Datasheet CAT4201 (ON Semiconductor) - 9

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CAT4201 APPLICATION INFORMATION Capacitor Selection Input Voltage Range
A 10 mF ceramic capacitor C2 across the LED(s) keeps the The minimum supply voltage required to maintain LED ripple current within ±15% of nominal for most adequate regulation is set by the cathode terminal voltage of applications. If needed, a larger capacitor can be used to the LED string (i.e., the VBAT voltage minus the LED string further reduce the LED current ripple. Any resistance in voltage). When the LED cathode terminal falls below 3 V, series with the LED (0.5 W or more) contributes to reduce a loss of regulation occurs. the ripple current. The capacitor voltage rating should be For applications which may occasionally need to equivalent to the maximum expected supply voltage so as to experience supply “dropout” conditions, it is recommended allow for “Open−LED” fault conditions. The capacitor that the CTRL input be used to sense the LED cathode value is independent of the switching frequency or the voltage. The CTRL pin can either be tied directly to the overall efficiency. cathode terminal (for Lamp Replacement) or connected via A 4.7 mF ceramic input capacitor C1 is recommended to a pass−transistor for PWM lighting applications. minimize the input current ripple generated on the supply. Figure 23 shows the regulation performance obtained in Using a larger capacitor value further reduces the ripple dropout, when the CTRL pin is configured to sense the LED noise appearing on the supply rail. cathode voltage. If a constant capacitance is needed across temperature and voltage, X5R or X7R dielectric capacitors are recommended. 400
Schottky Diode
The peak repetitive current rating of the Schottky diode 300 300 mA must be greater than the peak current flowing through the inductor. Also the continuous current rating of the Schottky [mA] must be greater than the average LED current. The voltage 200 rating of the diode should be greater than the peak supply voltage transient preventing any breakdown or leakage. 150 mA ON Semiconductor Schottky diode MBR0540 (40 V, 100 LED CURRENT 500 mA rated) is recommended. Schottky diodes rated at 400 mA (or higher) continuous current are fine for most applications. 0 0 1 2 3 4 5 6 NOTE: Schottky diodes with extremely low forward voltages (V CTRL VOLTAGE [V] F) are not recommended, as they may cause an increase in the LED current.
Figure 23. “Dropout” Configured LED Current
(as shown in Typical Application on page 1)
Dimming Methods Inductor Selection
Two methods for PWM dimming control on the LEDs are A 22 mH minimum inductor value is required to provide described below. The first method is to PWM on the control suitable switching frequency across a wide range of input pin, the other method is to turn on and off a second resistor supply values. For LED current of 150 mA or less, a 33 mH connected to the RSET pin and connected in parallel with R1. or 47 mH inductor is more suitable. Inductor values below
PWM on CTRL Pin
22 mH should not be used. A PWM signal from a microprocessor can be used for An inductor with at least 700 mA current rating must be dimming the LEDs when tied to the CTRL pin. The duty used. Minor improvements in efficiency can be achieved by cycle which is the ratio between the On time and the total selecting inductors with lower series resistance. cycle time sets the dimming factor. The recommended PWM frequency on the CTRL pin is between 100 Hz and 2 kHz.
Table 8. SUMIDA INDUCTORS Part Number L (mH) I Rated (A) LED Current (A)
CDRH6D26−220 22 1.0 0.35 CDRH6D28−330 33 0.92 0.35 CDRH6D28−470 47 0.8 0.35 CDRH6D28−560 56 0.73 0.35
Figure 24. PWM at 1 kHz on CTRL Pin www.onsemi.com 9