TNY375-380
BYPASS/MULTI-FUNCTION Pin Undervoltage
auto-restart alternately enables and disables the switching of
the power MOSFET until the fault condition is removed.
Figure ꢀ illustrates auto-restart circuit operation in the presence
of an output short circuit.
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 1 second
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
overheating 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, typically
extending hold-up time and minimizing the size of bulk
capacitor required. The on-time extension is disabled during
the startup of the power supply, and after auto-restart, 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 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 2ꢀ mA 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. After the line undervoltage condition goes
away and the BYPASS/MULTI-FUNCTION pin has stabilized at
ꢀ.8ꢀ V, switching is initiated. Once MOSFET switching is
enabled, the DC line voltage is ignored unless the power supply
enters auto-restart mode in the event of a fault condition. 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 1 second 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 typical 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-PK enters
into auto-restart operation. An internal counter clocked by the
oscillator is reset every time the EN/UV pin is pulled low. If the
EN/UV pin is not pulled low for 8192 switching cycles
(or 32 ms), the power MOSFET switching is normally disabled
for 1 second (except in the case of line undervoltage condition,
in which case it is disabled until the condition is removed). The
The line undervoltage circuit also detects when there is no
external resistor connected to the EN/UV pin (less than ~1 mA
into the pin). In this case the line undervoltage function is
disabled.
TinySwitch-PK Operation
V
300
DRAIN
TinySwitch-PK 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 (applicable when On-Time Extension is disabled).
Since the highest current limit level and frequency of a
TinySwitch-PK 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-PK 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
1000
2000
0
Time (ms)
Figure 5. Auto-Restart Operation.
4
Rev. C 09/12
www.powerint.com
POWERINT [ Power Integrations ]
