TNY253/254/255
V
EN
CLOCK
V
0 V
IN
DC
MAX
I
DRAIN
V
DRAIN
0 V
0
.2
.4
.6
.8
1
Time (ms)
V
DRAIN
Figure 7. TinySwitch Power-Up Timing Diagram.
PI-2261-061198
V
IN
Figure 6. TinySwitch Operation at Light Load.
130 kHz Switching Frequency (TNY255)
12 V
The switching frequency (with no cycle skipping) is set at
130kHz. This allows the TNY255 to deliver 10W while still
usingthesamesize, lowcosttransformer(EE16)asusedbythe
TNY253/254 for lower power applications.
0 V
V
DRAIN
BYPASS Pin Capacitor
The BYPASS pin uses a small 0.1 µF ceramic capacitor for
decoupling the internal power supply of the TinySwitch.
12 V
0 V
500
0
100
200
Time (ms)
Figure 8. TinySwitch Power Down Timing Diagram.
300
400
Application Examples
Television Standby
TinySwitch is an ideal solution for low cost, high efficiency
standby power supplies used in consumer electronic products
suchasTVs. Figure9showsa7.5 V,1.3 Wflybackcircuitthat
uses TNY253 for implementing a TV standby supply. The
circuit operates from the DC high voltage already available
fromthemainpowersupply. Thisinputvoltagecanrangefrom
120 to 375VDC depending on the input AC voltage range that
the TV is rated for. Capacitor C1 filters the high voltage DC
supply, and is necessary only if there is a long trace length from
the source of the DC supply to the inputs of the TV standby
circuit. The high voltage DC bus is applied to the series
combination of the primary winding of T1 and the integrated
high voltage MOSFET inside the TNY253. The low operating
frequency of the TNY253 (44kHz), allows a low cost snubber
circuitC2andR1tobeusedinplaceofaprimaryclampcircuit.
In addition to limiting the DRAIN turn off voltage spike to a
safevalue, theRCsnubberalsoreducesradiatedvideonoiseby
loweringthedv/dtoftheDRAINwaveform,whichiscriticalfor
video applications such as TV and VCR. On fixed frequency
PWM and RCC circuits, use of a snubber will result in an
undesirablefixedACswitchinglossthatisindependentofload.
TheON/OFFcontrolontheTinySwitcheliminatesthisproblem
by scaling the effective switching frequency and therefore,
switching loss linearly with load. Thus the efficiency of the
supply stays relatively constant down to a fraction of a watt of
output loading.
The secondary winding is rectified and filtered by D1 and C4 to
create the 7.5V output. L1 and C5 provide additional filtering.
The output voltage is determined by the sum of the optocoupler
U2 LED forward drop (~ 1 V) and Zener diode VR1 voltage.
The resistor R2, maintains a bias current through the Zener to
improve its voltage tolerance.
C
5
7/01
POWERINT [ Power Integrations ]
