AN557 datasheet(43/52 Pages) STMICROELECTRONICS

부품명	AN557

상세설명	EASY APPLICATION DESIGN WITH THE L4970A

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AN557 APPLICATION NOTE

blocked and the device begins to work starting in soft start. Switching off the mains voltage, the regu-

lator generating the negative volatge is still in regulation when the input voltage of the switching con-

verter has already dropped below the turn on threshold.

Carefull attention must be given therefore to calculate the input capacitor of the two sections in order

to avoid possible malfunctioning during the turning on and turning off.

b) a solution that presents a cheaper costs and tha doesn’t use a negative reference voltage is the follow-

ing: Setting the cursor "P" to the adjustable resistance at 0V, using R1 and R2 the maximum output

voltage can be fixed. In this case we set R1 = 24Kohm and R2 = 4K7ohm.

In R1 the maximum flowing current will be limited at 1mA; with 1mA flowing in R1, Vo = 30V. Now by

reducing the current in R1 the output voltage Vo can be adjusted till to 0V.

Figure 59.

Figure 60.

AN557 데이터시트(PDF) 43 Page - STMicroelectronics