AN2242

3 Cross Regulation and Standby

13/31

Table 6.

Output Voltage Measurement at Standby Load

In Table 6. the output voltage measurements during standby operation are shown. Even in this

load condition the circuit is able to regulate the output voltages within the required tolerance.

Figure 9.

Input Power vs. Mains Voltage During Standby

It is clearly visible that, while delivering the required standby load, (5V at 50mA, 1.8V, 3.3V and

12V at 0mA) the input power consumption is below 800mW at both the nominal input

voltage ranges and remains below 900mW at 265Vac. Figure 10. represents the input power

variation as a function of the 5V current.

During the standby operation the circuit works in burst mode, thus the number of switching

cycles decreases and, therefore it brings to an equivalent continuous switching frequency

reduction. This minimizes all frequency-related losses and maximizes the efficiency with

respect to other controllers without this feature. The L6668 enters automatically in burst mode

when its internal circuitry detects a light load by monitoring the voltage on pin #10 (Comp).

When it falls by 50mV below the threshold programmed via the divider connected at to pin #9

(SKIPADJ), the IC is disabled and its consumption is reduced to minimize the Vcc capacitor

discharge. The soft-start capacitor is not discharged. The control voltage now will increase as a

result of the feedback reaction to the energy delivery stop, the threshold will be exceeded and

the IC will restart switching again. In this way the converter works in burst-mode with a constant

at 115Vac - 60Hz

at 230Vac

Voltage [V] Current [A]

Deviation

Voltage [V]

Current [A]

Deviation

11.80

0

-1.67%

OK

11.70

0

-2.50%

OK

1.92

0

6.67%

OK

1.92

0

6.67%

OK

4.71

0.047

-5.80%

OK

4.73

0.047

-5.40%

OK

3.36

0

1.82%

OK

3.36

0

1.82%

OK

Vin= 115Vac

lin= 0.017Arms

Pin=0.51W

Vaux=9.68V

EFF.= 43.5%

Vin= 230Vac

lin= 0.0217Arms

Pin=0.75W

Vaux=10V

EFF.= 29.8%

Pout=0.22W

Pout=0.22W

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Mains voltage

Pin= [W]

0.49

0.51

0.75

0.88

Vaux= [V]

9.67

9.68

10

10.04

90Vac

115Vac

230Vac

265Vac