CAT4106 Datasheet, PDF (11/15 Pages) ON Semiconductor – 6 W Quad Channel DC/DC LED Driver with Diagnostics

CAT4106
The external resistor R6 value needed to set a VFMIN
level of 18 V is calculated as follows:
ǒ Ǔ R6 + R7
18
1.2
V
V
*
1
For R7 = 20 kW, R6 = 280 kW.
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:
ǒ Ǔ R4 + R5
24
1.2
V
V
*
1
For R5 = 20 kW, R4 = 380 kW.
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 normal. If a disconnected LED string
is reconnected, the FAULT flag remains on and the channel
disabled until the device has been re−enabled with the EN
pin going from low to high.
If all LED channels are detected as being open−circuit,
then the boost converter will limit the output voltage to the
VFMAX setting. This eliminates the need for an external
protection zener.
Enable and PWM Dimming Control
EN/PWM input signal provides two independent
functions. The first function is to enable and disable the
entire device. The second function is to apply PWM
dimming on the output channels while the chip remains fully
enabled. Applying logic high on the EN/PWM input will
power up the device. The device will continue to remain
powered up, even in the presence of PWM signals being
applied. To disable the device into complete system
shutdown mode, a logic low must be applied to the
EN/PWM input for typically 5 ms.
The duty cycle applied at the EN/PWM is directly applied
to all the output channels. Each time the input is taken low,
all output channels will immediately be switched off and the
channels will resume normal operation when the PWM is
taken back high. The response time of the channels when
switching ON or OFF is typically 0.2 ms.
During PWM dimming, the recommended minimum
pulse width interval (either High or Low) is 0.2 ms. The
recommended maximum pulse width during PWM
dimming is 2.5 ms, however this only applies to the Low
pulse interval. Pulse durations extending past 2.5 ms may
cause the device to enter full shutdown mode. The LED
channel response time is much longer if the device has been
in shutdown mode.
For most applications, a maximum dimming resolution
can be achieved with PWM clock frequencies in the range
of 100 Hz to 2 kHz. Pulse width intervals of 1 ms, allows up
to 1000:1 dimming ratio at 1 kHz PWM frequency.
One or Two LED String Applications
The CAT4106 can be used to drive one or two strings of
LEDs by connecting together some LED pins. Also for
applications requiring LED current greater than 175 mA,
LED channels can be tied together, assuming the supply is
suitable to drive the load. Figure 27 shows a typical
application for driving a single string of LEDs. The LED
current is equal to four times the channel current.
V_L
L
D
C2
V_IN
C1
C3
VIN ^SW VFMAX
VFMIN
CAT4106
R4 R6
R5 _R7
OFF... ON... DIM...
EN/PWM
LED1
RSET
R2
LED2
LED3
VCTRL
LED4
FB
GND
FAULT
PGND
R8
V_IN
R1 R3
LED Fault Detection
(open drain pull−down)
Figure 27. Application Circuit for One LED String
Figure 28 shows a typical application for driving two
strings of LEDs. The LED current is equal to two times the
channel current.
V_L
L
D
C2
V_IN
C1
C3
VIN ^SW VFMAX
VFMIN
CAT4106
R4 R6
R5 R7
OFF... ON... DIM...
EN/PWM
LED1
RSET
R2
VCTRL
LED2
LED3
LED4
FB
FAULT
GND PGND
V_IN
R8
R1 R3
LED Fault Detection
(open drain pull−down)
Figure 28. Application Circuit for Two LED Strings
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