®
RT8480
High Voltage Boost/SEPIC Controller
General Description
Features
The RT8480 is a current mode PWM controller designed
to drive an external MOSFET for high current LED
applications. With a low side current sense amplifier
threshold of 190mV, the LED current is programmable
with one external current sense resistor.
High Voltage Capability : VIN Up to 36V, VOUT is
limited by External MOSFET Switch
Boost Operation
Current Mode PWM with Programmable Switching
Frequency
Easy Dimming Control : Analog or Digital
Converting to Analog with One External Capacitor
True PWM Dimming : External FET Driver is BuildIn
Programmable Soft-Start to Avoid Inrush Current
Programmable Over Voltage Protection
VIN Under Voltage Lockout and Thermal Shutdown
16-Lead SOP Package
RoHS Compliant and Halogen Free
With programmable operating frequency up to 800kHz,
the external inductor and capacitors can be small while
maintaining high frequency.
Dimming can be done by either analog or digital. A built-in
clamping comparator and filter allow easy low noise analog
dimming conversion from digital signal with only one
external capacitor. An unique True PWM dimming control
is made easy with MOSFET under LED string. A very high
dimming ratio can be achieved by adopting both analog/
digital dimming and True PWM dimming together.
The RT8480 is available in a SOP-16 package.
Applications
Ordering Information
RT8480
Package Type
S : SOP-16
Lead Plating System
G : Green (Halogen Free and Pb Free)
Note :
Richtek products are :
RoHS compliant and compatible with the current requirements of IPC/JEDEC J-STD-020.
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
General Industrial High Power LED Lighting
Desk Lights and Room Lighting
Building and Street Lighting
Industrial Display Backlight
Pin Configurations
(TOP VIEW)
GBIAS
GATE
PWMOUT
ISW
PWMDIM
ISP
ISN
VC
2
16
15
3
14
4
5
13
12
6
7
11
10
8
9
GND
VCC
RSET
OVP
EN
SS
DCTL
ACTL
SOP-16
RT8480GS : Product Number
RT8480
GSYMDNN
YMDNN : Date Code
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8480-03
June 2016
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT8480
Typical Application Circuit
L1
47µH
VIN
6V to 36V
VOUT
CIN
10µF
RT8480
15
5V
RRSET
30k
Analog
Dimming
RVC
1.8k
GATE 2
VCC
12 EN
ISW 4
14 RSET
ISP
9 ACTL
10 DCTL
8 VC
ISN
OVP
11
CSS
0.1µF
CVC
10nF
D1
SS
1 GBIAS
CB
1µF
PWMOUT
COUT
1µF
M1
14 LEDs
RSW
0.05
6
RSENSE
7
R1
13
3
VOUT
R2
PWMDIM 5
GND
16
Figure 1. Analog Dimming in Boost Configuration
L1
47µH
VIN
6V to 36V
VOUT
CIN
10µF
RT8480
15
PWM
Dimming control
CSS
0.1µF
CB
1µF
COUT
1µF
M1
14 LEDs
RSW
0.05
ISW 4
6
ISP
10 DCTL
RSENSE
8 VC
11
SS
1 GBIAS
9 ACTL
RVC
1.8k
CVC
10nF
GATE 2
VCC
12 EN
14 RSET
5V
RRSET
30k
D1
ISN
OVP
PWMOUT
7
R1
13
VOUT
3
R2
PWMDIM 5
GND
16
CA
0.47µF
Figure 2. PWM to Analog Dimming in Boost Configuration
L1
47µH
VIN
6V to 36V
VOUT
CIN
10µF
RT8480
15
RRSET
30k
9 ACTL
10 DCTL
RVC
1.8k
CVC
10nF
VCC
12 EN
14 RSET
5V
CSS
0.1µF
D1
GATE 2
M1
ISP
14 LEDs
RSW
0.05
ISW 4
PWMOUT
COUT
1µF
3
M2
6
7
8 VC
ISN
5
11
PWMDIM
SS
13
1 GBIAS
OVP
GND
CB
16
1µF
RSENSE
R1
VOUT
R2
Figure 3. True PWM Dimming in Boost Configuration
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is a registered trademark of Richtek Technology Corporation.
DS8480-03
June 2016
RT8480
L1
47µH
VIN
10V to 36V
CIN
10µF
RT8480
15
5V
RRSET
30k
Analog
GATE 2
VCC
12 EN
ISW 4
14 RSET
ISP
CSS
0.1µF
ISN
8 VC
11
SS
1 GBIAS
RVC
1.8k
VOUT
100V
M1
RSW
0.05
100k
6
190
9 ACTL
10 DCTL
Dimming
CVC
10nF
D1
CB
1µF
OVP
PWMOUT
7
R1
13
3
VOUT
R2
PWMDIM 5
GND
16
Figure 4. Constant Voltage Output of Boost Converter
L1
47µH
VIN
10V to 36V
CIN
10µF
RT8480
15
5V
RRSET
30k Analog
Dimming
GATE 2
VCC
12 EN
ISW 4
14 RSET
ISP
CSS
0.1µF
ISN
8 VC
11
SS
1 GBIAS
CB
1µF
M1
RSW
0.05
D1
L2
47µH
VOUT
24V
24k
6
190
9 ACTL
10 DCTL
RVC
1.8k
CVC
10nF
1µF / 100V
OVP
PWMOUT
7
R1
13
3
VOUT
R2
PWMDIM 5
GND
16
Figure 5. Constant Voltage Output of SEPIC Converter
L1
47µH
VIN
10V to 36V
CIN
10µF
RT8480
GATE 2
15
VCC
12 EN
5V
ISW 4
14 RSET
RRSET
30k Analog
Dimming
RVC
1.8k
CVC
10nF
PWMOUT
ISP
9 ACTL
10 DCTL
8 VC
11
CSS
0.1µF
1µF / 100V
RSW
0.05
L2
47µH
3
VOUT
24V
COUT
1µF
M2
6
190
ISN
SS
1 GBIAS
CB
1µF
M1
D1
PWMDIM
GND
16
OVP
7
5
R1
13
VOUT
R2
Figure 6. True PWM Dimming in SEPIC Application
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8480-03
June 2016
is a registered trademark of Richtek Technology Corporation.
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3
RT8480
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
GBIAS
Internal gate driver bias pin. A good bypass capacitor is required.
2
GATE
External MOSFET switch gate driver output pin.
3
PWMOUT
4
ISW
5
PWMDIM
Output pin for the PWM dimming MOSFET driver.
External MOSFET switch current sense pin. Connect the current sense resistor
between external N-MOSFET switch and the ground.
Control input pin for the PWM dimming MOSRET driver.
6
ISP
7
ISN
8
VC
9
ACTL
LED current sense amplifier positive input.
LED current sense amplifier negative input. Voltage threshold between ISP and ISN is
190mV.
PWM control loop compensation pin.
Analog dimming control pin. The effective programming voltage range of the pin is
between 0.3V and 1.2V.
10
DCTL
PWM dimming control pin, By adding a 0.47F filtering capacitor on the ACTL pin, the
PWM dimming signal on the DCTL pin can be averaged and converted into analog
dimming signal on the ACTL pin following the formula below. VACTL = 1.2V x PWM
Dimming Duty Cycle.
11
SS
Soft-start pin. A capacitor of at least 100nF is required for proper soft-start.
12
EN
Chip enable (active high). When this pin voltage is low, the chip is in shutdown mode.
13
OVP
Over voltage protection pin. The PWM converter turns off when the voltage of the pin
goes to higher than 1.2V.
14
RSET
Switching frequency set pin connect a resistor from RSET to GND. fRSET = 30k will set
fSW = 380kHz.
15
VCC
The power supply pin of the chip. For good bypass, a low ESR capacitor is required.
16
GND
Ground.
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is a registered trademark of Richtek Technology Corporation.
DS8480-03
June 2016
RT8480
Functional Block Diagram
VOC
8.5V
EN
+
1.4V
RSET
VCC
+
-
Shutdown
-
GBIAS
S
OSC
GATE
-
5V (typ.)
5V
+
R
OVP
1.2V
+
R
-
R
100k
GBIAS
R
PWMOUT
PWMDIM
+
-
-
110mV
+
VC
ISW
ISP
ISN
GM
+
6µA
SS
1.2V
DCTL
+
+
-
-
GND
ACTL
VISP – VISN
(mV)
190
0
0.25
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
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June 2016
1.25
VACTL (V)
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RT8480
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, VCC ------------------------------------------------------------------------------------------------- 38V
GBIAS, GATE, PWMDIM, PWMOUT ---------------------------------------------------------------------------------- 10V
ISW
DC ------------------------------------------------------------------------------------------------------------------------------- 1V
< 200ns ------------------------------------------------------------------------------------------------------------------------ 6V
ISP, ISN
DC ------------------------------------------------------------------------------------------------------------------------------- 2V
< 200ns ------------------------------------------------------------------------------------------------------------------------ 6V
DCTL, ACTL, OVP Pin Voltage ------------------------------------------------------------------------------------------ 8V (Note 2)
EN Pin Voltage --------------------------------------------------------------------------------------------------------------- 20V
Power Dissipation, PD @ TA = 25°C
SOP-16 ------------------------------------------------------------------------------------------------------------------------ 1.176W
Package Thermal Resistance (Note 3)
SOP-16, θJA ------------------------------------------------------------------------------------------------------------------ 85°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 Mode) ------------------------------------------------------------------------------------------------ 2kV
MM (Machine Mode) -------------------------------------------------------------------------------------------------------- 200V
Recommended Operating Conditions
(Note 5)
Supply Input Voltage Range, VCC --------------------------------------------------------------------------------------- 6V to 36V
Junction Temperature Range ---------------------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VCC = 24V, No Load, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Overall
Supply Current
ICC
VC 0.4V (switching off)
--
6
7.2
mA
Shutdown Current
ISHDN
VEN 0.7V
--
12
--
A
EN Threshold
Voltage
Logic-High
VIH
2
--
--
Logic-Low
VIL
--
--
0.5
--
--
3
A
170
190
210
mV
VEN 3V
EN Input Current
V
Current Sense Amplifier
Input Threshold (VISP VISN)
ISP / ISN Input Current
VC Output Current
IISP / IISN
VISP VISN = 0V
--
20
--
A
IVC
VISP VISN = 190mV,
0.5V VC 2.4V
--
20
--
A
--
0.7
--
V
VC Threshold for PWM Switch
Off
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is a registered trademark of Richtek Technology Corporation.
DS8480-03
June 2016
RT8480
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
VACTL = 1.2V
--
1
--
VACTL = 0.3V
--
10
--
VACTL_ON
--
1.25
--
V
VACTL_OFF
--
0.25
--
V
--
--
0.5
A
300
380
460
kHz
--
280
--
ns
IGBIAS = 20mA
--
8.2
--
V
IGate = 20mA
--
7.2
--
IGate = 100A
--
7.5
--
IGate = 100A
--
0.5
--
V
1nF load at GATE
--
15
--
ns
ILIM_SW
--
90
--
mV
Logic-High
VPWMDIMH
2
--
--
Logic-Low
VPWMDIML
--
--
0.5
LED Dimming
ACTL Input Current
IACTL
LED Current On Threshold at
ACTL
LED Current Off Threshold at
ACTL
DCTL Input Current
IDCTL
0.3V VDCTL 6V
fSW
RRSET = 30k
A
PWM Control
Switching Frequency
Minimum OFF Time
(Note 6)
Switch Gate Driver
GBIAS Voltage
VGBIAS
Gate Voltage High
VGate_H
Gate Voltage Low
VGate_L
GATE Drive Rise and Fall Time
PWM Switch Current Limit
Threshold
PWM Dimming Gate Driver
PWMDIM
Threshold
Voltage
PWMOUT Output Voltage
V
V
VPWMOUTH
IPWMOUT = 1mA
--
7.5
--
VPWMOUTL
IPWMOUT = 100A
--
0.45
--
1nF load at PWMOUT
--
40
--
ns
1.12
1.18
1.24
V
PWMOUT Drive Rise and Fall
Time
OVP and Soft-Start
V
OVP Threshold
VOVP_th
OVP Input Current
IOVP
0.7V VOVP 1.5V
--
--
0.1
A
Soft-Start Current
ISS
VSS 2V
--
6
--
A
Thermal Protection
Thermal Shutdown
TSD
--
145
--
C
Thermal Shutdown Hysteresis
TSD
--
10
--
C
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8480-03
June 2016
is a registered trademark of Richtek Technology Corporation.
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RT8480
Note 1. Stresses beyond those listed under “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, ACTL and DCTL can go up to 36V.
Note 3. θJA is measured at TA = 25°C on a low effective thermal conductivity single-layer test board per JEDEC 51-3.
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.
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is a registered trademark of Richtek Technology Corporation.
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RT8480
Typical Operating Characteristics
Efficiency vs. Input Voltage
LED Current vs. ACTL PWM Duty
100.0
350
97.5
300
250
LED Current (mA)
Efficiency (%)
95.0
92.5
90.0
87.5
85.0
200
150
100
50
82.5
VIN = 24V
15 LEDs, IOUT = 300mA
80.0
12
15
18
21
24
27
30
33
0
0
36
20
40
Input Voltage (V)
LED Current vs. Input Voltage
RSET vs. Switching Frequency
Switching Frequency (kHz)1
LED Current (mA)
100
1200
350
300
250
200
150
100
50
1000
800
600
400
200
VIN = 24V
VIN = 24V to 36V
0
0
12
16
20
24
28
32
36
10
20
30
40
50
60
70
80
90
100
(kΩ)
RSET (K)
Input Voltage (V)
Switching Frequency vs. Input Voltage
VISP – VISN Threshold vs. Input Voltage
220
840
205
760
Switching Frequency (kHz)1
VISP – VISN Threshold (mV) 1
80
PWM Duty (%)
400
190
175
160
145
130
115
RRSET = 20kΩ
680
600
RRSET = 36kΩ
520
440
360
RRSET = 10kΩ
280
100
6
9
12
15
18
21
24
27
30
33
Input Voltage(V)
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
DS8480-03
60
June 2016
36
39
6
9
12
15
18
21
24
27
30
33
36
39
Input Voltage (V)
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RT8480
Supply Current vs. Input Voltage
10
390
9
380
8
Supply Current (mA)
Switching Frequency (kHz)1
Switching Frequency vs. Temperature
400
370
360
350
340
330
320
7
6
5
4
3
2
310
1
VIN = 24V
300
-50
-25
0
25
50
75
100
0
125
6
9
12
15
9
9
8
7
6
5
4
3
2
1
0
25
50
27
30
33
36
39
75
100
8
7
6
5
4
3
2
1
VIN = 24V
0
24
Shutdown Current vs. Input Voltage
10
Shutdown Current (μA)1
Supply Current (mA) 1
Supply Current vs. Temperature
10
-25
21
Input Voltage (V)
Temperature (°C)
-50
18
VEN = 0V
0
125
6
9
12
15
Temperature (°C)
18
21
24
27
30
33
36
39
33
36
39
Input Voltage (V)
OVP vs. Input Voltage
Soft-Start Current vs. Input Voltage
1.20
8.0
1.19
7.0
6.5
6.0
OVP (V)
Soft-Start Current (μA)
7.5
5.5
5.0
1.18
OVP_H
1.17
OVP_L
1.16
4.5
4.0
1.15
3.5
1.14
3.0
6
9
12
15
18
21
24
27
30
33
Input Voltage(V)
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10
36
39
6
9
12
15
18
21
24
27
30
Input Voltage (V)
is a registered trademark of Richtek Technology Corporation.
DS8480-03
June 2016
RT8480
GBIAS Voltage vs. Input Voltage
10
0.235
9
0.230
8
GBIAS Voltage (V)
ACTL Threshold (V) 1
ACTL Threshold vs. Input Voltage
0.240
0.225
0.220
0.215
0.210
7
6
5
4
0.205
3
0.200
2
6
9
12
15
18
21
24
27
30
33
36
39
1nF Load
6
9
12
15
Input Voltage(V)
ISW Threshold (mV)
Gate Output Voltage (V)
27
30
33
36
39
140
8
Gate_High
7
6
5
4
3
2
Gate_Low
1
130
120
110
100
90
80
70
60
0
50
6
9
12
15
18
21
24
27
30
33
36
39
6
ACTL
(5V/Div)
EN
(5V/Div)
GATE
(5V/Div)
GATE
(5V/Div)
VOUT
(50V/Div)
I LED
(500mA/Div)
VOUT
(50V/Div)
I LED
(500mA/Div)
Time (500μs/Div)
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
June 2016
12
15
18
21
24
27
30
33
36
39
Power On from EN
PWM Dimming Response
VIN = 24V, fPWM = 1kHz, Duty = 50%
9
Input Voltage (V)
Input Voltage(V)
DS8480-03
24
150
1nF Load
9
21
ISW Threshold vs. Input Voltage
Gate Output Voltage vs. Input Voltage
10
18
Input Voltage (V)
VIN = 24V, IOUT = 360mA, CSS = 0.1μF
Time (10ms/Div)
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RT8480
Application Information
The RT8480 is a constant frequency, current mode
controller which drives an external MOSFET for PWM LED
applications, DC/DC Boost, SEPIC and Flyback converter.
When using an external load switch, the PWMDIM input
not only drives PWMOUT, but also enables controller GATE
switching and error amplifier operation. This feature provides
extremely fast, true PWM load switching with no transient
overvoltage.
In normal operation with PWMDIM high, GATE goes high
and the power MOSFET is turned on. When the oscillator
sets the PWM latch, the power MOSFET is turned off
when the VC current comparator resets the latch. When
the load current increases, a fall in the ISN voltage relative
to the reference voltage at ISP causes the VC pin to rise
and the average inductor current will therefore rise until it
equals the load current. When PWMDIM goes low,
PWMOUT goes low, VC opens and GATE switching is
disabled. Lowering PWMOUT and disabling GATE causes
the output capacitor, COUT, to hold the output voltage
constant in the absence of load current.
Power on
sequence
VIN
UVLO
PWM
EN must be turned
on later than VIN
and PWM signal EN must be turned
off early than VIN
and PWM signal
No Soft-Start
Soft-Start
if PWM turns
on later
EN
VOUT
Figure 7. Power On Sequence Control by EN
Power on
sequence
Power off
sequence
UVLO
PWM
EN
VIN must be
turned off parlier
than EN and PWM
signal
Soft-Start
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12
No Soft-Start
if PWM turns
on late
Figure 8. Power On Sequence Control by VIN
EN and/or VIN should
be pulled low once
PWM pulls low for
over 10ms
Power Sequence
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
VIN must be
turned on later
than EN and
PWM signal
VOUT
EN/VIN
Please refer to below Figure 7 and 7. The recommended
power-on sequence suggests the PWM to be ready before
EN and/or VIN is ready. If not, the soft-start function will
be disabled. As for power-off sequence, EN/VIN must be
pulled low within 10ms to prevent “Hard-Start” as shown
Figure 9.
Abnormal Power
on sequence
VIN
Input UVLO
The input operating voltage range of the RT8480 is 6V to
36V. An input capacitor at the VCC pin can reduce ripple
voltage. It is recommended to use a ceramic 10μF or larger
capacitance as the input capacitor. This IC provides an
Under Voltage Lockout (UVLO) function to enhance the
stability when startup. The UVLO threshold of input rising
voltage is set at 4.5V typically with a 0.7V hysteresis.
Abnormal Power
on sequence
Power off
sequence
PWM
10ms
Figure 9. To Prevent “Hard-Start” Sequence
is a registered trademark of Richtek Technology Corporation.
DS8480-03
June 2016
RT8480
Soft-Start
The soft-start of the RT8480 can be achieved by connecting
a capacitor from the SS pin to GND. The built-in soft-start
circuit reduces the start-up current spike and output
voltage overshoot. The soft-start time is determined by
the external capacitor charged by an internal 6μA constant
charging current. The SS pin directly limits the rate of
voltage rise on the VC pin, which in turn limits the peak
switch current.
The soft-start interval is set by the soft-start capacitor
selection according to the equation :
tSS = CSS
2.4V
6 A
(s)
A typical value for the soft-start capacitor is 0.1μF. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch. The soft-start capacitor
is discharged when EN/UVLO falls below its threshold
during an over-temperature event or during a GBIAS under
voltage event.
GBIAS Regulator Operation
The GBIAS pin requires a capacitor for stable operation
and to store the charge for the large GATE switching
currents. Choose a 10V rated low ESR, X7R or X5R
ceramic capacitor for best performance. The value of the
capacitor is determined primarily by the stability of the
regulator rather than the gate charge of the switching
N-MOSFET. A 1μF capacitor will be adequate for most
applications.
Place the capacitor close to the IC to minimize the trace
length to the GBIAS pin and also to the IC ground. An
internal current limit on the GBIAS protects the RT8480
from excessive on-chip power dissipation.
If the input voltage, VIN, is less than 8V, then the GBIAS
pin should be connected to the input supply. Be aware
that if GBIAS supply is used to drive extra circuits besides
RT8480, typically the extra GBIAS load should be limited
to less than 10mA.
Loop Compensation
The RT8480 uses an internal error amplifier via the
compensation pin (VC) to optimize the loop response for
specific application. The external inductor, output capacitor,
Copyright © 2016 Richtek Technology Corporation. All rights reserved.
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June 2016
and compensation resistor and capacitor determine the
loop stability. The inductor and output capacitor are chosen
based on performance, size and cost. The compensation
resistor and capacitor at VC are selected to optimize
control loop response and stability.
An external resistor in series with a capacitor is connected
from the VC pin to GND to provide a pole and a zero for
proper loop compensation. The typical compensation for
the RT8480 is 1.8kΩ and 10nF.
LED Current Setting
The maximum current is programmed by placing an
appropriate valued sense resistor at the LED string. When
the voltage of ACTL is higher than 1.25V, the LED current
can be calculated by the following equation :
190mV
ILED(MAX) =
(mA)
RSENSE
where R SENSE is the resistor between the external
regulating N-MOSFET and GND.
The ACTL pin should be tied to a voltage higher than 1.25V
to get the full-scale 190mV (typical) threshold across the
sense resistor. The ACTL pin can also be used to dim the
LED current to zero, although relative accuracy decreases
with the decreasing voltage sense threshold. When the
ACTL pin voltage is less than 1.25V, the LED current is :
ILED =
(VACTL 0.25) 190mV
RSENSE
(mA)
The ACTL pin can also be connected with a thermistor to
provide over-temperature protection for the LED load, or
with a resistive voltage divider to VIN to reduce output
power and switching current when VIN is low.
Brightness Control
For LED applications where a wide dimming range is
required, two competing methods are available: analog
dimming and PWM dimming. The easiest method is to
simply vary the DC current through the LED by analog
dimming.
The RT8480 features both analog and digital dimming
control. Analog dimming is linearly controlled by an
external voltage (0.25V to 1.25V) at the ACTL pin. Digital
dimming can be implemented by driving a PWM signal at
the DCTL pin for linear current regulator. A very high
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RT8480
Dimming frequency can be sufficiently adjusted from
100Hz to 30kHz. However, LED current cannot be 100%
proportional to duty cycle especially for high frequency
and low duty ratio because of physical limitation caused
by internal switching frequency.
Typically, in order to avoid visible flicker, PWM dimming
signal should be greater than 120Hz. Assuming inductor
and capacitor sizing close to discontinuous operation, two
fOSC cycles are sufficient for proper PWM operation. Thus,
the minimum dimming duty can be as low as 1% for the
frequency range from 100Hz to 300Hz. For the dimming
frequency from 300Hz to 1kHz, the duty is about 5%. If
the frequency is increased to 1kHz to 30kHz, the duty
will be about 10%.
1/fPWM
Duty/fPWM
Table 1. Switching Frequency vs RT Value (1%
Resistors)
f OSC (kHZ)
RRSET (k)
800
10.6
600
15.81
500
20.26
300
35.8
200
47.6
Frequency vs. RRSET
900
800
Frequency (kHz)1
contrast ratio can be obtained via true digital PWM
dimming, which is achieved by driving ACTL pin with a
PWM signal. The recommended PWM frequency rangle
is 100Hz to 10kHz.
700
600
500
400
300
200
10
15
20
25
30
35
40
45
50
RRSET (Ω)
( )
Figure 11. Switching Frequency vs RRSET
N>2
1/fOSC
N : the number of fOSC cycles per PWM
cycle
Figure 10. PWM Dimming Parameters
Programmable Switching Frequency
The RSET frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz for optimized
efficiency and performance or external component size.
Higher frequency operation allows for smaller component
size but increases switching losses and gate driving
current, and may not allow sufficiently high or low duty
cycle operation. Lower frequency operation gives better
performance but with larger external component size. For
an appropriate RRSET resistor value see Table 1 or Figure
11. An external resistor from the RSET pin to GND is
required-do not leave this pin open.
Input Over Current Protection
The resistor, RSW, between the source of the external
switching N-MOSFET and GND should be selected to
provide adequate switch current.
The RT8480 senses the inductor current through ISW pin
in the switch on period. The duty cycle depends on the
current sense signal summed with the internal slope
compensation and compared to the VC pin signal. The
external N-MOSFET will be turned off when the current
signal is larger than the VC pin signal. In the off period,
the inductor current will descend. The external N-MOSFET
is turned on by the oscillator at the beginning of the next
cycle. To drive the application without exceeding the 90mV
(typical) current limit threshold on the ISW pin of the
RT8480, it is recommended to select a resistor that gives
a switch current of at least 20% greater than the required
LED current according to :
V 0.1V
RSW =( IN
)
VOUT IOUT
The ISW pin input to the RT8480 should be a Kelvin
connection to the positive terminal of RSW.
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is a registered trademark of Richtek Technology Corporation.
DS8480-03
June 2016
RT8480
Output Over Voltage Protection Setting
Power MOSFET Selection
The RT8480 is equipped with an Over Voltage Protection
(OVP) function. When the voltage at the OVP pin exceeds
a threshold of approximately1.18V, the power switch will
be turned off. The power switch can be turned on again
once the voltage at the OVP pin drops below 1.18V. The
output voltage could be clamped at a certain voltage level
set by the following equation :
R1
VOUT, OVP = 1.18 (1
)
R2
For applications operating at high input or output voltages,
the power N-MOSFET switch is typically chosen for drain
voltage, VDS, rating and low gate charge. Consideration of
switch on-resistance, R DS(ON), is usually secondary
because switching losses dominate power loss. The
GBIAS regulator on the RT8480 has a fixed current limit
to protect the IC from excessive power dissipation at high
VIN, so the N-MOSFET should be chosen such that the
product of Qg at 5V and switching frequency does not
exceed the GBIAS current limit.
where R1 and R2 are the voltage divider resistors from
VOUT to GND with the divider center node connected to
the OVP pin
If at least one string is in normal operation, the controller
will automatically ignore the open strings and continue to
regulate the current for the string(s) in normal operation.
Over Temperature Protection
The RT8480 provides an over temperature protection (OTP)
function to prevent the excessive power dissipation from
overheating the device. The OTP function will shut down
switching operation when the die junction temperature
exceeds 145°C. The chip will automatically start to switch
again when the die junction temperature is reduced by
approximately 10°C.
Inductor Selection
The inductor used with the RT8480 should have a
saturation current rating appropriate to the maximum
switch current. Choose an inductor value based on
operating frequency, input and output voltage to provide a
current mode ramp of approximately 20mV magnitude on
the ISW pin during the switch on-time. The following
equations are useful to estimate the inductor value :
L=
(VOUT VIN ) (VIN )2
2 IOUT f (VOUT )2
where,
VOUT = Maximum output voltage.
VIN = Minimum input voltage.
f = Operating frequency.
Moreover, to obtain better conversion efficiency, GATE
high level must be higher than the gate threshold voltage
Vgs(th) of the power N-MOSFET. If GATE high level is
lower, an external totem pole circuit has to be added.
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for RT8480
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are also important
parameters for Schottky diode selection. Choose a
suitable Schottky diode with reverse voltage rating greater
than the maximum output voltage. 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). If using
the PWM feature for dimming, it is important to consider
diode leakage, which increases with temperature, from
the output during the PWM low interval. Therefore, a
Schottky diode with sufficiently low leakage current is
suggested.
ID, PEAK = 1.2 IOUT (
VOUT VIN
)
VOUT
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most of the RT8480 applications, a 10μF 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.
IOUT = Sum of current from all LED strings.
η is the efficiency of the power converter.
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DS8480-03
June 2016
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15
RT8480
1.2
Maximum Power Dissipation (W)1
In Boost Application, the output capacitor is typically a
ceramic capacitor and is selected based on the output
voltage ripple requirements. The minimum value of the
output capacitor, COUT, is approximately given by the
following equation :
I
(VOUT VIN )
COUT = OUT
VRIPPLE VOUT f
For LED applications, the equivalent resistance of the LED
is typically low and the output filter capacitor should be
sized to attenuate the current ripple. Use of X7R type
ceramic capacitors is recommended. Lower operating
frequencies will require proportionately higher capacitor
values.
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 :
by the following formula :
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
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16
50
75
100
125
Figure 12. Derating Curve for RT8480 Package
Layout Consideration
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guidelines are suggested as follows :
The power components L1, D1, CIN, M1 and COUT must
be placed as close to each other as possible to reduce
the ac current loop area. At least one via to the ground
plane immediately under the exposed pad. The ground
trace on the top layer of the PC board should be as
wide and short as possible to minimize series resistance
and inductance.
Place L1 and D1 connected to N-MOSFET as close to
each other as possible. The trace should be as short
and wide as possible.
The input capacitor, CIN, must be placed as close to the
VCC pin as possible.
Place the compensation components as close to the
VC pin as possible to avoid noise pick up.
VOUT
D1
PD(MAX) = (125°C − 25°C) / (85°C/W) = 1.176W for
SOP-16 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. For the RT8480 package, the derating
curve in Figure 12 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
25
Ambient Temperature (°C)
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
For recommended operating condition specifications of
the RT8480, the maximum junction temperature is 125°C
and TA is the ambient temperature. The junction to ambient
thermal resistance, θJA, is layout dependent. For SOP16 packages, the thermal resistance, θJA, is 85°C/W on a
standard JEDEC 51-7 single-layer thermal test board. The
maximum power dissipation at TA = 25°C can be calculated
1.0
0
PD(MAX) = (TJ(MAX) − TA) / θJA
thermal resistance.
Single-Layer PCB
1.1
L1
VIN
M1
GND
M2
GBIAS
GATE
PWMOUT
ISW
PWMDIM
ISP
ISN
VC
16
2
15
3
14
4
13
5
12
6
11
7
10
8
9
GND
VCC
RSET
OVP
EN
SS
DCTL
ACTL
VOUT
Analog Dimming
Figure 13. PCB Layout Guide
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DS8480-03
June 2016
RT8480
Outline Dimension
H
A
M
B
J
F
C
I
D
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
9.804
10.008
0.386
0.394
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
0.178
0.254
0.007
0.010
I
0.102
0.254
0.004
0.010
J
5.791
6.198
0.228
0.244
M
0.406
1.270
0.016
0.050
16–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.
DS8480-03
June 2016
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