CAT4139

© Catalyst Semiconductor, Inc.

7

Doc. No. MD-5032 Rev. A

Characteristics subject to change without notice

DEVICE OPERATION

The CAT4139 is a fixed frequency (1MHz), low noise,

inductive boost converter that provides a constant

current with excellent line and load regulation. The

device uses a high-voltage CMOS power switch

between the SW pin and ground to energize the

inductor. When the switch is turned off, the stored

energy in the inductor is released into the load via the

Schottky diode.

The on/off duty cycle of the power switch is internally

adjusted and controlled to maintain a constant

regulated voltage of 0.3V across the feedback resistor

connected to the feedback pin (FB). The value of the

resistor sets the LED current accordingly (0.3V/R1).

During the initial power-up stage, the duty cycle of the

internal power switch is limited to prevent excessive

in-rush currents and thereby provide a “soft-start”

mode of operation.

In the event of an “Open LED” fault condition, where the

feedback control loop becomes open, the output voltage

will continue to increase. Once this voltage exceeds 24V,

an internal protection circuit will become active and place

the device into a very low power safe operating mode.

Thermal overload protection circuitry has been

included to prevent the device from operating at

unsafe junction temperatures above 150ºC.

In the

event of a thermal overload condition the device will

automatically shutdown and wait till the junction

temperatures cools to 130ºC before normal operation

is resumed.

APPLICATION INFORMATION

EXTERNAL COMPONENT SELECTION

CAPACITORS

The CAT4139 only requires small ceramic capacitors

of 4.7μF on the input and 1µF on the output. Under

normal condition, a 4.7µF input capacitor is sufficient.

For applications with higher output power, a larger

input capacitor of 10µF may be appropriate. X5R and

X7R capacitor types are ideal due to their stability

across temperature range.

Inductor

A 22µH inductor is recommended for most of the

CAT4139 applications. In cases where the efficiency is

critical, inductances with lower series resistance are

preferred. Inductors with current rating of 800mA or

higher are recommended for most applications.

Sumida CDRH6D28-220 22µH inductor has a rated

current of 1.2A and a series resistance (D.C.R.) of

128mΩ typical.

Schottky Diode

The current rating of the Schottky diode must exceed

the peak current flowing through it. The Schottky diode

performance is rated in terms of its forward voltage at

a given current. In order to achieve the best efficiency,

this forward voltage should be as low as possible. The

response time is also critical since the driver is

operating at 1MHz. Central Semiconductor Schottky

rectifier CMSH1-40 (1A rated) is recommended for

most applications.

LED Current Setting

The LED current is set by the external resistor R1

connected between the feedback pin (FB) and ground.

The formula below gives the relationship between the

resistor and the current:

R1 = 0.3V / LED current

Table 1. Resistor R1 and LED Current

LED current (mA)

R1 [Ω]

20

15

25

12

30

10

100

3

300

1