®
RT8477
High Voltage High Current LED Driver
General Description
Features
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.
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
The RT8477 is now available in the SOP-8 package.
Applications
Ordering Information
Desk Lights and Room Lighting
Industrial Display Backlight
RT8477
Package Type
S : SOP-8
Pin Configurations
(TOP VIEW)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
8
VCC
2
7
DRV
ISN
3
6
GND
CTL
4
5
SENS
RoHS compliant and compatible with the current requireSOP-8
ments of IPC/JEDEC J-STD-020.
CREG
ISP
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
RT8477GS : Product Number
RT8477
GSYMDNN
YMDNN : Date Code
Simplified Application Circuit
D1
R1
VIN
C1
C4
RT8477
C5
1µF
VCC
...
R4
10
ISP
LEDs
L1
ISN
Analog Dimming
CTL
CREG
GND
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
December 2014
R3
SENS
C2
DS8477-02
M1
DRV
C3
R2
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1
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
Positive Input Current Sense.
3
ISN
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
S
OSC
-
VCC
4.5V
CREG
R
+
DRV
R
5V
LDO
+
-
SENS
Soft-Start
GM
+
CTL
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, currentmode control scheme to provide excellent line and load
regulation.
by the control loop.
The control loop has a current sense amplifier which
senses the voltage between the ISP and ISN pins.
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.
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
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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
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.
DS8477-02
December 2014
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
4.5
5
5.5
V
mA
Overall
Regulator Output Voltage
VCREG
ICREG = 20mA
Supply Current
I VCC
VCTL = 3V
--
--
3
VIN Under Voltage Lockout
Threshold
VUVLO
VIN Rising
--
4.25
4.5
VIN Falling
--
4.2
--
4.5 Common Mode 20V
95
100
105
I ISP
VISP = 24V
--
150
--
I ISN
VISN = 24V
--
50
--
Input Current of CTL Pin
ICTL
0.2V VCTL 1.2V
--
1
2
A
LED Current Off Threshold at CTL
VCTL_OFF
--
0.33
0.5
V
LED Current On Threshold at CTL
VCTL_ON
--
2
2.5
V
V
Current Sense Amplifier
Input Threshold (VISP VISN)
Input Current
mV
A
LED Dimming
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8477-02
December 2014
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RT8477
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
330
380
430
kHz
--
--
100
%
--
200
--
ns
4.5
5
5.5
V
Gate Driver Source
1
2.5
--
A
Gate Driver Sink
1
3.5
--
A
--
2
--
ms
100
150
--
mV
PWM Converter
Switch Frequency
f SW
Maximum Duty Cycle
DMAX
(Note 6)
Minimum On-Time
Gate High Voltage
VGATE_H
Soft-Start Time
Sense Current Limit Threshold
IGATE = 20mA
(Note 7)
ISENS_LIM
Over-Temperature Protection
Thermal Shutdown Temperature
TSD
--
150
--
C
Thermal Shutdown Hysteresis
T SD
--
20
--
C
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may 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.
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is a registered trademark of Richtek Technology Corporation.
DS8477-02
December 2014
RT8477
Typical Application Circuit
D1
R1
VIN
4.5V to 50V
C1
10µF
0.1
RT8477
1
C5
1µF
ISP
VCC
ISN
4 CTL
8
CREG
Analog Dimming
DRV
C2
1µF 6
GND
SENS
C4
1µF
LEDs
...
R4
10
2
L1
22µH
3
7
M1
R3 51
5
C3
1nF
R2
0.03
Figure 1. Buck Configuration
L1
22µH
VIN
VOUT
50V(Max)
C1
10µF
C5
1µF
1 VCC
RT8477
7
DRV
SENS
4 CTL
8
CREG
C2
1µF 6
GND
5
M1
ISN
LEDs
R3 51
C3
1nF
ISP
C4
1µF
...
R4
10
Analog Dimming
R1
0.1
D1
R2
0.03
51
2
3
VF
(VF > VLEDs)
Figure 2. Boost Configuration
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8477-02
December 2014
is a registered trademark of Richtek Technology Corporation.
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5
RT8477
Typical Operating Characteristics
Efficiency vs. Input Voltage
Efficiency vs. Input Voltage
100
100
Buck, LED Current = 2A, L = 22μH
VOUT
VOUT
VOUT
VOUT
98
94
21V
18V
15V
12V
96
92
VOUT = 9V
90
88
86
Boost, LED Current = 1A, L = 22μH
98
94
Efficiency (%)
Efficiency (%)
96
=
=
=
=
VOUT = 6V
92
90
VOUT
VOUT
VOUT
VOUT
88
86
84
84
82
82
80
22V
28V
35V
40V
80
5
15
25
35
45
55
5
10
15
Input Voltage (V)
20
25
30
Input Voltage (V)
LED Current vs. VCTL
Supply Current vs. VCC
450
1.5
LED Current = 300mA, LED = 6pcs
400
1.4
350
LED Current (mA)
Supply Current (mA)
=
=
=
=
1.3
1.2
300
250
200
150
100
1.1
50
0
1.0
0
5
10
15
20
25
30
35
40
45
0.3
50
0.6
0.9
1.2
VCC (V)
ISP - ISN Threshold vs. Temperature
1.8
2.1
2.4
2.7
3
45
50
Frequency vs. VCC
120
410
110
400
Frequency (kHz)1
ISP - ISN Threshold (mV)
1.5
VCTL (V)
100
90
80
390
380
370
VCC = 24V
70
360
-40
-10
20
50
80
110
Temperature (°C)
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140
0
5
10
15
20
25
30
35
40
VCC (V)
is a registered trademark of Richtek Technology Corporation.
DS8477-02
December 2014
RT8477
Power On from VIN
Power Off from VIN
VIN
(20V/Div)
VIN
(20V/Div)
VOUT
(10V/Div)
VOUT
(10V/Div)
IOUT
(2A/Div)
VIN = 30V, IOUT = 2A, LED = 42pcs, L = 47μH
Time (2.5ms/Div)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS8477-02
December 2014
IOUT
(2A/Div)
VIN = 30V, IOUT = 2A, LED = 42pcs, L = 47μH
Time (25ms/Div)
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RT8477
Application Information
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.
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 by the control loop.
operation when the die junction temperature exceeds
150°C. The chip will automatically start to switch again
when the die junction temperature cools off.
Inductor Selection
The converter operates in discontinuous conduction mode
when the inductance value is less than the value LBCM.
With an inductance greater than LBCM, the converter
operates in Continuous Conduction Mode (CCM). The
inductance LBCM is determined by the following equations.
For Buck application :
LBCM
LED Current Setting
The LED current can be calculated by the following
equation :
V(ISP ISN)
ILED(MAX) =
R1
where V(ISP − ISN) is the voltage between ISP and ISN
(100mV typ. if CTL dimming is not applied) and the R1 is
the resister between ISP and ISN.
Sense Resistor Selection
The resistor, R2, between the Source of the external NMOSFET and GND should be selected to provide adequate
switch current to drive the application without exceeding
the current limit threshold set by the SENSE pin sense
threshold of RT8477. The Sense resistor value can be
calculated according to the formula below :
R2
Current Limlit Threshold Minimum Value
IOCP
where IOCP is about 1.33 to 1.5 times of inductor peak
current IPEAK.
The placement of R2 should be close to the source of the
N-MOSFET and the IC GND of the RT8477. The SENSE
pin input to RT8477 should be a Kelvin sense connection
to the positive terminal of R2.
Over-Temperature Protection
The RT8477 has Over-Temperature Protection (OTP)
function to prevent the excessive power dissipation from
overheating. The OTP function will shut down switching
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VOUT
VIN VOUT
2 IOUT f
VIN
For Boost application :
LBCM
VIN
VOUT VIN
2 IOUT f VOUT
where
VOUT = output voltage.
VIN = input voltage.
f = operating frequency.
IOUT = LED current.
Choose an inductance based on the operating frequency,
input voltage and output voltage to provide a current mode
ramp signal during the MOSFET on period for PWM control
loop regulation. The inductance also determines the
inductor ripple current. Operating the converter in CCM is
recommended, which will have the smaller inductor ripple
current and hence the less conduction losses from all
converter components.
As a design example, to design the peak to peak inductor
ripple to be ±30% of the output current, the following
equations can be used to estimate the size of the needed
inductance :
For Buck application :
L=
VOUT
VIN VOUT
2 0.3 IOUT f
VIN
For Boost application :
L=
VIN
VOUT VIN
2 0.3 IOUT f VOUT
is a registered trademark of Richtek Technology Corporation.
DS8477-02
December 2014
RT8477
For Buck application :
VOUT VIN VOUT
2 L f
VIN
For Boost application :
IPEAK = IOUT +
IPEAK =
VOUT IOUT
VIN
VOUT VIN
+
2 L f VOUT
VIN
where
η is the efficiency of the power converter.
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for RT8477
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are important parameters
of the Schottky diode that must be considered. The diode's
average current rating must exceed the average output
current. The diode conducts current only when the power
switch is turned off (typically less than 50% duty cycle).
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most RT8477 applications, a 4.7μF ceramic capacitor is
sufficient. A value higher or lower may be used depending
on the noise level from the input supply and the input
current to the converter. In Buck application, the output
capacitor is typically ceramic and selection is mainly
based on the output voltage ripple requirements. The
output ripple, ΔVOUT, is determined by the following
equation :
1
VOUT IL ESR +
8
f
C
OUT
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-8 package, the thermal resistance, θJA, is 188°C/W
on a standard JEDEC 51-7 four-layer thermal test board.
The maximum power dissipation at TA = 25°C can be
calculated by the following formula :
PD(MAX) = (125°C − 25°C) / (188°C/W) = 0.53W for
SOP-8 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 4 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Maximum Power Dissipation (W)1
The inductor must also be selected with a saturation
current rating greater than the maximum inductor current
during normal operation. The maximum inductor current
can be calculated by the following equations.
1.0
Four-Layer PCB
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 4. Derating Curve of Maximum Power Dissipation
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
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December 2014
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9
RT8477
Layout Considerations
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guide lines are as follows :
The power components M1, L1, D1 and C4 must be
placed as close to each other as possible to reduce the
ac current loop area. The PCB trace between power
components must be as short and wide as possible
due to large current flow through these traces during
operation.
Place M1, L1 and D1 as close to each other as possible.
The trace should be as short and wide as possible.
The input capacitor C5 must be placed as close to VCC
pin as possible.
Keep the ISP and ISN with
the Kelvin sense connection
VIN power trace to ISP
must be wide and short.
ISP
VIN
R1
C1
ISN
D1
...
Locate input capacitor as
close VCC as possible.
GND
L1
C2
8
CREG
ISP
2
7
DRV
ISN
CTL
3
6
GND
4
5
SENS
VCC
C5
GND
C4
R4
R3
C3
Power trace must be wide
and short when compared
to the normal trace.
M1
Place these components as
close as possible.
R2
GND
Normal trace.
Figure 5. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
DS8477-02
December 2014
RT8477
Outline Dimension
H
A
M
J
B
F
C
I
D
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
4.801
5.004
0.189
0.197
B
3.810
3.988
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.508
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.170
0.254
0.007
0.010
I
0.050
0.254
0.002
0.010
J
5.791
6.200
0.228
0.244
M
0.400
1.270
0.016
0.050
8-Lead SOP Plastic Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot
assume responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be
accurate and reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third
parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
DS8477-02
December 2014
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