Functional Pin Description
Pin No. Pin Name
Supply Voltage Input. For good bypass, a low ESR capacitor is required.
Positive Input Current Sense.
Negative Input Current Sense. Voltage threshold between ISP and ISN is 100mV.
Analog Dimming Control Input. Effective programming range is 0.33V to 2V.
Current Sense Input for LED Current. Connect the current sense resistor between
external N-MOSFET switch and the ground.
External MOSFET Switch Gate Driver Output.
8 CREG Regulator Output. Placed 1F capacitor to stabilize the 5V regulator output.
Function Block Diagram
The RT8477 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications. This device uses a fixed frequency, current-
mode control scheme to provide excellent line and load
The control loop has a current sense amplifier which
senses the voltage between the ISP and ISN pins.
A PWM comparator then turns off the external power
switch when the sensed power switch current exceeds
the internal compensated voltage. The power switch will
not be reset by the oscillator clock in each cycle. If the
comparator does not turn off the switch in a cycle, the
power switch will be on for more than a full switching period
until the comparator is tripped. In this manner, the
programmed voltage across the sense resistor is regulated
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
by the control loop.
The current through the sense resistor is set by the
programmed voltage and the sense resistance. The voltage
across the sense resistor can be programmed by the
analog or digital signal at the CTL pin with good dimming
linearity. The max sense threshold of 100mV can be
obtained with CTL pin voltage greater than 2V (max
dimming point). The sense threshold is intentionally forced
to zero by an internal comparator when the CTL pin voltage
is less than around 0.33V (min dimming point). Because
of that, the actual sense threshold right before cut off may
vary from part to part over process variation.
The RT8477 provides protection functions which include
over-temperature, and switch current limit to prevent
is a registered trademark of Richtek Technology Corporation.
DS8477-00 December 2013