TNY284-290
external resistor. This facilitates powering of TinySwitch-4
externally through a bias winding to decrease the no-load
consumption to well below ꢀ0 mW.
is pulled low. If the ENABLE/UNDERVOLTAGE pin is not pulled
low for 64 ms, the power MOSFET switching is normally
disabled for 2.ꢀ seconds (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 6 illustrates auto-restart circuit operation in the
presence of an output short-circuit.
BYPASS/MULTI-FUNCTION Pin Undervoltage
The BYPASS/MULTI-FUNCTION pin undervoltage circuitry
disables the power MOSFET when the BYPASS/MULTI-
FUNCTION pin voltage drops below 4.9 V in steady state
operation. Once the BYPASS/MULTI-FUNCTION pin voltage
drops below 4.9 V in steady state operation, it must rise back to
ꢀ.8ꢀ V to enable (turn-on) the power MOSFET.
In the event of a line undervoltage condition, the switching of
the power MOSFET is disabled beyond its normal 2.ꢀ seconds
until the line undervoltage condition ends.
Over Temperature Protection
The thermal shutdown circuitry senses the die temperature.
The threshold is typically set at 142 °C with 7ꢀ °C hysteresis.
When the die temperature rises above this threshold the power
MOSFET is disabled and remains disabled until the die
temperature falls by 7ꢀ °C, at which point it is re-enabled. A
large hysteresis of 7ꢀ °C (typical) is provided to prevent over-
heating of the PC board due to a continuous fault condition.
Adaptive Switching Cycle On-Time Extension
Adaptive switching cycle on-time extension keeps the cycle on
until current limit is reached, instead of prematurely terminating
after the DCMAX signal goes low. This feature reduces the
minimum input voltage required to maintain regulation, extending
hold-up time and minimizing the size of bulk capacitor required.
The on-time extension is disabled during the start-up of the
power supply, until the power supply output reaches regulation.
Current Limit
The current limit circuit senses the current in the power
MOSFET. When this current exceeds the internal threshold
(ILIMIT), the power MOSFET is turned off for the remainder of that
cycle. The current limit state machine reduces the current limit
threshold by discrete amounts under medium and light loads.
Line Undervoltage Sense Circuit
The DC line voltage can be monitored by connecting an
external resistor from the DC line to the ENABLE/
UNDERVOLTAGE pin. During power-up or when the switching
of the power MOSFET is disabled in auto-restart, the current
into the ENABLE/UNDERVOLTAGE pin must exceed 2ꢀ μA to
initiate switching of the power MOSFET. During power-up, this
is accomplished by holding the BYPASS/MULTI-FUNCTION pin
to 4.9 V while the line undervoltage condition exists. The
BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to ꢀ.8ꢀ 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 2.ꢀ seconds 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.
Auto-Restart
In the event of a fault condition such as output overload, output
short-circuit, or an open loop condition, TinySwitch-4 enters
into auto-restart operation. An internal counter clocked by the
oscillator is reset every time the ENABLE/UNDERVOLTAGE pin
The line undervoltage circuit also detects when there is no
external resistor connected to the ENABLE/UNDERVOLTAGE
pin (less than ~2 μA into the pin). In this case the line
undervoltage function is disabled.
V
300
DRAIN
TinySwitch-4 Operation
TinySwitch-4 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-4 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-4 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.
200
100
0
10
V
DC-OUTPUT
5
0
2500
5000
0
Time (ms)
Figure 6. Auto-Restart Operation.
4
Rev. A 09/12
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