CAT3614

http://onsemi.com

10

LED Current Setting

Figure 2 shows the timing diagram necessary at the

EN/DIM input for setting the LED currents.

The EN/DIM set up time requires the signal to be held

high for 10

ms or longer to ensure the initialization of the

driver at power−up. Each subsequent pulse on the EN/DIM

(300 ns to 200

ms pulse duration) steps down the LED

current from full scale of 31 mA to zero with a 1 mA

resolution. Consecutive pulses should be separated by

300 ns or longer. Pulsing beyond the 0 mA level restores the

current level back to full scale and the cycle repeats. Pulsing

frequencies from 5 kHz up to 1 MHz can be supported

during dimming operations. When the EN/DIM is held low

for 1.5 ms or more, the CAT3614 enters the shutdown mode

and draws “zero” current.

For applications with three LEDs or less, any unused LED

pins should be tied to VOUT, as shown on Figure 28.

Protection Mode

If an LED becomes open−circuit, the output voltage

VOUT is internally limited to about 5.5 V. This is to prevent

the output pin from exceeding its absolute maximum rating.

The driver enters a thermal shutdown mode as soon as the

die temperature exceeds about +165

°C. When the device

temperature drops down by about 20

°C, the device resumes

normal operation.

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

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

must be connected to the ground plane underneath. The use

of multiple vias improves the package heat dissipation.

Figure 28. Three LED Application

Figure 29. Single Flash LED Application

GND

LED4

EN/DIM

CAT3614

VIN

VOUT

3 V to

5.5 V

C1+

ENABLE/

DIMMING

GND

LED2

LED3

LED1

LED4

EN/DIM

CAT3614

VIN

VOUT

1 mF

1 mF

3 V to

5.5 V

C1−

C1+

C2−

C2+

1 mF

1 mF

Flash

LED

ENABLE/

DIMMING

120 mA

1 mF

1 mF

1 mF

LED3

LED2

LED1

1 mF

C1− C2+

C2−