CAT4201

http://onsemi.com

9

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.

0

100

200

300

400

01

2

3

4

5

6

CTRL VOLTAGE [V]

150 mA

300 mA

Figure 23. “Dropout” Configured LED Current

(as shown in Typical Application on page 1)

Inductor Selection

For 350 mA LED current drive levels, a 22

mH inductor

value is recommended 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.

The inductor must have a maximum current rating which

equals or exceeds twice the programmed LED current. For

example, when driving LEDs at 350 mA, 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 7. SUMIDA INDUCTORS

Part Number

L (mH)

I Rated (A)

LED Current (A)

CDRH6D26−100

10

1.5

0.35

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

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.

Central Semiconductor Schottky diode CMDSH05−4 (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

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