FAN5608 Fairchild Semiconductor, FAN5608 Datasheet - Page 6

• Manufacturer Part Number: FAN5608
• Description: Serial/Parallel LED Driver with Current-Regulated/ Step-Up DC/DC Converter
• Manufacturer: Fairchild Semiconductor

PRODUCT SPECIFICATION
Digital Control
The FAN5608’s digital decoder allows selection of the fol-
lowing modes of operation: OFF, 5mA, 10mA, 20mA per
branch.
Analog Control
Inputs A1 and A2 are used to control the LED currents.
Inputs B1 and B2 should be connected to GND (logic level
“0”). An external resistor (R) is connected from A1 and/or
A2 to a stable voltage source (V
current, I
and the graph below:
Where V
PWM Control in Digital Mode
The logic level HIGH, VH and logic level LOW, VL of the
PWM signal should be:
(V
The frequency of the PWM signal should be within 50Hz to
1kHz range; it can go up to 30kHz at any input if the other
input is kept HIGH. In the case of FAN5608MPX and
FAN5608DMPX, the B1 and B2 inputs are internally con-
nected to GND and the PWM signal can be applied to A1
and A2 inputs only. Consequently, the maximum LED
current, for 100% duty cycle, is 5mA on each channel.
6
IN
I
LED
A
B
– 0.7V) < VH < V
25
20
15
10
5
0
1.25
I
LED
Ref
LED
= 1.22V, V
1.5
=
. The I
R=10Kohm


OFF
V
---------------------------------------- -
0
0
1.75
External
LED
R
Ref
IN
2
can be determined using the formula
–
and 0 < VL < 0.6V
< V
V
5mA
2.25
V
Ref
1
0
External
External


External
×
2.5
Multiplication Ratio
(V)
R=100kohm
< (V
10mA
) to control the LED
2.75
0
1
IN
– 0.7V)
3
3.25
20mA
1
1
3.5
PWM Control in Analog Mode
The logic level HIGH, VH and logic level LOW, VL of the
PWM signal should be:
V
The frequency of the PWM signal should be within 50Hz
to 1kHz range. The VH sets the maximum LED current
while the duty cycle sets the average current between 0 and
I
If the analog inputs A1 and/or A2 are driven in digital mode
by an open drain output, it is important to choose the appro-
priate value of the pull-up resistor. Its resistance should be
low enough to ensure less than 0.7V dropout, hence V
(V
Open-Circuit Protection
A built-in over voltage protection circuit prevents the device
from being damaged when it is powered up with no load.
This circuit reduces the boost converter duty cycle, to a
minimum thus limiting the output voltage to a safe value
when no load condition is detected. If one of the two enable
branches is accidentally disconnected, the converter contin-
ues the operation, however, the current in the remaining
branch is no longer regulated and the actual branch current
will be determined by the input voltage, the inductor value
and the switching frequency.
However, the FAN5608 can be damaged when a full load
(more than six LEDs, driven by 20mA) is suddenly discon-
nected from V
unlikely event, an external 24V Zener diode can be con-
nected between V
Shutdown Mode
Each branch can be independently disabled by applying
LOW logic level voltage to the A and B inputs. When both
branches are disabled, the FAN5608 enters Shutdown mode
and the supply current is reduced to less than 1µA.
LEDmax
Ref
IN
< VH < (V
– 0.7V) as required for HIGH logic level:
.
OUT
R
IN
pull up
OUT
. To protect the FAN5608 against this
– 0.7V) and 0 < VL < 0.6V
–
and GND.
<
700mV
----------------- -
60µA
=
11.66kΩ
REV. 1.0.3 6/28/04
FAN5608
A
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