CAT4238 Datasheet, PDF (7/12 Pages) Catalyst Semiconductor – High Efficiency 10 LED Boost Converter

CAT4238
TYPICAL CHARACTERISTICS
(V_IN = 3.6 V, C_IN = 4.7 mF, C_OUT = 0.22 mF, L = 47 mH with 10 LEDs at 20 mA, T_AMB = 25°C, unless otherwise specified.)
60
1.0
50
0.8
40
30
0.6
20
VOUT = 35 V
0.4
10
−40°C
−25°C
85°C
125°C
0
3.0
3.5
4.0
4.5
5.0
5.5
INPUT VOLTAGE (V)
Figure 16. Maximum Output Current vs. Input
Voltage
0.2
3.0
3.5
4.0
4.5
5.0
INPUT VOLTAGE (V)
Figure 17. Shutdown Voltage vs. Input Voltage
Application Information
External Component Selection
Capacitors
The CAT4238 only requires small ceramic capacitors of
4.7 mF on the input and 0.22 mF on the output. Under normal
condition, a 4.7 mF input capacitor is sufficient. For
applications with higher output power, a larger input
capacitor of 10 mF may be appropriate. X5R and X7R
capacitor types are ideal due to their stability across
temperature range.
Inductor
A 47 mH inductor is recommended for most of the
CAT4238 applications. In cases where the efficiency is
critical, inductances with lower series resistance are
preferred. Inductors with current rating of 300 mA or higher
are recommended for most applications. Sumida
CDC5D23B−470 47 mH inductor has a rated current of
490 mA and a series resistance (D.C.R.) of 420 mW typical.
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 diode CMDSH05−4
(500 mA rated) is recommended for most applications.
LED Current Setting
The LED current is set by the external resistor R₁
connected 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
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