TNY274-280
In addition, there is a 6.4 V shunt regulator clamping the
BYPASS/MULTI-FUNCTION pin at 6.4 V when current
is provided to the BYPASS/MULTI-FUNCTION pin
through an external resistor. This facilitates powering of
TinySwitch-III
externally through a bias winding to decrease
the no-load consumption to well below 50 mW.
BYPASS/MULTI-FUNCTION Pin Under-Voltage
The BYPASS/MULTI-FUNCTION pin under-voltage 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 5.85 V to enable (turn-on) the power MOSFET.
Over Temperature Protection
The thermal shutdown circuitry senses the die temperature. The
threshold is typically set at 142 °C with 75 °C hysteresis. When
the die temperature rises above this threshold the power MOSFET
is disabled and remains disabled until the die temperature falls
by 75 °C, at which point it is re-enabled. A large hysteresis of
75 °C (typical) is provided to prevent overheating of the PC
board due to a continuous fault condition.
Current Limit
The current limit circuit senses the current in the power MOSFET.
When this current exceeds the internal threshold (I
LIMIT
), 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.
The leading edge blanking circuit inhibits the current limit
comparator for a short time (t
LEB
) 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.
300
200
100
0
10
5
0
V
DC-OUTPUT
V
PI-4098-082305
Auto-Restart
In the event of a fault condition such as output overload, output
short circuit, or an open loop condition,
TinySwitch-III
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 64 ms, the power MOSFET
switching is normally disabled for 2.5 seconds (except in the
case of line under-voltage 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.
In the event of a line under-voltage condition, the switching of
the power MOSFET is disabled beyond its normal 2.5 seconds
until the line under-voltage condition ends.
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 DC
MAX
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 startup of the power supply, until the power supply
output reaches regulation.
Line Under-Voltage 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 25
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 under-voltage condition exists. The
BYPASS/MULTI-FUNCTION pin then rises from 4.9 V to
5.85 V when the line under-voltage condition goes away. When the
switching of the power MOSFET is disabled in auto-restart mode
and a line under-voltage condition exists, the auto-restart counter
is stopped. This stretches the disable time beyond its normal
2.5 seconds until the line under-voltage condition ends.
The line under-voltage circuit also detects when there is
no external resistor connected to the EN/UV pin (less than
~1
µA
into the pin). In this case the line under-voltage function
is disabled.
TinySwitch-III
Operation
TinySwitch-III
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 DC
MAX
limit is
reached. Since the highest current limit level and frequency of
a
TinySwitch-III
design are constant, the power delivered to the
DRAIN
0
2500
5000
Time (ms)
Figure 5. Auto-Restart Operation.
4
E
2/06
POWERINT [ POWER INTEGRATIONS, INC. ]
