iW1706

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

iW1706

soft-start cycle. The controller continues attempting start-up

until the fault condition is removed.

9.10 PCL, OCP and SRS Protection

Peak-current limit (PCL), over-current protection (OCP) and

sense-resistor short protection (SRSP) are features built-in

to the iW1706. With the I

monitor the peak primary current. This allows for cycle by

cycle peak current control and limit. When the primary peak

current multiplied by the I

over current (OCP) is detected and the IC will immediately

turn off the base driver until the next cycle. The output

driver will send out a switching pulse in the next cycle, and

the switching pulse will continue if the OCP threshold is

not reached; or, the switching pulse will turn off again if the

OCP threshold is reached. If the OCP occurs for several

consecutive switching cycles, the iW1706 shuts down.

If the I

of the over current condition not being detected. Thus,

the IC is designed to detect this sense-resistor-short fault

after startup and shut down immediately. The V

discharged since the IC remains biased. Once V

below the UVLO threshold, the controller resets itself and

then initiates a new soft-start cycle. The controller continues

attempting to startup, but does not fully startup until the fault

condition is removed.

9.11 Dynamic Base Current Control

One important feature of the iW1706 is that it directly drives

a BJT switching device with dynamic base current control to

optimize performance. The BJT base current ranges from

13 mA to 40 mA, and is dynamically controlled according

to the power supply load change. The higher the output

power, the higher the base current. Specifically, the base

current is related to V

0

10

20

30

40

0.1

0

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

50

Figure 9.5: Base Drive Current vs. V

IPK

9.12 Cable Drop Compensation

The iW1706 incorporates an innovative method to

compensate for any IR drop in the secondary circuitry

including cable and cable connector. A 6-W adapter with

12 V DC output has 1.26% deviation at 0.5 A load current

due to the drop across a 24 AWG, 1.8 meter DC cable

without cable compensation. The iW1706 compensates

for this voltage drop by providing a voltage offset to the

feedback signal based on the amount of load current

detected.

The “Cable Comp” specified in the Table in Section 11.0

refers to the voltage increment at PCB end from no-load

to full-load conditions in the CV mode, with the assumption

that the secondary diode voltage drop can be ignored at

the point when the secondary voltage is sensed. Also, the

“Cable Comp” is specified based on the nominal output

voltage of 5 V. For different output voltage, the actual

voltage increment needs to be scaled accordingly.

To calculate the amount of cable compensation needed,

take the resistance of the cable and connector and multiply

by the maximum output current.