Operation description LED7706
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
V
OUT
=
N
ROWS max i
=
1
( m
LEDS
Σ
j
=
1
V
F , j
)
+
400 mV 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 (400 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
Row with the highest voltage drop across LEDs
V
IN
12/46
Boost controller
Current generators section
I
LED max
Σ
V
F
V
BOOST
400 mV
Leading generator
AM00599v1
8 1
The LED7706 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. When turned off, the LED7706 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
.
