LED7706 Operation description
The LED7706 is a monolithic LEDs driver for the backlight of LCD panels and it consists of a boost converter and six PWM-dimmable current generators.
The boost section is based on a constant switching frequency, peak current-mode architecture. The boost output voltage is controlled such that the lowest row's voltage, referred to SGND, is equal to an internal reference voltage (400 mV typ. see
Figure 5 ). The
input voltage range is from 4.5 V up to 36 V. In addition, the LED7706 has an internal LDO that supplies the internal circuitry of the device and is capable to deliver up to 40 mA. The input of the LDO is the VIN pin.
The LDO5 pin is the LDO output and the supply for the power MOSFET driver at the same time. The AVCC pin is the supply for the analog circuitry and should be connected to the
LDO output through a simple RC filter in order to improve the noise rejection.
Figure 4.
AVCC filtering
C
LDO5 C
AVCC
AM00597v1
Two loops are involved in regulating the current sunk by the generators.
The main loop is related to the boost regulator and uses a constant frequency peak currentmode architecture to regulate the power rail that supplies the LEDs (
Figure 5 ), while an
internal current loop regulates the same current (flowing through the LEDs) at each row according to the set value (RILIM pin).
Figure 5.
Main loop and current loop diagram
V IN
COMP
Slope
LX ROWx
PWM
SGND
RILIM
Minimum voltage drop selector
E/A
0.4V
Error amplifier
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