iW1707

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

iW1707

9.7 Variable Frequency Operation Mode

At each of the switching cycles, the falling edge of V

SENSE

will be checked. If the falling edge of V

the off-time will be extended until the falling edge of V

SENSE

is detected. The maximum allowed transformer reset time is

110 μs. When the transformer reset time reaches 110 μs,

the iW1707 shuts off.

9.8 Internal Loop Compensation

The iW1707 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.

9.9 Voltage Protection Features

The secondary maximum output DC voltage is limited by

the iW1707. When the V

OVP threshold at point 1 indicated in Figure 9.3 the iW1707

shuts down.

The iW1707 protects against input line undervoltage

by setting a maximum T

Since output power is

proportional to the squared V

output power as V

by knowing when the maximum T

detects that the minimum V

The maximum t

monitors the voltage on the V

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

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.

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 iW1707. 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 iW1707 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 iW1707 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