IW1697 Datasheet, PDF (10/12 Pages) Dialog Semiconductor – Low-Power Off-Line Digital Green-Mode PWM Controller

iW1697
Low-Power Off-Line Digital Green-Mode PWM Controller
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 iW1697 shuts down.
If the I_SENSE resistor is shorted there is a potential danger
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_CC will be
discharged since the IC remains biased. Once V_CC drops
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 iW1697 is that it directly drives
a BJT switching device with dynamic base current control to
optimize performance. The BJT base current ranges from
10 mA to 31 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_IPK, as shown in Figure 9.5.
35
30
25
20
15
10
5
0_{0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1}
V_IPK (V)
Figure 9.5: Base Drive Current vs. V_IPK
9.12 Cable Drop Compensation
The iW1697 incorporates an innovative method to
compensate for any IR drop in the secondary circuitry
including cable and cable connector. A 2.5 W adapter with
5 V DC output has 3% deviation at 0.5 A load current due
to the drop across a 24 AWG, 1.8 meter DC cable without
cable compensation. The iW1697 compensates for this
voltage drop by providing a voltage offset to the feedback
signal based on the amount of load current detected.
To calculate the amount of cable compensation needed, take
the resistance of the cable and connector and multiply by the
maximum output current.
Rev. 1.1
iW1697
February 1, 2012
Page 10