LNK302/304-306
Pre-load Resistor R4
Quick Design Checklist
In high-side, direct feedback designs where the minimum load
is <3 mA, a pre-load resistor is required to maintain output
regulation. This ensures sufficient inductor energy to pull the
inductor side of the feedback capacitor C3 to input return via
D2. The value of R4 should be selected to give a minimum
output load of 3 mA.
As with any power supply design, all LinkSwitch-TN designs
should be verified for proper functionality on the bench. The
following minimum tests are recommended:
1) Adequate DC rail voltage – check that the minimum DC
inputvoltagedoesnotfallbelow70VDCatmaximumload,
minimum input voltage.
2) Correct Diode Selection – UF400x series diodes are
recommended only for designs that operate in MDCM at
an ambient of 70 °C or below. For designs operating in
continuousconductionmode(CCM)and/orhigherambients,
then a diode with a reverse recovery time of 35 ns or better,
such as the BYV26C, is recommended.
3) Maximum drain current – verify that the peak drain current
is below the data sheet peak drain specification under
worst-case conditions of highest line voltage, maximum
overload (just prior to auto-restart) and highest ambient
temperature.
4) Thermal check – at maximum output power, minimum
input voltage and maximum ambient temperature, verify
that the LinkSwitch-TN SOURCE pin temperature is
100 °C or below. This figure ensures adequate margin due
to variations in RDS(ON) from part to part. Abattery powered
thermocouplemeterisrecommendedtomakemeasurements
whentheSOURCEpinsareaswitchingnode.Alternatively,
the ambient temperature may be raised to indicate margin
to thermal shutdown.
In designs with an optocoupler the Zener or reference bias
current provides a 1 mA to 2 mA minimum load, preventing
“pulse bunching” and increased output ripple at zero load.
LinkSwitch-TN Layout Considerations
In the buck or buck-boost converter configuration, since the
SOURCEpinsinLinkSwitch-TNareswitchingnodes,thecopper
area connected to SOURCE should be minimized to minimize
EMI within the thermal constraints of the design.
In the boost configuration, since the SOURCE pins are tied
to DC return, the copper area connected to SOURCE can be
maximized to improve heatsinking.
The loop formed between the LinkSwitch-TN, inductor (L1),
freewheeling diode (D1), and output capacitor (C2) should
be kept as small as possible. The BYPASS pin capacitor
C1 (Figure 6) should be located physically close to the
SOURCE (S) and BYPASS (BP) pins. To minimize direct
coupling from switching nodes, the LinkSwitch-TN should be
placed away from AC input lines. It may be advantageous to
place capacitors C4 and C5 in-between LinkSwitch-TN and the
AC input. The second rectifier diode D4 is optional, but may
be included for better EMI performance and higher line surge
withstand capability.
InaLinkSwitch-TNdesignusingabuckorbuckboostconverter
topology, the SOURCE pin is a switching node. Oscilloscope
measurements should therefore be made with probe grounded
to a DC voltage, such as primary return or DC input rail, and
not to the SOURCE pins. The power supply input must always
be supplied from an isolated source (e.g. via an isolation
transformer).
28-8
Rev. I 11/08
POWERINT [ Power Integrations ]
