Rev. F 08/16

4

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TNY253/254/255

TNY253/254/255

Figure 4. TinySwitch Operation at Heavy Load.

Figure 5. TinySwitch Operation at Medium Load.

V

DRAIN

V

EN

CLOCK

DC

DRAIN

I

MAX

PI-2255-061298

V

DRAIN

V

EN

CLOCK

DC

DRAIN

I

MAX

PI-2259-061298

The response time of TinySwitch ON/OFF control scheme is

veryfastcomparedtonormalPWMcontrol. Thisprovideshigh

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-

atedcomponents,noneofwhicharenecessarywithTinySwitch.

Current Limit Operation

Each switching cycle is terminated when the DRAIN current

reachesthecurrentlimitof theTinySwitch. For agivenprimary

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,

thepreferredsnubbingschemesareRCDordiode-Zenerclamps.

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.