AN1729 datasheet(9/10 Pages) STMICROELECTRONICS

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AN1729 APPLICATION NOTE

The value of the capacitor C4 has been multiplied by 1.2 to account for its tolerance, the value of the quiescent

current is the minimum from the datasheet of the controller and also the maximum hysteresis of the on/off VCC

thresholds has been considered.

The charge time of the capacitor C4 has to be less then TburstMax-Tdisch-C4-Max. Following the previous consid-

erations and using eq. 2, a minimum Ist-up, of 1.2 mA is needed. As a result, the values of 10kΩ, for the resis-

tance RH3, and 15V, for the zener voltage of the zener diode DZ1, were chosen.

Experimental results

In this section the results of experimental tests on a board manufactured according to the schematic in figure 3

are reported. In table 2 the measured standby consumption at different values of the input voltage are shown:

Table 2. Standby Input Power Measurements

Fig. 6 shows the measured waveforms of the output voltages and of the Voltage at the VCC pin.

Channel 1 is the output of the voltage regulator and its value is constant (~5V). Channel 2 is the input of the

voltage regulator and here we can see that during the idle phase the voltage decreases linearly (the output ca-

pacitor is discharged with a constant current ISTB), during the switching phase (note, it is actually very short) this

output voltage rises up from VOminstb (Ch2Min = 7.48V) to VOMax (Ch2Max = 8.68V).

Figure 6. Standby waveforms: steady state with 10 mA load current

Channel 3 shows the voltage on VCC pin of the controller L6565. As soon as this voltage reaches the UVLO on-

threshold the system starts switching but, as soon as the voltage regulator input voltage reaches a value such

that the small signal bipolar Q3 is turned on, the gate driver of the L6565 is disabled, the switching is stopped

and the VCC voltage decreases until the UVLO off-threshold. At this point the start-up network is on again and

Vin

88Vac

110Vac

220Vac

264Vac

Pin-stand-by

320mW

390mW

682mW

835mW

AN1729 数据表(PDF) 9 Page - STMicroelectronics