Current Limit Operation
The primary-side controller has a current limit threshold ramp that is
inversely proportional to the time from the end of the previous
primary switching cycle (i.e. from the time the primary MOSFET turns
off at the end of a switching cycle).
This characteristic produces a primary current limit that increases as
the switching frequency (load) increases (Figure 6).
This algorithm enables the most efficient use of the primary switch
with the benefit that this algorithm responds to digital feedback
information immediately when a feedback switching cycle request is
At high load, switching cycles have a maximum current approaching
100% ILIM. This gradually reduces to 30% of the full current limit as
load decreases. Once 30% current limit is reached, there is no
further reduction in current limit (since this is low enough to avoid
audible noise). The time between switching cycles will continue to
increase as load reduces.
The normalized current limit is modulated between 100% and 95% at
a modulation frequency of fM this results in a frequency jitter of
~7 kHz with average frequency of ~100 kHz.
In the event a fault condition occurs (such as an output overload,
output short-circuit, or external component/pin fault), the
InnoSwitch3-Pro enters auto-restart (AR) or latches off. The latching
condition is reset by bringing the PRIMARY BYPASS pin below ~ 3 V
or by going below the UNDER/OVER INPUT VOLTAGE pin UV (IUV-)
In auto-restart, switching of the power MOSFET is disabled for t .AR(OFF)
There are 2 ways to enter auto-restart:
1. Continuous secondary requests at above the overload detection
frequency (~110 kHz) for longer than 82 ms (tAR).
2. No requests for switching cycles from the secondary for > tAR(SK).
The second is included to ensure that if communication is lost, the
primary tries to restart. Although this should never be the case in
normal operation, it can be useful when system ESD events (for
example) causes a loss of communication due to noise disturbing the
secondary controller. The issue is resolved when the primary restarts
after an auto-restart off-time.
The first auto-restart off-time is short (t ).AR(OFF)SH This short auto-
restart time is to provide quick recovery under fast reset conditions.
The short auto-restart off-time allows the controller to quickly check to
determine whether the auto-restart condition is maintained beyond
The auto-restart is reset as soon as an AC reset occurs.
In the event that there are two consecutive cycles where the ILIM is
reached within ~500 ns (the blanking time + current limit delay time),
the controller will skip 2.5 cycles or ~25 ms (based on full frequency
of 100 kHz). This provides sufficient time for the transformer to reset
with large capacitive loads without extending the start-up time.
Input Line Voltage Monitoring
The UNDER/OVER INPUT VOLTAGE pin is used for input undervoltage
and overvoltage sensing and protection.
A 4 MΩ resistor is tied between the high-voltage DC bulk capacitor
after the bridge (or to the AC side of the bridge rectifier for fast AC
reset) and the UNDER/OVER INPUT VOLTAGE pin to enable this
functionality. This function can be disabled by shorting the UNDER/
OVER INPUT VOLTAGE pin to primary GND.
At power-up, after the primary bypass capacitor is charged and the
ILIM state is latched, and prior to switching, the state of the UNDER/
OVER INPUT VOLTAGE pin is checked to confirm that it is above the
brown-in and below the overvoltage shutdown thresholds.
In normal operation, if the UNDER/OVER INPUT VOLTAGE pin current
falls below the brown-out threshold and remains below brown-in for
longer than tUV-, the controller enters auto-restart. Switching will only
resume once the UNDER/OVER INPUT VOLTAGE pin current is above
the brown-in threshold.
In the event that the UNDER/OVER INPUT VOLTAGE pin current is
above the overvoltage threshold, the controller will also enter
auto-restart. Again, switching will only resume once the UNDER/
OVER INPUT VOLTAGE pin current has returned to within its normal
The input line UV/OV function makes use of a internal high-voltage
(VV) MOSFET on the UNDER/OVER INPUT VOLTAGE pin to reduce
power consumption. The controller samples the input line at light
load conditions when the time between switching cycles is 50 msec
or more. At >50 msec between switching cycles, the high-voltage
MOSFET will remain on making sensing continuous.
At start-up, the primary-side initially switches without any feedback
information (this is very similar to the operation of a standard
TOPSwitch™, TinySwitch™ or LinkSwitch™ controllers).
If no feedback signals are received during the auto-restart on-time
(tAR), the primary goes into auto-restart mode. Under normal
conditions, the secondary controller will power-up via the FORWARD
pin or from the OUTPUT VOLTAGE pin and take over control. From
this point onwards the secondary controls switching.
Rev. E 08/18