RT8477 Datasheet PDF - Richtek

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RT8477
Richtek

Part Number RT8477
Description High Voltage High Current LED Driver
Page 11 Pages


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®
RT8477
High Voltage High Current LED Driver
General Description
The RT8477 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications with wide input voltage (4.5V to 50V) and
output voltage (up to 50V) ranges. With internal 380kHz
operating frequency, the size of the external PWM inductor
and input/output capacitors can be minimized. High
efficiency is achieved by a 100mV current sensing control.
LED Dimming control can be done by analog.
The RT8477 is now available in the SOP-8 package.
Ordering Information
RT8477
Package Type
S : SOP-8
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
RoHS compliant and compatible with the current require-
ments of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Features
Buck, Boost Constant Current Converter
High Voltage : VIN up to 50V, VOUT up to 50V
380kHz Fixed Switching Frequency
Analog or PWM Control Signal for LED Dimming
Internal Soft-Start to Avoid Inrush Current
Under-Voltage Lockout
Thermal Shutdown
RoHS Compliant and Halogen Free
Applications
Desk Lights and Room Lighting
Industrial Display Backlight
Pin Configurations
(TOP VIEW)
VCC
ISP
ISN
CTL
2
3
4
8 CREG
7 DRV
6 GND
5 SENS
SOP-8
Marking Information
RT8477
GSYMDNN
RT8477GS : Product Number
YMDNN : Date Code
Simplified Application Circuit
VIN
Analog Dimming
C1
RT8477
VCC
ISP
CTL
CREG
C2
GND
ISN
DRV
SENS
D1
R1
LEDs
C4
L1
R3
C3
M1
R2
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8477-00 December 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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RT8477
Functional Pin Description
Pin No. Pin Name
Pin Function
1 VCC
Supply Voltage Input. For good bypass, a low ESR capacitor is required.
2 ISP
3 ISN
Positive Input Current Sense.
Negative Input Current Sense. Voltage threshold between ISP and ISN is 100mV.
4 CTL
Analog Dimming Control Input. Effective programming range is 0.33V to 2V.
5
SENS
Current Sense Input for LED Current. Connect the current sense resistor between
external N-MOSFET switch and the ground.
6 GND
Ground.
7 DRV
External MOSFET Switch Gate Driver Output.
8 CREG Regulator Output. Placed 1F capacitor to stabilize the 5V regulator output.
Function Block Diagram
VCC
CREG
4.5V
OSC
-
+
5V
LDO
Soft-Start
CTL
S
R
R
-
GM
+
+
-
DRV
SENS
ISN
ISP
GND
Opertation
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
regulation.
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.
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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
abnormal situations.
is a registered trademark of Richtek Technology Corporation.
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RT8477
Absolute Maximum Ratings (Note 1)
Supply Input Voltage, VCC ------------------------------------------------------------------------------------ 0.3V to 60V
ISP, ISN ------------------------------------------------------------------------------------------------------------ 0.3V to 60V
SENS, DRV, CREG Pin Voltage ----------------------------------------------------------------------------- 0.3V to 5.5V
CTL Pin Voltage ------------------------------------------------------------------------------------------------- 0.3V to 20V (Note 2)
Power Dissipation, PD @ TA = 25°C
SOP-8 -------------------------------------------------------------------------------------------------------------- 0.53W
Package Thermal Resistance (Note 3)
SOP-8, θJA -------------------------------------------------------------------------------------------------------- 188°C/W
Junction Temperature ------------------------------------------------------------------------------------------- 150°C
Lead Temperature (Soldering, 10 sec.) --------------------------------------------------------------------- 260°C
Storage Temperature Range ---------------------------------------------------------------------------------- 65°C to 150°C
ESD Susceptibility (Note 4)
HBM (Human Body Model) ------------------------------------------------------------------------------------ 2kV
Recommended Operating Conditions (Note 5)
Supply Input Voltage, VCC ------------------------------------------------------------------------------------ 4.5V to 50V
Junction Temperature Range ---------------------------------------------------------------------------------- 40°C to 125°C
Ambient Temperature Range ---------------------------------------------------------------------------------- 40°C to 85°C
Electrical Characteristics
(VCC = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min Typ Max Unit
Overall
Regulator Output Voltage
Supply Current
VIN Under Voltage Lockout
Threshold
Current Sense Amplifier
VCREG
IVCC
VUVLO
ICREG = 20mA
VCTL = 3V
VIN Rising
VIN Falling
4.5 5
-- --
-- 4.25
-- 4.2
5.5
3
4.5
--
V
mA
V
Input Threshold (VISP VISN)
Input Current
LED Dimming
IISP
IISN
4.5 Common Mode 20V 95 100 105 mV
VISP = 24V
VISN = 24V
-- 150 --
-- 50 --
A
Input Current of CTL Pin
ICTL 0.2V VCTL 1.2V
-- 1 2 A
LED Current Off Threshold at CTL VCTL_OFF
LED Current On Threshold at CTL VCTL_ON
-- 0.33 0.5
-- 2 2.5
V
V
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
DS8477-00 December 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
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RT8477
Parameter
Symbol
Test Conditions
Min Typ Max Unit
PWM Converter
Switch Frequency
Maximum Duty Cycle
Minimum On-Time
fSW
DMAX
(Note 6)
330 380 430 kHz
-- -- 100 %
-- 200 --
ns
Gate High Voltage
Gate Driver Source
VGATE_H IGATE = 20mA
4.5 5 5.5
1 2.5 --
V
A
Gate Driver Sink
1 3.5 --
A
Soft-Start Time
(Note 7)
-- 2 -- ms
Sense Current Limit Threshold
Over-Temperature Protection
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
ISE NS_LI M
TS D
TSD
100 150
--
mV
-- 150 --
-- 20 --
C
C
Note 1. Stresses beyond those listed Absolute Maximum Ratingsmay cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in
the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may
affect device reliability.
Note 2. If connected with a 20kΩ serial resistor, CTL can go up to 40V.
Note 3. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
Note 4. Devices are ESD sensitive. Handling precaution is recommended.
Note 5. The device is not guaranteed to function outside its operating conditions.
Note 6. When the natural maximum duty cycle of the switching frequency is reached, the switching cycle will be skipped (not
reset) as the operating condition requires to effectively stretch and achieve higher on cycle than the natural maximum
duty cycle set by the switching frequency.
Note 7. Guaranteed by design, not subjected to production test.
Copyright ©2013 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8477-00 December 2013



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