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White LED Step-Up Converter
PT6937
APPLICATION INFORMATION
INDUCTOR SELECTION
A 22µH inductor is recommended for most PT6937 applications. Although small size and high
efficiency are major concerns, the inductor should have low core losses at 1MHz and low DCR (copper
wire resistance).
CAPACITOR SELECTION
The small size of ceramic capacitors makes them ideal for PT6937 applications. X5R and X7R types
are recommended because they retain their capacitance over wider voltage and temperature ranges
than other types such as Y5V or Z5U. A 1µF input capacitor and a 0.22µF output capacitor are
sufficient for most PT6937 applications.
DIODE SELECTION
Schottky diodes, with their low forward voltage drop and fast reverse recovery, are the ideal choices for
the PT6937 applications. The forward voltage drop of a Schottky diode represents the conduction
losses in the diode, while the diode capacitance (CT or CD) represents the switching losses. For diode
selection, both forward voltage drop and diode capacitance need to be considered. Schottky diodes
with higher current ratings usually have lower forward voltage drop and larger diode capacitance,
which can cause significant switching losses at the 1.2MHz switching frequency of the PT6937. A
Schottky diode rated at 100mA to 200mA is sufficient for most PT6937 applications.
LED CURRENT CONTROL
The LED current is controlled by the feedback resistor (R1 in Figure 1). The feedback reference is
95mV. The LED current is 95mV/R1. In order to have accurate LED current, precision resistors are
preferred (1% is recommended). The formula and table for R1 selection are shown below.
R1=95mV / I
LED
(1)
ILED (mA)
5
10
12
15
20
R1 (Ω)
19.1
9.53
7.87
6.34
4.75
PT6937 V1.1
-4-
March, 2006
PTC [ PRINCETON TECHNOLOGY CORP ]
