TNY256 データシートの表示（PDF） - Power Integrations, Inc

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TNY256

Energy Efficient/ Low Power Off-line Switcher

Power Integrations, Inc 

TNY256

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 µA 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.

300

DRAIN

VOLTAGE

200

100

0

20

10

OUTPUT

VOLTAGE

0

0

250

500

Time (ms)

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 DCMAX by the oscillator. Since the

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

VEN

CLOCK

DMAX

IDRAIN

VEN

CLOCK

DMAX

IDRAIN

VDRAIN

Figure 6. TNY256 Operation at Heavy Load.

4

B

8/99

VDRAIN

PI-2373-120998

Figure 7. TNY256 Operation at Light Load.

PI-2377-120998

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