FAN5608_06 Datasheet, PDF (5/13 Pages) Fairchild Semiconductor – Serial / Parallel LED Driver with Current-Regulated, Step-Up DC/DC Converter

Electrical Specifications
Recommended operating conditions and component values are per the Figure 18 test circuit. Typical values are at
25°C and VIN=3.6V. Boldface indicates specifications over ambient operating temperature (-40°C to +85°C).
Parameter
Conditions
Min. Typ. Max. Units
Supply current in OFF mode
LED current accuracy
Current matching⁽³⁾
Switching frequency
Internal reference voltage, V_A
Current multiplication ratio
Power efficiency (AVG)⁽⁴⁾
Input A1, A2 threshold
Input B1, B2 threshold
Input A1, A2 current
Input B1, B2 current
A input LOW, B input LOW
A input HIGH, B input HIGH
A input HIGH, B input HIGH
Analog Control Mode
V_IN > 3V
Digital Control Mode
Analog Control Mode
Digital Control Mode
Digital Control Mode
Digital Control Mode
HIGH
LOW
HIGH
LOW
V_A = V_IN
V_A =0
0.1
1
µA
18
20
22
mA
3
%
500
KHz
1.2
V
850 1000 1150
80
%
V_IN -0.7
0
V_IN
0.6
V
1.2
0.6 x V_IN
0
V_IN
V
0.3 x V_IN
50
80
µA
0.1
0.1
µA
Notes:
3. Current matching is the absolute value of the difference in current of the two LEDs channels, divided by the
average current in the two channels.
4. Power efficiency is the ratio between the electrical power to the LEDs and the total power consumed from the
input power supply. Although this definition leads to a lower value than the boost converter efficiency, it more
accurately reflects the system performance in an application.
Circuit Description
When power is applied to the V_IN pin, the system is
enabled, the bandgap reference acquires its nominal
voltage (1.2V), and the soft-start cycle begins. Once
“power good” is achieved (when 0.5mA voltage flows
through the LEDs), the soft-start cycle stops, and the
boost voltage increases to generate the current selected
by the input control pins (A1, A2, B1, B2).
If CH1 is not selected, the CH1 output pin is HIGH, its
output measurement is approximately equal to V_OUT,
and its LEDs are OFF. The FAN5608 provides tightly
regulated output currents for the selected LED’s
channel. An internal feedback loop determines which
LED string requires the highest output voltage to sustain
the pre-set current. It also adjusts the boost regulator
based on CH1 and CH2 feedback voltage and A1, A2,
B1, B2 input settings. System efficiency decreases if the
channels’ strings of LEDs have different forward voltage.
If one channel is used, the other channel should be
disabled by connecting the corresponding DAC inputs to
logic LOW. If the external output capacitor V_OUT is
shorted to GND, the internal Schottky diode can be
damaged, a condition which should be avoided.
LED Brightness Control
The control inputs are A1 and B1 for CH1 and A2 and
B2 for CH2. B1 and B2 are digital inputs that require
LOW (GND) and HIGH V_IN control signals. In analog
mode, A1 and A2 are connected to an external stable
voltage source via an external resistor, and B1 and B2
inputs are connected to ground. The current flowing
through the resistor is scaled by a factor of
approximately 1000.
LED dimming can be controlled with static or dynamic
inputs from CH1 and CH2. CH1 and CH2 outputs are
driven directly by A1, B1 and A2, B2 inputs, respectively.
Either channel can be configured as analog or digital.
Inputs A1 and A2 are analog inputs that can be
connected to an external regulated voltage source,
which can be calculated by:
EQ 1
where current multiplication ratio is according to the
Electrical Specifications table.
Inputs B1 and B2 are digital inputs and can only be set
by external logic of “0” (LOW) or “1” (HIGH).
© 2006 Fairchild Semiconductor Corporation
FAN5608 Rev. 1.0.5 • 8/8/06
5
www.fairchildsemi.com