CAT3603

http://onsemi.com

9

Basic Operation

At power−up, the CAT3603 starts operating in 1x mode

where the output will be approximately equal to the input

supply voltage (minus any internal voltage losses). If the

output voltage is sufficient to regulate all LED currents, the

device remains in 1x operating mode.

If the input voltage falls to a level where the regulated

currents cannot be maintained, the device automatically

switches into 1.5x mode.

In 1.5x mode, the output is approximately equal to 1.5

times the input supply voltage (minus any internal voltage

losses), and high enough to achieve the nominal LED

current.

The above sequence is reinitialized every time the chip is

powered−up or is taken out of shutdown mode (via EN pin).

LED Current Setting

connected between the RSET pin and ground. Table 5 lists

for standard 1% precision surface mount resistors.

Table 5. RSET Resistor Selection

LED Current [mA]

1

649

2

287

5

102

10

49.9

15

32.4

20

23.7

30

15.4

The unused LED channels must be disabled by connecting

the respective LED pins to VOUT. A disabled channel sinks

only 0.2 mA typical. When the following equation is true on

any channel, the driver turns off the LED channel:

Note: The CAT3603 is designed to drive LEDs with forward

voltage greater than 1 V and is not compatible with resistive

loads less than 5 k

W.

External Components

The driver requires a total of four external 1

mF ceramic

capacitors: two for decoupling input and output, and two for

the charge pump. Both capacitor types X5R and X7R are

recommended for the LED driver application. In the 1.5x

charge pump mode, the input current ripple is kept very low

by design, and an input bypass capacitor of 1

mF is sufficient.

In 1x mode, the device operating in linear mode does not

introduce switching noise back onto the supply.

Recommended Layout

In 1.5x charge pump mode, the driver switches internally

at a high frequency of 1 MHz. It is recommended to

minimize trace length to all four capacitors. A ground plane

should cover the area under the driver IC as well as the

bypass capacitors. Short connection to ground on capacitors

CIN and COUT can be implemented with the use of multiple

via. A copper area matching the TDFN exposed pad (GND)

must be connected to the ground plane underneath. The use

of multiple via improves the package heat dissipation.

Figure 22. Recommended Layout