Preliminary
RT9272
Function Block Diagram
OVP
LX
VCC
VCC
OVP
Buffer
EN
GT
EN
EN
EN
ON
FB
VRF
VRF
GND
Dimming Controller
Boost Controller
CH1
CH2
GND
SP
GT
1ms
5u
FB1
FB2
Operation
RT9272 operates at a wide input voltage range suitable for Li-ion battery and has a constant high frequency for smaller
inductor and capacitors. The block diagram refers the operation circuit. The scheme is divided into three parts: dimming
controller, boost controller and the logic selection function. By SP pin, it can choose the different channel to turn on CH1
or CH2. As the input signal change, internal MOSFET of LX pin will turn off 1ms to prevent the over voltage damage by
the output voltage instant change.
The Dimming Controller:
RT9272 uses the PWM signal on EN pin to achieve dimming control. If the period of EN is less than 3ms in 50% duty, EN
is always pulled high. Then EN signal is sent to the next boost controller to enable the function. This action is to
differentiate between shut down and dimming control. VRF offers the reference voltage to error amplifier of next function.
The internal soft start is designed for enable control and dimming control to avoid the inrush current.
The Boost Controller:
RT9272 uses current mode control scheme to provide better performance of line and load regulation. The boost controller
includes of error amplifier, summing comparator, logic control and protection function.As SP switches, the channel will be
changed for 2LEDs output or 3LEDs output. The difference between the feedback voltage and the reference voltage is
send to the negative input of comparator. The comparator also senses the sensing current from inductor and slope
compensation. To keep the output in regulation, the output of summing comparator is sent to the logic function, and then
to control the duty ratio of the MOSFET switch. If the error amplifier's output increases, the duty of the switch will be larger
to deliver more current to the output.
DS9272-09 August 2007
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RICHTEK [ RICHTEK TECHNOLOGY CORPORATION ]
