CAT4106

http://onsemi.com

10

Device Operation

In a typical application, the CAT4106 drives an LED array

consisting of up to four separate strings. A single external

resistor value is used to select the drive level in all output

channels. Each channel provides a very well regulated and

tightly matched bias current on all LED strings.

An integrated DC/DC boost converter is used to generate

the high voltage output required to power the LED strings.

Up to a maximum of 6 W of LED power can be delivered

when powered from a 12 V supply.

To offer improved performance, the device also allows the

user to set the operating voltage level on the output drive

channels. A pair of external resistors is used to control the

nominal channel voltage, during normal operation. The

resulting output voltage (LED anodes) will be equal to the

sum of the adjusted channel voltage plus the maximum

forward voltage present on any string.

The CAT4106 provides four tightly matched current sinks

to accurately regulate LED current in each channel. Up to

175 mA per channel can be programmed by selecting a

suitable value for the external R1 resistor (connected

between RSET and GND pins).

LED Current Setting

The CAT4106 provides four tightly matched current sinks

to accurately regulate LED current in each channel. The

LED current is set by the external resistor R1 connected

between the RSET pin and ground. The following formula

gives the relationship between the resistor value and the

LED current per channel.

R1

[ 103

1.2 V

LED current per channel

Table 6. RESISTOR R1 AND LED CURRENT

LED Current per Channel (mA)

R1 (

W)

10

12.4 k

20

6.19 k

30

4.12 k

50

2.49 k

100

1.24 k

150

820

Setting the Channel Voltage

Each LED channel typically requires less than 0.5 V of

headroom for full load operation. The lowest channel

operating voltage can be controlled and programmed via the

external resistor network connected between, CTRL, FB

and GND as shown in Figure 24.

Lowest LED pin voltage

+ V

FB

R2

) R3

R3

For most applications, a gain of 2x (R2 equals R3) is

recommended. The CTRL output pin has optimal

performance when loaded with around 15

mA, which

equates to an R3 resistor value of 20 k

W.

LED Detection

The CAT4106 detects failure conditions related to the

LED load, such as if one of the LED strings has been

disconnected (Open−LED) or if one LED string has a lower

LED pin voltage than expected (Short−LED). Either

condition is flagged by turning on the open−drain output on

the FAULT pin. The Open−LED detection is active in

normal operation. The Short−LED detection is only active

during power−up, just after the EN/PWM input goes from

low to high. If in normal operation (LEDs are in regulation)

a short−LED condition occurs, it will not be detected.

Figure 25. Cold Power−Up Short−LED Detection

Figure 26. Normal Operation Open−LED Detection

The test condition in Figure 25 is with one LED pin

shorted to the output (top LED anode). The test condition in

Figure 26 is with one LED pin floating or disconnected. In

both cases, the CAT4106 is set to drive 50 mA per channel.

Setting VFMIN Level (Short−LED)

The VFMIN level represents the minimum level expected

for the LED string voltage “window of operation”. This

voltage setting is based on the number of series LEDs being

normal operation.

Example: Consider a string of 6 LEDs in series, with each

± 0.5 V. Since the

expected string voltage would be 18 V. Any string voltage

of less than 18 V is considered as a short−circuit fault in one

or more of the series LEDs.