iW1707

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

iW1707

9.1 Pin Detail

Pin 1 – V

CC

Power supply for the controller during normal operation.

The controller will start up when V

and will shut-down when the V

A decoupling capacitor of 0.1 μF or so should be connected

between the V

Pin 2 – V

SENSE

Sense signal input from auxiliary winding. This provides the

secondary voltage feedback used for output regulation.

Pin 3 – ASU

Control signal for active startup device. This signal is pulled

low after start-up is finished to cut off the active device.

This pin can be left unconnected if an active start-up device

is not needed in the application circuit.

Pin 4 – I

SENSE

Primary current sense. Used for cycle-by-cycle peak current

control and limit.

Pin 5 – GND

Ground.

Pin 6 – OUTPUT

Base drive for the external power BJT switch.

9.2 Active Start-up and Adaptively

Controlled Soft Start

The iW1707 features an innovative proprietary soft-start

scheme to achieve fast yet smooth build-up of output voltage

with a wide range of output loads, including capacitive loads

typically from 330 μF to 6,000 μF, and for output voltage

covering typically from 5 V to 12 V. In addition, the active

start-up schemes enable shortest possible turn-on delay

without sacrificing no-load power loss.

Refer to Figure 3.1 and Figure 3.2 for active start-up circuits

using external depletion NFET and BJT respectively.

Prior to start-up, the ENABLE signal is low, and the ASU

pin voltage closely follows the V

in Figure 9.1. Consequently, the depletion NFET or the

BJT is turned on, allowing the start-up current to charge

the V

is charged to a voltage higher than the start-up threshold

V

commences the soft start function. During the soft-start

process, the primary-side peak current is limited cycle-by-

cycle by the I

can break down into several stages based on the output

voltage levels, which is indirectly sensed by V

the primary side. At different stages, the iW1707 adaptively

controls the switching frequency and primary-side peak

current such that the output voltage can always build up

very fast at the early stages and smoothly transition to the

desired regulation voltage at the final stage, regardless of

any capacitive and resistive loads that the applications may

incur. With a lowest system cost, this adaptively controlled

soft start feature makes the iW1707 ideal in network power

adapter applications such as ADSL modems where the

adapter needs to drive large capacitive loads with 12 V

output voltage.

Figure 9.1: Start-up Sequencing Diagram

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 startup

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.

If at any time the V

lockout (UVLO) threshold V

shutdown. At this time ENABLE signal becomes low, the

ENABLE

Start-up

Sequencing

ASU