CAT4137_11 Datasheet, PDF (10/15 Pages) ON Semiconductor – CMOS Boost Converter - White LED Driver

CAT4137
Application Information
External Component Selection
Capacitors
The CAT4137 only requires small ceramic capacitors of
1 mF on the input and 0.22 mF on the output. The output
capacitor should be rated at 30 V or greater. Under normal
conditions, a 1 mF input capacitor is sufficient. For
applications with higher output power, a larger input
capacitor of 2.2 mF or 4.7 mF may be appropriate. X5R and
X7R capacitor types are ideal due to their stability across
temperature range.
Inductor
A 22 mH 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 24 shows how different inductor types affect
the efficiency across the load range.
100
3 LEDs
VIN = 3.6 V
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 24. 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 1 MHz.
Central Semiconductor Schottky CMDSH2−3 (200 mA
rated) or the CMDSH−3 (100 mA 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
+
0.3 V
LED current
Table 5. RESISTOR R1 AND LED CURRENT
LED Current (mA)
R1 (W)
5
60
10
30
15
20
20
15
25
12
30
10
http://onsemi.com
10