TNY253/254/255
V
EN
CLOCK
DC
V
EN
CLOCK
MAX
DC
MAX
I
DRAIN
IDRAIN
VDRAIN
VDRAIN
PI-2255-061298
PI-2259-061298
Figure 4. TinySwitch Operation at Heavy Load.
Figure 5. TinySwitch Operation at Medium Load.
The response time of TinySwitch ON/OFF control scheme is
very fast compared to normal PWM control. This provides high
line ripple rejection and excellent transient response.
Power Up/Down
TinySwitch requires only a 0.1 µF capacitor on the BYPASS
pin. Because of the small size of this capacitor, the power-up
delay is kept to an absolute minimum, typically 0.3 ms (Fig-
ure 7). Due to the fast nature of the ON/OFF feedback, there is
no overshoot at the power supply output. During power-down,
the power MOSFET will switch until the rectified line voltage
drops to approximately 12 V. The power MOSFET will then
remain off without any glitches (Figure 8).
Bias Winding Eliminated
TinySwitch does not require a bias winding to provide power
to the chip. Instead it draws the power directly from the
DRAIN pin (see Functional Description above). This has two
main benefits. First for a nominal application, this eliminates
the cost of an extra bias winding and associated components.
Secondly, for charger applications, the current-voltage char-
acteristic often allows the output voltage to fall to low values
while still delivering power. This type of application normally
requires a forward-bias winding which has many more associ-
ated components, none of which are necessary with TinySwitch.
Current Limit Operation
Each switching cycle is terminated when the DRAIN current
reaches the current limit of the TinySwitch. For a given primary
inductance and input voltage, the duty cycle is constant. How-
ever, duty cycle does change inversely with the input voltage
providing “voltage feed-forward” advantages: good line ripple
rejection and relatively constant power delivery independent
of the input voltage.
44 kHz Switching Frequency (TNY253/254)
Switching frequency (with no cycle skipping) is set at 44 kHz.
This provides several advantages. At higher switching frequen-
cies, the capacitive switching losses are a significant proportion
of the power losses in a power supply. At higher frequencies,
the preferred snubbing schemes are RCD or diode-Zener clamps.
However, due to the lower switching frequency of TinySwitch ,
it is possible to use a simple RC snubber (and even just a capaci-
tor alone in 115 VAC applications at powers levels below 4 W).
Secondly, a low switching frequency also reduces EMI filtering
requirements. At 44 kHz, the first, second and third harmon-
ics are all below 150 kHz where the EMI limits are not very
restrictive. For power levels below 4 W it is possible to meet
worldwide EMI requirements with only resistive and capaci-
tive filter elements (no inductors or chokes). This significantly
reduces EMI filter costs.
Finally, if the application requires stringent noise emissions
(such as video applications), then the TNY253/254 will allow
more effective use of diode snubbing (and other secondary
snubbing techniques). The lower switching frequency allows
RC snubbers to be used to reduce noise, without significantly
impacting the efficiency of the supply.
130 kHz Switching Frequency (TNY255)
The switching frequency (with no cycle skipping) is set at
130 kHz. This allows the TNY255 to deliver 10 W while still
using the same size, low cost transformer (EE16) as used by
the TNY253/254 for lower power applications.
4
Rev E
02/12
POWERINT [ POWER INTEGRATIONS, INC. ]
