TNY274-280
Power Up/Down
Functional Description above). 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
(50 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 1 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 0.1 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-
FUNCTIONpincapacitorvaluewhenselectingdifferentcurrent
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 (100 V). Figures 10 and 11 show the power-up
timing waveform in applications with and without an external
resistor (4 MΩ) connected to the EN/UV pin.
Understartupandoverloadconditions,whentheconductiontime
is less than 400 ns, the device reduces the switching frequency
to maintain control of the peak drain current.
Current Limit Operation
Each switching cycle is terminated when the DRAIN current
reaches the current limit of the device. Current limit operation
providesgoodlineripplerejectionandrelativelyconstantpower
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 under-voltage 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 0.1 µF for decoupling the internal power
supplyofthedevice.Alargercapacitorsizecanbeusedtoadjust
the current limit. For TNY275-280, a 1 µF BP/M pin capacitor
will select a lower current limit equal to the standard current
limit of the next smaller device and a 10 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 850 mA typical. The TNY274 MOSFET
does not have the capability for increased current limit so this
feature is not available in this device.
Figure 12 illustrates a typical power-down timing waveform.
Figure 13 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
E
2/06
7
POWERINT [ Power Integrations ]
