CAT4106HV4-GT2 Datasheet, PDF (11/17 Pages) ON Semiconductor – 6W Quad Channel DC/DC LED Driver with Diagnostics

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 4.
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 μA, which
equates to an R3 resistor value of 20 kΩ.
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 5. Cold Power-Up Short-LED Detection
Figure 6. Normal Operation Open-LED Detection
CAT4106
The test condition in Figure 5 is with one LED pin
shorted to the output (top LED anode). The test
condition in Figure 6 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 used and the expected
minimum forward voltage V_F during normal operation.
Example: Consider a string of 6 LEDs in series, with
each LED having a V_F range of 3.5 V ± 0.5 V. Since
the minimum V_F of each LED is 3.0 V the overall
lowest 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.
The external resistor R6 value needed to set a VFMIN
level of 18 V is calculated as follows:
R6
=
R7
×
⎜⎜⎝⎛
18
1.2
V
V
− 1_{⎟⎟⎠⎞}
For R7 = 20 kΩ, R6 = 280 kΩ.
SETTING VFMAX LEVEL (OPEN-LED)
The VFMAX level represents the maximum level
expected for the LED string voltage “window of
operation”. This voltage setting is based on the
number of series LEDs being used and the expected
maximum V_F during normal operation.
Example: Consider a string of 6 LEDs in series, with
each LED having a V_F range of 3.5 V ± 0.5 V. Since
the maximum V_F of each LED is 4.0 V, the overall
maximum expected string voltage would be 24 V. Any
string voltage which appears greater than 24 V would
be considered as containing an open-circuit in one or
more of the series LEDs. The external resistor R4
value needed to set a VFMAX level of 24 V is
calculated as follows:
R4
=
R5
×
⎜⎜⎝⎛
24
1.2
V
V
− 1_{⎟⎟⎠⎞}
For R5 = 20 kΩ, R4 = 380 kΩ.
In normal operation, the LED string which has the
largest V_F will be used to set the VCTRL/VFB voltage
levels. If the largest string voltage tries to exceed
VFMAX setting, it will no longer be allowed to control
the voltage level of VCTRL/VFB (i.e it will be ignored)
and subsequently the next largest LED string voltage
will then be used in the control loop. All remaining
functional LED channels will continue to operate as
© 2009 SCILLC. All rights reserved.
11
Characteristics subject to change without notice
Doc. No. MD-5037, Rev. A