TNY263-268
the appropriate current limit. At high loads, when the EN/UV
pin is high (less than 240
μA
out of the pin), a switching cycle
with the full current limit occurs. At lighter loads, when EN/UV
is high, a switching cycle with a reduced current limit occurs.
At near maximum load, TinySwitch-II will conduct during
nearly all of its clock cycles (Figure 6). At slightly lower load, it
will “skip” additional cycles in order to maintain voltage
regulation at the power supply output (Figure 7). At medium
loads, cycles will be skipped and the current limit will be
reduced (Figure 8). At very light loads, the current limit will be
reduced even further (Figure 9). Only a small percentage of
cycles will occur to satisfy the power consumption of the
power supply.
The response time of the TinySwitch-II ON/OFF control
scheme is very fast compared to normal PWM control. This
provides tight regulation and excellent transient response.
Power Up/Down
The TinySwitch-II requires only a 0.1
μF
capacitor on the
BYPASS pin. Because of its small size, the time to charge this
capacitor is kept to an absolute minimum, typically 0.6 ms.
Due to the fast nature of the ON/OFF feedback, there is no
overshoot at the power supply output. When an external
resistor (2 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 of TinySwitch-II in applications with and without an
external resistor (2 MΩ) connected to the EN/UV pin.
During power-down, when an external resistor is used, the
power MOSFET will switch for 50 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 12 illustrates a typical power-down timing waveform of
TinySwitch-II. Figure 13 illustrates a very slow power-down
timing waveform of TinySwitch-II as in standby applications.
The external resistor (2 MΩ) is connected to the EN/UV pin in
this case to prevent unwanted restarts.
PI-2749-050301
V
EN
CLOCK
D
MAX
I DRAIN
V DRAIN
Figure 6.
TinySwitch-II Operation at Near Maximum Loading.
The TinySwitch-II does not require a bias winding to provide
power to the chip, because it draws the power directly from
the DRAIN pin (see Functional Description above). This has
V
EN
CLOCK
V
EN
CLOCK
D
D
MAX
MAX
I DRAIN
I DRAIN
V DRAIN
V DRAIN
PI-2667-090700
PI-2377-091100
Figure 7.
TinySwitch-II Operation at Moderately Heavy Loading.
Figure 8.
TinySwitch-II Operation at Medium Loading.
5
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Rev. H 02/09
POWERINT [ POWER INTEGRATIONS, INC. ]
