iW1696

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

iW1696

9.6 Multi-Mode PWM/PFM Control and

Quasi-Resonant Switching

The iW1696 uses an adaptive multi-mode PWM /PFM

control to dramatically improve the light-load efficiency and

thus the overall average efficiency.

During the constant voltage (CV) operation, the iW1696

normally operates in a pulse-width-modulation (PWM)

mode during heavy load conditions. In the PWM mode, the

switching frequency keeps around constant. As the output

load I

the controller adaptively transitions to a pulse-frequency-

modulation (PFM) mode. During the PFM mode, the BJT

is turned on for a set duration under a given instantaneous

rectified AC input voltage, but its off time is modulated by

the load current. With a decreasing load current, the off time

increases and thus the switching frequency decreases.

As the load current is further reduced, the iW1696 transitions

to a deep PFM mode (DPFM) which reduces the switching

frequency to a very low level.

While operating in the adaptive multi-mode PWM/PFM

control, iW1696 also incorporates a unique quasi-resonant

switching in both CV and CC operations. Together these

innovative digital control architecture and algorithms enable

iW1696 to achieve highest overall efficiency and lowest

EMI.

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

75 μs. When the transformer reset time reaches 75 μs, the

iW1696 immediately shuts off.

9.8 Internal Loop Compensation

The iW1696 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 iW1696. When the V

OVP threshold at point 1 indicated in figure 9.3 the iW1696

immediately shuts down.

The iW1696 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

Also, the iW1696

monitors the voltage on the V

on this pin is below UVLO threshold the IC shuts down

immediately. The output voltage is monitored through the

V

iW1696 will shut 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 iW1696. 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 iW1696 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 iW1696 is that it directly drives

a BJT switching device with dynamic base current control to

optimize performance. The BJT base current ranges from