TNY284-290
capacitor to be effective as a high frequency filter, the capacitor
should be located as close as possible to the SOURCE and
BYPASS/MULTI-FUNCTIONAL pins of the device.
practically eliminates audible noise. Vacuum impregnation of
the transformer should not be used due to the high primary
capacitance and increased losses that result. Higher flux
densities are possible, however careful evaluation of the audible
noise performance should be made using production
transformer samples before approving the design.
Peak Output Power Table
230 VAC 1ꢀ5
8ꢀ-26ꢀ VAC
ILIMIT ILIMIT+1
Product
Ceramic capacitors that use dielectrics such as ZꢀU, when
used in clamp circuits, may also generate audio noise. If this is
the case, try replacing them with a capacitor having a different
dielectric or construction, for example a film type.
ILIMIT-1 ILIMIT ILIMIT+1 ILIMIT-1
TNY284P
9.1 W 10.9 W 9.1 W 7.1 W 8.ꢀ W 7.1 W
10.8 W 12 W 1ꢀ.1 W 8.4 W 9.3 W 11.8 W
11.8 W 1ꢀ.3 W 19.4 W 9.2 W 11.9 W 1ꢀ.1 W
1ꢀ.1 W 19.6 W 23.7 W 11.8 W 1ꢀ.3 W 18.ꢀ W
TNY285P
TNY286P
TNY287P
TNY288P
TNY289P
TNY290P
TinySwitch-4 Layout Considerations
Layout
19.4 W 24 W
28 W 1ꢀ.1 W 18.6 W 21.8 W
See Figure 17 for a recommended circuit board layout for
TinySwitch-4.
23.7 W 28.4 W 32.2 W 18.ꢀ W 22 W 2ꢀ.2 W
28 W 32.7 W 36.6 W 21.8 W 2ꢀ.4 W 28.ꢀ W
Single Point Grounding
Use a single point ground connection from the input filter
capacitor to the area of copper connected to the SOURCE pins.
Table 3.
Minimum Practical Power at Three Selectable Current Limit Levels.
For best performance of the OVP function, it is recommended
that a relatively high bias winding voltage is used, in the range of
1ꢀ V - 30 V. This minimizes the error voltage on the bias
winding due to leakage inductance and also ensures adequate
voltage during no-load operation from which to supply the
BYPASS/MULTI-FUNCTIONAL pin for reduced no-load
consumption.
Bypass Capacitor (CBP)
The BYPASS/MULTI-FUNCTIONAL pin capacitor must be
located directly adjacent to the BYPASS/MULTI-FUNCTIONAL
and SOURCE pins.
If a 0.1 μF bypass capacitor has been selected it should be a
high frequency ceramic type (e.g. with X7R dielectric). It must
be placed directly between the ENABLE and SOURCE pins to
filter external noise entering the BYPASS pin. If a 1 μF or 10 μF
bypass capacitor was selected then an additional 0.1 μF
capacitor should be added across BYPASS and SOURCE pins
to provide noise filtering (see Figure 17).
Selecting the Zener diode voltage to be approximately 6 V
above the bias winding voltage (28 V for 22 V bias winding)
gives good OVP performance for most designs, but can be
adjusted to compensate for variations in leakage inductance.
Adding additional filtering can be achieved by inserting a low
value (10 W to 47 W) resistor in series with the bias winding
diode and/or the OVP Zener as shown by R7 and R3 in Figure 16.
The resistor in series with the OVP Zener also limits the
maximum current into the BYPASS/MULTI-FUNCTIONAL pin.
ENABLE/UNDERVOLTAGE Pin
Keep traces connected to the ENABLE/UNDERVOLTAGE pin
short and, as far as is practical, away from all other traces and
nodes above source potential including, but not limited to, the
bypass, drain and bias supply diode anode nodes.
Reducing No-load Consumption
As TinySwitch-4 is self-powered from the BYPASS/MULTI-
FUNCTIONAL pin capacitor, there is no need for an auxiliary or
bias winding to be provided on the transformer for this purpose.
Typical no-load consumption when self-powered is <1ꢀ0 mW at
26ꢀ VAC input. The addition of a bias winding can reduce this
down to <ꢀ0 mW by supplying the TinySwitch-4 from the lower
bias voltage and inhibiting the internal high-voltage current
source. To achieve this, select the value of the resistor (R8 in
Figure 16) to provide the data sheet DRAIN supply current. In
practice, due to the reduction of the bias voltage at low load,
start with a value equal to 405 greater than the data sheet
maximum current, and then increase the value of the resistor to
give the lowest no-load consumption.
Primary Loop Area
The area of the primary loop that connects the input filter
capacitor, transformer primary and TinySwitch-4 should be kept
as small as possible.
Primary Clamp Circuit
A clamp is used to limit peak voltage on the DRAIN pin at
turn-off. This can be achieved by using an RCD clamp or a
Zener (~200 V) and diode clamp across the primary winding.
To reduce EMI, minimize the loop from the clamp components
to the transformer and TinySwitch-4.
Thermal Considerations
The SOURCE pins are internally connected to the IC lead frame
and provide the main path to remove heat from the device.
Therefore all the SOURCE pins should be connected to a
copper area underneath the TinySwitch-4 to act not only as a
single point ground, but also as a heat sink. As this area is
connected to the quiet source node, this area should be
maximized for good heat sinking. Similarly for axial output
diodes, maximize the PCB area connected to the cathode.
Audible Noise
The cycle skipping mode of operation used in TinySwitch-4 can
generate audio frequency components in the transformer. To
limit this audible noise generation the transformer should be
designed such that the peak core flux density is below
3000 Gauss (300 mT). Following this guideline and using the
standard transformer production technique of dip varnishing
10
Rev. A 09/12
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