TNY274-280
Power Up/Down
Functional Description). This has two main benefits. First, for a
nominal application, this eliminates the cost of a bias winding
and associated components. Secondly, for battery charger
applications, the current-voltage characteristic often allows the
output voltage to fall close to zero volts while still delivering
power. TinySwitch-III accomplishes this without a forward bias
winding and its many associated components. For applications
that require very low no-load power consumption (10 mW), a
resistor from a bias winding to the BYPASS/MULTI-FUNCTION
pin can provide the power to the chip. The minimum
recommended current supplied is ꢀ mA. The BYPASS/MULTI-
FUNCTION pin in this case will be clamped at 6.4 V. This
method will eliminate the power draw from the DRAIN pin,
thereby reducing the no-load power consumption and
improving full-load efficiency.
The TinySwitch-III requires only a 0.ꢀ μF capacitor on the
BYPASS/MULTI-FUNCTION pin to operate with standard
current limit. Because of its small size, the time to charge this
capacitor is kept to an absolute minimum, typically 0.6 ms. The
time to charge will vary in proportion to the BYPASS/MULTI-
FUNCTION pin capacitor value when selecting different current
limits. Due to the high bandwidth of the ON/OFF feedback,
there is no overshoot at the power supply output. When an
external resistor (4 MΩ) is connected from the positive DC input
to the EN/UV pin, the power MOSFET switching will be delayed
during power up until the DC line voltage exceeds the threshold
(ꢀ00 V). Figures ꢀ0 and ꢀꢀ show the power up timing waveform
in applications with and without an external resistor (4 MΩ)
connected to the EN/UV pin.
Current Limit Operation
Under startup and overload conditions, when the conduction
time is less than 400 ns, the device reduces the switching
frequency to maintain control of the peak drain current.
Each switching cycle is terminated when the DRAIN current
reaches the current limit of the device. Current limit operation
provides good line ripple rejection and relatively constant power
delivery independent of input voltage.
During power down, when an external resistor is used, the
power MOSFET will switch for 64 ms after the output loses
regulation. The power MOSFET will then remain off without any
glitches since the undervoltage function prohibits restart when
the line voltage is low.
BYPASS/MULTI-FUNCTION Pin Capacitor
The BYPASS/MULTI-FUNCTION pin can use a ceramic
capacitor as small as 0.ꢀ μF for decoupling the internal power
supply of the device. A larger capacitor size can be used to
adjust the current limit. For TNY271-280, a ꢀ μF BP/M pin
capacitor will select a lower current limit equal to the standard
current limit of the next smaller device and a ꢀ0 μF BP/M pin
capacitor will select a higher current limit equal to the standard
current limit of the next larger device. The higher current limit
level of the TNY280 is set to 810 mA typical. The TNY274
MOSFET does not have the capability for increased current limit
so this feature is not available in this device.
Figure ꢀ2 illustrates a typical power down timing waveform.
Figure ꢀ3 illustrates a very slow power down timing waveform
as in standby applications. The external resistor (4 MΩ) is
connected to the EN/UV pin in this case to prevent unwanted
restarts.
No bias winding is needed to provide power to the chip
because it draws the power directly from the DRAIN pin (see
7
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Rev. I 01/09
POWERINT [ Power Integrations ]
