TNY263-268
that cycle. The current limit state machine reduces the current
limit threshold by discrete amounts under medium and light
loads.
BYPASS pin then rises from 4.8 V to 5.8 V when the line
undervoltage condition goes away. When the switching of the
power MOSFET is disabled in auto-restart mode and a line
undervoltage condition exists, the auto-restart counter is
stopped. This stretches the disable time beyond its normal
850 ms until the line undervoltage condition ends.
The leading edge blanking circuit inhibits the current limit
comparator for a short time (tLEB) after the power MOSFET is
turned on. This leading edge blanking time has been set so
that current spikes caused by capacitance and secondary-
side rectifier reverse recovery time will not cause premature
termination of the switching pulse.
The line undervoltage circuit also detects when there is no
external resistor connected to the EN/UV pin (less than
~2 μA into the pin). In this case the line undervoltage function
is disabled.
Auto-Restart
In the event of a fault condition such as output overload,
output short circuit, or an open loop condition, TinySwitch-II
enters into auto-restart operation. An internal counter clocked
by the oscillator gets reset every time the EN/UV pin is pulled
low. If the EN/UV pin is not pulled low for 50 ms, the power
MOSFET switching is normally disabled for 850 ms (except in
the case of line undervoltage condition, in which case it is
disabled until the condition is removed). The auto-restart
alternately enables and disables the switching of the power
MOSFET until the fault condition is removed. Figure 5
illustrates auto-restart circuit operation in the presence of an
output short circuit.
TinySwitch-II Operation
TinySwitch-II devices operate in the current limit mode. When
enabled, the oscillator turns the power MOSFET on at the
beginning of each cycle. The MOSFET is turned off when the
current ramps up to the current limit or when the DCMAX limit is
reached. Since the highest current limit level and frequency of
a TinySwitch-II design are constant, the power delivered to the
load is proportional to the primary inductance of the transformer
and peak primary current squared. Hence, designing the supply
involves calculating the primary inductance of the transformer
for the maximum output power required. If the TinySwitch-II is
appropriately chosen for the power level, the current in the
calculated inductance will ramp up to current limit before the
DCMAX limit is reached.
In the event of a line undervoltage condition, the switching of
the power MOSFET is disabled beyond its normal 850 ms time
until the line undervoltage condition ends.
Enable Function
TinySwitch-II senses the EN/UV pin to determine whether or
not to proceed with the next switching cycle as described
earlier. The sequence of cycles is used to determine the
current limit. Once a cycle is started, it always completes the
cycle (even when the EN/UV pin changes state half way
through the cycle). This operation results in a power supply in
which the output voltage ripple is determined by the output
capacitor, amount of energy per switch cycle and the delay of
the feedback.
Line Undervoltage Sense Circuit
The DC line voltage can be monitored by connecting an
external resistor from the DC line to the EN/UV pin. During
power-up or when the switching of the power MOSFET is
disabled in auto-restart, the current into the EN/UV pin must
exceed 49 μA to initiate switching of the power MOSFET.
During power-up, this is accomplished by holding the BYPASS
pin to 4.8 V while the line undervoltage condition exists. The
The EN/UV pin signal is generated on the secondary by
comparing the power supply output voltage with a reference
voltage. The EN/UV pin signal is high when the power supply
output voltage is less than the reference voltage.
V
300
DRAIN
200
In a typical implementation, the EN/UV pin is driven by an
optocoupler. The collector of the optocoupler transistor is
connected to the EN/UV pin and the emitter is connected to
the SOURCE pin. The optocoupler LED is connected in series
with a Zener diode across the DC output voltage to be
regulated. When the output voltage exceeds the target
regulation voltage level (optocoupler LED voltage drop plus
Zener voltage), the optocoupler LED will start to conduct,
pulling the EN/UV pin low. The Zener diode can be replaced
by a TL431 reference circuit for improved accuracy.
100
0
10
V
DC-OUTPUT
5
0
ON/OFF Operation with Current Limit State Machine
The internal clock of the TinySwitch-II runs all the time. At the
beginning of each clock cycle, it samples the EN/UV pin to
decide whether or not to implement a switch cycle, and based
on the sequence of samples over multiple cycles, it determines
1000
2000
0
Time (ms)
Figure 5. TinySwitch-II Auto-Restart Operation.
4
Rev. H 02/09
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