CAT4201TD-GT3 Datasheet, PDF (9/15 Pages) ON Semiconductor – 350mA 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 3V, 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 2 shows the regulation performance obtained
in dropout, when the CTRL pin is configured to sense
the LED cathode voltage.
400
300
300mA
200
150mA
100
0
0 12 34 56
CTRL VOLTAGE [V]
Figure 2. “Dropout” configured LED Current
(as shown in Typical Application on page 1)
Inductor Selection
For 350mA LED current drive levels, a 22µH inductor
value is recommended to provide suitable switching
frequency across a wide range of input supply values.
For LED current of 150mA or less, a 33µH or 47µH
inductor is more suitable.
The inductor must have a maximum current rating
which equals or exceeds twice the programmed LED
current. For example, when driving LEDs at 350mA,
an inductor with at least 700mA current rating must be
used. Minor improvements in efficiency can be
achieved by selecting inductors with lower series
resistance.
Part Number
CDRH6D26-100
CDRH6D26-220
CDRH6D28-330
CDRH6D28-470
CDRH6D28-560
L
(µH)
10
22
33
47
56
I rated
(A)
1.5
1.0
0.92
0.8
0.73
LED current
(A)
0.35
0.35
0.35
0.35
0.35
Table 3. Sumida inductors
Capacitor Selection
A 10μF 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 (0.5Ω 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 4.7μF 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.
Central Semiconductor Schottky diode CMDSH05-4
(40V, 500mA rated) is recommended. Schottky diodes
rated at 400mA (or higher) continuous current are fine
for most applications.
Note: Schottky diodes with extremelly low forward voltages (V_F) are
not recommended, as they may cause an increase in the
LED current.
© 2008 SCILLC. All rights reserved.
9
Characteristics subject to change without notice
Doc. No. MD-5025 Rev. E