RT8474 - Richtek

®
RT8474
High Voltage Multiple-Topology LED Driver with Dimming Control
General Description
Features
The RT8474 is a current-mode LED driver supporting wide
input voltage range from 4.5V to 50V and output voltage
up to 50V. With internal 500kHz 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 from either analog or PWM signal. The RT8474
provides an internal soft-start function to avoid inrush
current and thermal shutdown to prevent the device from
overheat.

High Voltage : VIN Up to 50V, VOUT Up to 50V

Support Multiple-Topologies (Buck / Boost / BuckBoost)
Built-In 2A Power Switch
Current-Mode PWM Control
500kHz 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 RT8474 is available in the SOP-8 (Exposed pad)
package.
Applications
Desk Lights and Room Lighting
Industrial Display Backlight

Ordering Information

RT8474
Package Type
SP : SOP-8 (Exposed-Option 2)
Marking Information
RT8474GSP : Product Number
Lead Plating System
G : Green (Halogen Free and Pb Free)
RT8474
GSPYMDNN
Note :
YMDNN : Date Code
Richtek products are :

RoHS compliant and compatible with the current require-
Pin Configurations
ments of IPC/JEDEC J-STD-020.

(TOP VIEW)
Suitable for use in SnPb or Pb-free soldering processes.
8
VCC
ISP
2
ISN
3
NC
4
GND
CREG
7
SW
6
CTL
5
VC
9
SOP-8 (Exposed Pad)
Simplified Application Circuit
D1
VIN
R3
C1
RT8474
VCC
C5
ISP
Analog Dimming
or PWM Dimming
ISN
L1
SW
R1
C2
C3
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DS8474-04 April 2015
C4
CTL
VC
R2
RSENSE
CREG
GND
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RT8474
Functional Pin Description
Pin No.
Pin Name
Supply Voltage Input. For good bypass, connect a low ESR capacitor
between this pin and GND.
1
VCC
2
ISP
3
ISN
4
NC
5
VC
6
CTL
7
SW
8
CREG
9 (Exposed Pad)
Pin Function
Positive Current Sense Input.
Negative Current Sense Input. Voltage threshold between ISP and ISN is
100mV.
No Internal Connection.
Compensation Node for Current Loop.
Analog Dimming Control Input. Effective programming range is 0.2V to
1.2V.
Switch Node of the PWM Converter.
Regulator Output for Internal Circuit. Place a 1F capacitor to stabilize the
5V output regulator.
Ground. The exposed pad must be soldered to a large PCB and connected
to GND for maximum power dissipation.
GND
Function Block Diagram
SW
S
OSC
-
VCC
4.5V
R
+
R
5V
LDO
CREG
+
-
VC
Soft-Start
GM
+
ISN
ISP
+
-
GND
Operation
CTL
The RT8474 is specifically designed to be operated in
Buck converter 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 and provides an output voltage at the
VC pin. A PWM comparator then turns off the internal
power switch when the sensed power switch current
exceeds the compensated VC pin 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
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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.
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. The RT8474 provides
protection functions which include over- temperature, and
switch current limit to prevent abnormal situations.
is a registered trademark of Richtek Technology Corporation.
DS8474-04 April 2015
RT8474
Absolute Maximum Ratings










(Note 1)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------SW Pin Voltage at Switching Off, ISP, ISN -------------------------------------------------------------------------CREG Voltage -------------------------------------------------------------------------------------------------------------CTL Voltage (Note 2) ---------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
−0.3V to 60V
−0.3V to 60V
−0.3V to 6V
−0.3V to 20V
SOP-8 (Exposed Pad) --------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 3)
SOP-8 (Exposed Pad), θJA ---------------------------------------------------------------------------------------------SOP-8 (Exposed Pad), θJC --------------------------------------------------------------------------------------------Junction Temperature ----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------Storage Temperature Range -------------------------------------------------------------------------------------------ESD Susceptibility (Note 4)
HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) -----------------------------------------------------------------------------------------------------
3.44W
Recommended Operating Conditions



29°C/W
2°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 5)
Supply Input Voltage ------------------------------------------------------------------------------------------------------ 4.5V to 50V
Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VCC = 5V, CIN = 1μF, 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
IVCC
VC  0.4V
--
--
3
VIN Under-Voltage Lockout
Threshold
VUVLO
VIN Rising
--
4.2
--
VIN Falling
--
3.8
--
VCTL  1.25V
97
100
103
mV
V
Current Sense Amplifier
Input Threshold (VISP VISN)
Input Current
IISP
VISP = 24V
--
200
--
A
Input Current
IISN
VISN = 24V
--
20
--
A
Output Current
IVC
2.4V > VC > 0.3V
--
±10
--
A
--
0.4
--
V
--
1
2
A
--
0.2
0.25
V
VC Threshold for CTL Switch Off
LED Dimming
Input Current of CTL Pin
ICTL
0.2V  VCTL  1.2V
LED Current off Threshold at CTL VCTL_OFF
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
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RT8474
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
440
490
540
kHz
--
--
100
%
Minimum On-Time
--
100
200
ns
SW RDS(ON)
--
0.15
--

2
2.5
--
A
--
5.7
--
ms
PWM Converter
Switch Frequency
fSW
Maximum Duty Cycle
DMAX
SW Current Limit
ILIM_SW
Soft-Start Time
Over-Temperature Protection
Thermal Shutdown Threshold
TSD
--
150
--
C
Thermal Shutdown Hysteresis
TSD
--
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, PWM 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. θJC is
measured at the exposed pad of the package.
Note 4. Devices are ESD sensitive. Handling precaution is recommended.
Note 5. The device is not guaranteed to function outside its operating conditions.
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is a registered trademark of Richtek Technology Corporation.
DS8474-04 April 2015
RT8474
Typical Application Circuit
Buck Configuration
VIN
4.5V to 50V
D1
R3
10
C1
RT8474
1 VCC
C5
1µF
Analog Dimming
or PWM Dimming
ISP
ISN
6 CTL
5 VC
R2
5.1M
R1
10k
C2
3.3nF
C3
1µF
2
R4
(Short
Option)
3
R5 (Short Option)
SW 7
8 CREG
RSENSE
100mV
C4
L1
GND
9 (Exposed Pad)
Note : VIN, VSW, VISP, VISN < 50V
Boost Configuration
L1
22µH
VIN
R2
10
Analog Dimming
or PWM Dimming
C1
10µF
R1
0.1
D1
VLED
50V (MAX)
C4
1µF
RT8474
1 VCC
SW
6 CTL
ISP
5 VC
ISN
7
C3
1µF
R3
10k
C5
3.3nF
C2
1µF
2
20R
51R
3
8 CREG
VZ
VF > VLED
GND
9 (Exposed Pad)
Note :
1. VIN, VSW, VISP, VISN < 50V
2. VLED : the voltage across the LED string
3. Vz : Zener diode breakdown voltage
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RT8474
Buck-Boost Configuration
L1
22µH
VIN
R2
10
C1
10µF
R1
0.1
RT8474
1 VCC
Analog Dimming
or PWM Dimming
D1
SW
7
VLED
C3
1µF
R3
10k
C5
3.3nF
C2
1µF
6 CTL
ISP
5 VC
ISN
2
3
20R
C4
1µF
51R
VZ
VF > VLED
8 CREG
GND
9 (Exposed Pad)
Note :
1. VSW < 50V, VIN + VLED < 50V
2. VLED : the voltage across the LED string
3. Vz : Zener diode breakdown voltage
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RT8474
Typical Operating Characteristics
LED Current vs. VCTL
450
95
400
90
350
LED Current (mA)
Efficiency (%)
Efficiency vs. Input Voltage
100
85
80
LED = 6pcs
LED = 5pcs
LED = 4pcs
LED = 3pcs
LED = 2pcs
LED = 1pcs
75
70
65
60
300
250
200
150
100
55
50
IOUT = 340mA, L = 47μH
50
RSENS = 300mΩ, LED = 6pcs
0
5
10
15
20
25
30
35
40
45
50
0
0.5
1
Input Voltage (V)
Supply Current vs. VCC
2.5
3
ISP-ISN Threshold vs. Temperature
104
ISP-ISN Threshold (mV)
2.25
Supply Current (mA)
2
105
2.30
2.20
2.15
2.10
2.05
2.00
1.95
103
102
101
100
99
98
97
96
ICC
VCC = 24V
95
1.90
0
5
10
15
20
25
30
35
40
45
-50
50
-25
0
25
50
75
100
VCC (V)
Temperature (°C)
SW RDS(ON) vs. VCC
SW RDS(ON) vs. Temperature
125
0.30
0.180
0.178
0.26
RDS(ON) ( Ω)
RDS(ON) (Ω)
1.5
VCTL (V)
0.176
0.174
0.22
0.18
0.172
VCC = 24V
VCC = 24V
0.14
0.170
0
5
10
15
20
25
30
35
40
45
VCC (V)
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50
-50
-25
0
25
50
75
100
125
Temperature (°C)
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RT8474
Frequency vs. VCC
Power On from VCC
535
534
VIN
(20V/Div)
Frequency (kHz)1
533
532
531
VOUT
(20V/Div)
530
529
528
IOUT
(200mA/Div)
527
526
VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs
525
0
5
10
15
20
25
30
35
40
45
50
Time (25ms/Div)
VCC (V)
Switching
Power Off from VCC
VIN
(20V/Div)
SW
SW
(20V/Div)
V IN
VOUT
(20V/Div)
IOUT
(200mA/Div)
V
VIN
= 24V,
24V, IIOUT
= 340mA,
340mA, LL == 47μH,
47μH, LED
LED == 6pcs
6pcs
IN =
OUT =
Time (50ms/Div)
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VIN
(10V/Div)
VOUT
(10V/Div)
IOUT
(200mA/Div)
V OUT
IOUT
VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs
Time (1μs/Div)
is a registered trademark of Richtek Technology Corporation.
DS8474-04 April 2015
RT8474
Application Information
The RT8474 is specifically designed to be operated in
Buck converter 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 and provides an output voltage at the
VC pin. A PWM comparator then turns off the internal
power switch when the sensed power switch current
exceeds the compensated VC pin 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 by the control loop.
Frequency Compensation
The RT8474 has an external compensation pin, allowing
the loop response to be optimized for specific applications.
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 value for the
RT8474 is 10k and 3.3nF.
LED Current Setting
The LED current can be calculated by the following
equation :
 VISP  VISN 
ILED(MAX) =
RSENSE
where (VISP − VISN) is the voltage between the ISP and
ISN pins (100mV typ. if CTL dimming is not applied) and
the RSENSE is the resister between the ISP and ISN pins.
Current Limit
The RT8474 can limit the peak switch current with its
internal over-current protection feature. In normal operation,
the power switch is turned off when the switch current
hits the loop-set value. The over-current protection function
will turn off the power switch independent of the loop control
when the peak switch current reaches around 2A.
Copyright © 2015 Richtek Technology Corporation. All rights reserved.
DS8474-04 April 2015
Over-Temperature Protection
The RT8474 has Over-Temperature Protection (OTP)
function to prevent the excessive power dissipation from
overheating. The OTP function will shut down switching
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
Choose an inductor that can handle the necessary peak
current without saturating and ensure that the inductor
has a low DCR (copper-wire resistance) to minimize I2R
power losses. A 4.7mH to 22mH inductor will meet the
demand of most of the RT8474 applications. Inductor
manufacturers specify the maximum current rating as the
current where the inductance falls to certain percentage
of its nominal value, typically 65%. In Multiple-Topology
application where the transition between discontinuous
and continuous modes occurs, the value of the required
output inductor, L, can be approximated by the following
equation :
For Buck application :

V
VOUT 
L =  OUT   1 
 f  IL   VIN(MAX) 
The ripple current ΔIL and peak current IPEAK can be
calculated :
V
V
IL =  OUT   1 OUT 
VIN 
 f L  
IL
IPEAK = IOUT +
2
For Boost application :
V
V
L =  IN   1 IN 
f


I
V
L 
OUT 

The ripple current ΔIL and peak current IPEAK can be
calculated :
V
V
IL =  IN   1 IN 
 f  L   VOUT 
I
 VOUT  IL
IPEAK =  OUT
+


VIN 
2

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RT8474
For Buck-Boost application :
Thermal Considerations
V
VOUT

L =  OUT   1


I

f
V
+
V
L
IN
OUT

 
For continuous operation, do not exceed the maximum
operation junction temperature 125°C. The maximum
power dissipation depends on the thermal resistance of
IC package, PCB layout, the rate of surroundings airflow
and temperature difference between junction to ambient.
The maximum power dissipation can be calculated by
following formula :
The ripple current ΔIL and peak current IPEAK can be
calculated :
V
VOUT

IL =  OUT   1
L
f
V

IN + VOUT 

 
  VIN + VOUT   IOUT  IL
IPEAK = 
 +
  VIN
2


where,
VOUT = output voltage.
PD(MAX) = ( TJ(MAX) − TA ) / θJA
where T J(MAX) is the maximum operation junction
temperature, TA is the ambient temperature and the θJA is
the junction to ambient thermal resistance.
VIN = input voltage.
IOUT = LED current.
f = switching frequency.
η = efficiency
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for RT8474
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).
For recommended operating conditions specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-8 (Exposed Pad) package, the thermal resistance
θJA is 29°C/W on the standard JEDEC 51-7 four-layer
thermal test board. The maximum power dissipation at TA
= 25°C can be calculated by following formula :
P D(MAX) = (125°C − 25°C) / (29°C/W) = 3.44W for
SOP-8 (Exposed Pad) package
The maximum power dissipation depends on operating
ambient temperature for fixed T J(MAX) and thermal
resistance θJA. The deration curve in Figure 1 allows the
designer to see the effect of rising ambient temperature
on the maximum power allowed.
Capacitor Selection
1

VOUT  IL  ESR +
8  f  COUT 

3.6
Maximum Power Dissipation (W)1
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most RT8474 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 :
Four-Layer PCB
3.0
2.4
1.8
1.2
0.6
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 1. Derating Curve of Maximum Power Dissipation
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DS8474-04 April 2015
RT8474
Layout Considerations

PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guide lines are as follows :
Place 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 the
VCC pin as possible.

Place the compensation components to the VC pin as
close as possible to avoid noise pickup.

The power components 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.
Keep the ISP and ISN with
the Kelvin sense connection.
VIN power trace to ISP
must be wide and short.
ISP
VIN
RSENSE
C1
ISN
R3
D1
Locate input capacitor as
close to VCC as possible.
C3
VCC
C5
GND
ISP
2
ISN
3
NC
4
GND
8
CREG
7
SW
6
CTL
5
VC
9
Power trace must be wide
and short when compared
to the normal trace.
L1
Place these components
as close as possible.
R1
Locate the compensation
components to VC pin as
close as possible.
C4
...
GND
Normal trace.
R2
C2
GND
Figure 2. PCB Layout Guide
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RT8474
Outline Dimension
H
A
M
EXPOSED THERMAL PAD
(Bottom of Package)
Y
J
X
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
4.000
0.150
0.157
C
1.346
1.753
0.053
0.069
D
0.330
0.510
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.170
0.254
0.007
0.010
I
0.000
0.152
0.000
0.006
J
5.791
6.200
0.228
0.244
M
0.406
1.270
0.016
0.050
X
2.000
2.300
0.079
0.091
Y
2.000
2.300
0.079
0.091
X
2.100
2.500
0.083
0.098
Y
3.000
3.500
0.118
0.138
Option 1
Option 2
8-Lead SOP (Exposed Pad) 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.
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DS8474-04 April 2015