CAT4137 Datasheet, PDF (9/16 Pages) Catalyst Semiconductor – CMOS Boost Converter - White LED Driver

CAT4137
APPLICATION INFORMATION
EXTERNAL COMPONENT SELECTION
Capacitors
The CAT4137 only requires small ceramic
capacitors of 1uF on the input and 0.22µF on the
output. The output capacitor should be rated at 30V
or greater. Under normal conditions, a 1µF input
capacitor is sufficient. For applications with higher
output power, a larger input capacitor of 2.2µF or
4.7µF may be appropriate. X5R and X7R capacitor
types are ideal due to their stability across
temperature range.
Inductor
A 22µH inductor is recommended for most of the
CAT4137 applications. In cases where the
efficiency is critical, inductances with lower series
resistance are preferred. Several inductor types
from various vendors can be used. Figure 1 shows
how different inductor types affect the efficiency
across the load range.
100
3 LEDs
VIN = 3.6V
90
80
SUMIDA CDRH3D16-220
MURATA LQH32CN220
70
PANASONIC ELJ-EA220
PANASONIC ELJ-PC220
60
5
10 15 20 25 30
LED CURRENT [mA]
Figure 1. Efficiency for various inductors
Schottky Diode
The current rating of the Schottky diode must
exceed the peak current flowing through it. The
Schottky diode performance is rated in terms of its
forward voltage at a given current. In order to
achieve the best efficiency, this forward voltage
should be as low as possible. The response time is
also critical since the driver is operating at 1MHz.
Central Semiconductor Schottky CMDSH2-3
(200mA rated) or the CMDSH-3 (100mA rated) is
recommended for most applications.
LED Current Setting
The LED current is set by the external resistor
between the feedback pin (FB) and ground. The
formula below gives the relationship between the
resistor and the current:
R1 = 0.3V/LED current
LED current (mA)
5
10
15
20
25
30
R1 (Ω)
60
30
20
15
12
10
Table 1. Resistor R1 and LED current
© Catalyst Semiconductor, Inc.
9
Characteristics subject to change without notice
Doc. No. MD-5012Rev. B