iW1677

Low-Power Off-Line Digital Green-Mode PWM Controller

Rev. 2.0

Datasheet

© 2016 Dialog Semiconductor

www.dialog-semiconductor.com

9 of 16

3-June-2016

9.2 Start-up

Refer to Figure 3.1 and Figure 3.2 for active start-up circuits using external depletion NFET and BJT respectively.

ENABLE signal becomes active and the iW1677 commences soft start function. During this start-up process an

adaptive soft-start control algorithm is applied, where the initial output pulses will be small and gradually get larger

to initiate a new soft-start process.

While the ENABLE signal initiates the soft-start process, it also pulls down the ASU pin voltage at the same time,

which turns off the depletion NFET or the BJT, thus minimizing the no-load standby power consumption. For the active

start-up scheme in Figure 3.2, the start-up resistors connected between the base of the BJT and DC input still conduct

current after start-up is finished. Their resistance needs to be large enough to minimize no-load power consumption,

meanwhile the BJT with ample gain should be selected in order to obtain a sufficient charge current for a fast start-up.

ENABLE

Start-up

Sequencing

ASU

Figure 9.1 : Start-up Sequencing Diagram

9.3 Understanding Primary Feedback

+

D1

Q1

N:1

+

–

Figure 9.2 : Simplified Flyback Converter