LYT0002/0004-0006
Output Capacitor C5
Feedback Capacitor C4
The primary function of capacitor C5 is to smooth the inductor
current. Select a low or ultra-low ESR series if electrolytic types
are used to ensure capacitor heating is minimized. Ceramic or
solid polymer types are also suitable but are typically higher
cost per unit capacitance.
Capacitor C4 can be a low-cost general purpose capacitor. It
provides a “sample and hold” function, charging to the sensed
current value during the off-time of LYTSwitch-0. Its value
should be 10 mF to 22 mF; smaller values cause poorer regulation
and lower efficiency. This capacitor also bypasses the switching
current during the free-wheeling period, reducing the sense
resistor dissipation.
Select the voltage rating to be the nearest above the LED string
voltage. Select the initial capacitance value based on the ripple
current parameter calculated in the design spreadsheet. The
capacitance value may be further increased to reduce the LED
ripple current dependent on the specification requirements of
the driver. For long life use 105 °C or above rated parts unless
the ambient temperature inside the lamp is less than 80 °C and
select a series with an appropriate lifetime rating. Note that
operating electrolytic capacitors below their rated temperature
specification will significantly extend their lifetime e.g., 105 °C
capacitor operated at 80 °C will increase lifetime by a factor of
2 to 3.
LYTSwitch-0 Layout Considerations
In the buck or buck-boost converter configuration, since the
SOURCE pins in LYTSwitch-0 are switching nodes, the copper
area connected to SOURCE should be minimized to minimize
EMI within the thermal constraints of the design.
In the boost and non-isolated flyback configuration, since the
SOURCE pins are tied to DC return, the copper area connected
to SOURCE can be maximized to improve heat sinking.
The loop formed between the LYTSwitch-0, inductor (L2),
free-wheeling diode (D1), and output capacitor (C5) should be
kept as small as possible. The BYPASS pin capacitor C3
(Figure 7a) should be located physically close to the SOURCE (S)
and BYPASS (BP) pins. To minimize direct coupling from
Sense Resistor R2
Sense resistor should be a 1% tolerance and either pulse rated
or overdesigned to avoid resistance drift with time. If using a
standard metal film type, overdesign power rating by 2-4 times.
The value of the resistor is provided in the design spreadsheet.
U1
C4
R3
C3
D
FB
BP
S
D1
R1
L1
R2
S
S
RF1
S
AC
INPUT
VR1
RV1
C1
C2
~
~
+
–
DC
OUTPUT
L2
BR1
C5
+
Optimize hatched copper areas (
) for heat sinking.
PI-7033-052913
Figure 7a. Recommended Printed Circuit Layout for LYTSwitch-0 in a Buck Converter Configuration using P Package.
7
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Rev. A 06/13
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