Datasheet

Operation description LED7707
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A dedicated circuit automatically selects the lowest voltage drop among all the rows and
provides this voltage to the main loop that, in turn, regulates the output voltage. In fact, once
the reference generator has been detected, the error amplifier compares its voltage drop to
the internal reference voltage and varies the COMP output. The voltage at the COMP pin
determines the inductor peak current at each switching cycle. The output voltage of the
boost regulator is thus determined by the total forward voltage of the LEDs strings (see
Figure 6):
Equation 1
where the first term represents the highest total forward voltage drop over N active rows and
the second is the voltage drop across the leading generator (700 mV typ.).
The device continues to monitor the voltage drop across all the rows and automatically
switches to the current generator having the lowest voltage drop.
Figure 6. Calculation of the output voltage of the boost regulator
5.1.2 Enable function
The LED7707 is enabled by the EN pin. This pin is active high and, when forced to SGND,
the device is turned off. This pin is connected to a permanently active 2.5 µA current source;
when sudden device turn-on at power-up is required, this pin must be left floating or
connected to a delay capacitor. Starting from an ON state, when the LED7707 is turned off,
it quickly discharges the Soft-Start capacitor and turns off the power-MOSFET, the current
generators and the LDO. The power consumption is thus reduced to 20 µA only.
In applications where the dimming signal is used to turn on and off the device, the EN pin
can be connected to the DIM pin as shown in Figure 7.
mV700)V(maxV
j,F
m
1j
N
1i
BST
LEDS
ROWS
+=
Σ
=
=
700 mV
I
LED
Row with the highest voltage
drop across LEDs
V
BOOST
V
IN
max Σ V
F
Leading
generator
Boost
controller
Current
generators
section
AM00583v1