IW1700 Datasheet, PDF (10/12 Pages) Dialog Semiconductor – Zero Power No-Load Off-Line Digital PWM Controller

iW1700
Zero Power No-Load Off-Line Digital PWM Controller
9.9 Internal Loop Compensation
The iW1700 incorporates an internal Digital Error Amplifier
with no requirement for external loop compensation. For a
typical power supply design, the loop stability is guaranteed
to provide at least 45 degrees of phase margin and -20 dB
of gain margin.
9.10 Voltage Protection Features
The secondary maximum output DC voltage is limited by the
iW1700. When the V_SENSE signal exceeds the output OVP
threshold at point 1 indicated in Figure 9.3 the iW1700 shuts
down.
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.
9.12 Dynamic Base Current Control
One important feature of the iW1700 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.
The iW1700 protects against input line undervoltage
by setting a maximum T_ON time. Since output power is
proportional to the squared V_INT_ON product, then for a given
output power, as V_IN decreases the T_ON will increase. Thus
by knowing when the maximum T_ON time occurs the iW1700
detects that the minimum V_IN is reached, and shuts down.
The maximum t_ON limit is set to 13.8 μs. Also, the iW1700
monitors the voltage on the V_CC pin and when the voltage
on this pin is below UVLO threshold the IC shuts down
immediately.
When any of these faults are met the IC remains biased
to discharge the V_CC supply. Once V_CC drops below UVLO
threshold, the controller resets itself and then initiates a new
soft-start cycle. The controller continues attempting start-up
until the fault condition is removed.
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
9.11 PCL, OCP and SRS Protection
9.13 Cable Drop Compensation
Peak-current limit (PCL), over-current protection (OCP) and
sense-resistor short protection (SRSP) are features built-in
to the iW1700. With the I_SENSE pin the iW1700 is able to
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_SENSE resistor is greater than 1.15 V,
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 iW1700 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
The iW1700 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 iW1700 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.
Rev. 1.2
iW1700
February 13, 2012

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