TNY284-290
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 MW) is connected from the positive DC input
to the ENABLE/UNDERVOLTAGE pin, the power MOSFET
switching will be delayed during power-up until the DC line
voltage exceeds the threshold (100 V). Figures 11 and 12 show
the power-up timing waveform in applications with and without
an external resistor (4 MW) connected to the ENABLE/
UNDERVOLTAGE pin. Under start-up 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.
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.
Figure 13 illustrates a typical power-down timing waveform.
Figure 14 illustrates a very slow power-down timing waveform
as in standby applications. The external resistor (4 MW) is
connected to the ENABLE/UNDERVOLTAGE 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
Functional Description). This has two main benefits. First, for a
nominal application, this eliminates the cost of a bias winding
40
and associated components. Secondly, for battery charger
applications, the current-voltage characteristic often allows the
output voltage to fall close to 0 V while still delivering power.
TinySwitch-4 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.
Current Limit Operation
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.
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
supply of the device. A larger capacitor size can be used to
adjust the current limit. For TNY285-290, a 1
μF
BYPASS/
MULTI-FUNCTIONAL pin capacitor will select a lower current
limit equal to the standard current limit of the next smaller
device and a 10
μF
BYPASS/MULTI-FUNCTIONAL 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 TNY290 is set to 850 mA typical. The TNY284 MOSFET
does not have the capability for increased current limit so this
feature is not available in this device.
Maximum Over Power (W)
TNY290
TNY280
35
30
25
20
85
100 115 130 145 160 175 190 205 220 235 250 265
Input Voltage (VAC)
Figure 15. Comparison of Maximum Overpower for TinySwitch-4 and
TinySwitch-III as a Function of Input Voltage (Data Collected from
RDK-295 20 W Reference Design).
PI-6788-052312
7
www.powerint.com
Rev. A 09/12
POWERINT [ POWER INTEGRATIONS, INC. ]
