CAT4201_15 Datasheet, PDF (9/12 Pages) ON Semiconductor – High Efficiency Step Down LED Driver

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