datasheet(14/56 Pages) STMICROELECTRONICS

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5.1.5

Switch-over feature

The switch-over (selectable by connecting the VBIAS pin as shown in Figure 11, left) helps to improve the

efficiency, especially at lower currents.

Figure 11. Switch-over selection

PGOOD

BOOT

VBIAS

PGND

GND

VOUT_prim

VOUT_iso

GND_iso

L6983I

RFB1

RFB2

CSEC

COUT

PGOOD

BOOT

VBIAS

PGND

GND

VOUT_prim

VOUT_iso

GND_iso

L6983I

RFB1

RFB2

CSEC

COUT

Switch-over

No switch-over

VBIAS can be also connected to an external power supply. The external power supply connected to the VBIAS pin

must be disabled when the voltage on the EN pin is lower than VWAKE_UP.

The effect on the efficiency for both the primary output and the secondary isolated output are shown in Figure 12.

Figure 12. Effect of the switch-over on the efficiency for primary output (left) and isolated secondary

output (right)

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0,5

1,5

2,5

Primary output current [A]

VBIAS = VOUTprim

VBIAS = GND

Efficiency variation

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

0,1

0,2

0,3

0,4

0,5

Isolated output current [A]

VBIAS = VOUTprim

VBIAS = GND

Efficiency variation

5.1.6

Spread spectrum

The spread spectrum feature, helpful to improve EMC performances, is selectable by connecting the RFSW

resistor to the VCC pin (please refer to Section 4.1 Frequency selection table). The internal dithering circuit

changes the switching frequency in a range of ± 5%.

∆FSW=10%∙Fsw

(1)

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

(2)

A visual explanation about how the switching frequency varies when the spread spectrum feature is used can be

observed in Figure 13.

L6983I

Primary side

DS14040 - Rev 1

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