LYT0002/0004-0006
The output current is regulated via the voltage across R2 during
the free-wheeling period when the internal MOSFET of U1 is off.
This voltage is filtered by capacitor (C4) and fed into the
FEEDBACK pin of U1. Regulation is maintained by skipping
switching cycles. As the output current rises, the voltage on
the FEEDBACK pin will rise. If this exceeds VFB then subsequent
cycles will be skipped until the voltage reduces below VFB.
Component Selection
Referring to Figure 6, the following considerations may be
helpful in selecting components for a LYTSwitch-0 design.
Optional Varistor (RV1)
The Metal Oxide Varistor (RV1) is used to suppress the line
surge in order to meet IEC61000-4-5 (differential input line
1.2/50 ms and differential ring wave input line surge). A MOV is
recommended for high PF designs with surge levels of 1 kV or
greater. High PF design requires lower input capacitance values
giving a greater voltage rise across limited input capacitance
during surge events. A MOV is typically not required if the
design will use high-input capacitance (mF’s vs. nF’s) (non-PF
application).
Open-loop protection is provided via the auto-restart function.
If no cycles are skipped during a 50 ms period LYTSwitch-0 will
enter auto-restart (LYT0004-0006), limiting the average output
power to approximately 6% of the maximum overload power.
The auto-restart function requires the value of C3 to be 100 nF
or greater such that the IC remains operational from half-line
cycle to half-line cycle.
Input Capacitance C1 and C2
Use a film capacitor if the input capacitance is less than 1 mF.
Make sure that the RMS current rating is not exceeded especially
if planning to use electrolytic capacitor. For universal or high-
line only input design use 400 V or 630 V rated capacitors, and
for low-line only use 250 V rated capacitors for lower cost and
smaller size.
For disconnected LED protection an optional Zener (not shown)
can be placed across the output. This will fuse short-circuit
and prevent the output voltage rising.
Key Application Considerations
LYTSwitch-0 Design Considerations
Free-wheeling Diode D1
Output Current Table
Diode D1 should be an ultrafast type. For MDCM, reverse
recovery time of ≤75 ns should be used in designs where the
diode temperature is 70 °C or below. Slower diodes are not
acceptable, as continuous mode operation will always occur
during start-up, causing high leading edge current spikes,
terminating the switching cycle prematurely, and preventing the
output from reaching regulation. If the diode temperature is
above 70 °C then a diode with a reverse recovery time of ≤35 ns
should be used.
Data sheet maximum output current table (Table 1) represents
the maximum practical continuous output current for both
mostly discontinuous conduction mode (MDCM) and continuous
conduction mode (CCM) of operation that can be delivered
from a given LYTSwitch-0 device under the following assumed
conditions:
1. Buck converter topology.
2. The minimum DC input voltage is equal to voltage output.
3. For CCM operation a KRP* of 0.4.
4. Output voltage of 54 VDC.
5. Efficiency of 90%.
6. A catch/free-wheeling diode with tRR ≈35 ns is used.
7. The part is board mounted with SOURCE pins soldered to a
sufficient area of copper to keep the SOURCE pin tempera-
ture at or below 100 °C.
For CCM an ultrafast diode with reverse recovery time ≤35 ns
should be used. Slower diodes cause excessive leading edge
current spikes, terminating the switching cycle prematurely and
preventing full power delivery.
Standard plastic or fast (tRR >75 ns) diodes should never be
used as the large reverse recovery currents can cause
excessive power dissipation in the diode and/or exceed the
maximum drain current specification of LYTSwitch-0.
*KRP is the ratio of ripple to peak inductor current.
LYTSwitch-0 Selection and Selection Between
MDCM and CCM Operation
Inductor L1
Select the LYTSwitch-0 device, free-wheeling diode and output
inductor that gives the lowest overall cost. In general, MDCM
provides the lowest cost and highest efficiency converter. CCM
designs require a larger inductor and ultrafast (tRR ≈35 ns)
free-wheeling diode in all cases. It is lower cost to use a larger
LYTSwitch-0 in MDCM than a smaller LinkSwitch-0 in CCM
because of the additional external component costs of a CCM
design. However, if the highest output current is required, CCM
should be employed following the guidelines below.
Choose any standard off-the-shelf inductor that meets the
design requirements. A “drum” or “dog bone” “I” core inductor
is recommended with a single ferrite element due to its low-cost
and very low audible noise properties. The typical inductance
value and RMS current rating can be obtained from the
LYTSwitch-0 PIXls design spreadsheet. The PIXls application is
part of the PI Expert design suite available for free download
from Power Integrations. Choose L1 greater than or equal to
the typical calculated inductance.
Topology Options
Note that the open magnetic path of non-shielded discrete
inductors may cause inductance value changes when placed
within metal enclosure requiring a larger value to be used.
LYTSwitch-0 can be used in all common topologies, with or
without an optocoupler and reference to improve output voltage
tolerance and regulation. Table 2 provide a summary of these
configurations.
6
Rev. A 06/13
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