TYN256P Power Integrations Inc., TYN256P Datasheet - Page 4

• Manufacturer Part Number: TYN256P
• Description: Energy Efficient/ Low Power Off-line Switcher
• Manufacturer: Power Integrations Inc.

short, or an open loop condition, TNY256 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 32 ms, the power MOSFET
switching is disabled for 128 ms (except in the case of line
under-voltage condition). 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 a temporary output short.
In the event of line under-voltage condition, the switching of the
power MOSFET is disabled beyond its normal 128 ms time
until the line under-voltage condition goes away.
Line Under-Voltage (UVLO) 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 50 A to initiate switching of the power MOSFET.
During power-up, this is implemented by holding the BYPASS
pin to 5.1 V while the line under-voltage condition exists. The
BYPASS pin then rises from 5.1 V to 5.8V when the line under-
voltage condition goes away. When the switching of the power
MOSFET is disabled in auto-restart mode and the line under-
voltage condition exists, the counter is stopped. This stretches
the disable time beyond its normal 128ms until the line under-
voltage condition goes away.
The line under-voltage circuit also detects when there is no
external resistor connected to the EN/UV pin. In this case the
line under-voltage function is disabled.
Figure 6. TNY256 Operation at Heavy Load.
V
I
CLOCK
DRAIN
4
DRAIN
D
MAX
V
EN
TNY256
B
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Figure 5. TNY256 Auto-Restart Operation.
TinySwitch Operation
TinySwitch 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. The maximum on-time of
the MOSFET is limited to DC
current limit and frequency of the TNY256 is constant, the
power delivered is proportional to the primary inductance of the
transformer and is relatively independent of the input voltage.
Therefore, the design of the power supply involves calculating
the primary inductance of the transformer for the maximum
power required. If the TNY256 is appropriately chosen for the
300
Figure 7. TNY256 Operation at Light Load.
200
100
20
10
0
0
V
0
I
CLOCK
D
DRAIN
DRAIN
MAX
V
EN
VOLTAGE
DRAIN
Time (ms)
MAX
250
VOLTAGE
by the oscillator. Since the
OUTPUT
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500