®
RT8474A
High Voltage Buck LED Driver with Open Detection
General Description
Features
The RT8474A 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
RT8474A 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
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
Adjustable OVP for LED Open Protection
Under-Voltage Lockout
Thermal Shutdown
RoHS Compliant and Halogen Free
The RT8474A is available in the SOP-8 (Exposed pad)
package.
Applications
Ordering Information
Desk Lights and Room Lighting
Industrial Display Backlight
RT8474A
Marking Information
Package Type
SP : SOP-8 (Exposed-Option 2)
RT8474AGSP : Product Number
Lead Plating System
G : Green (Halogen Free and Pb Free)
RT8474A
GSPYMDNN
YMDNN : Date Code
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.
Simplified Application Circuit
D1
VIN
C1
RT8474A
ISP
VCC
Analog Dimming
or PWM Dimming
ISN
CTL
VC
R2
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8474A-02
December 2013
R4
R3
OVP
CREG
C3
C4
L1
SW
R1
C2
RSENSE
GND
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT8474A
Pin Configurations
(TOP VIEW)
8
VCC
ISP
2
ISN
3
OVP
4
GND
CREG
7
SW
6
CTL
5
VC
9
SOP-8 (Exposed Pad)
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VCC
Supply Voltage Input. For good bypass, connect a low ESR capacitor
between this pin and GND.
2
ISP
Positive Current Sense Input.
3
ISN
Negative Current Sense Input. Voltage threshold between ISP and ISN is
100mV.
4
OVP
Over-Voltage Protection Sense Input.
5
VC
Compensation Node for Current Loop.
6
CTL
Analog Dimming Control Input. Effective programming range is 0.2V to
1.2V.
7
SW
Switch Node of the PWM Converter.
8
CREG
Regulator Output for Internal Circuit. Place a 1F capacitor to stabilize the
5V output regulator.
GND
Ground. The exposed pad must be soldered to a large PCB and connected
to GND for maximum power dissipation.
9 (Exposed Pad)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
is a registered trademark of Richtek Technology Corporation.
DS8474A-02
December 2013
RT8474A
Function Block Diagram
SW
OSC
-
VCC
4.5V
S
+
R
OVP
-
ISP - 1.2V
5V
LDO
CREG
R
+
+
-
VC
Soft-Start
GM
+
ISN
ISP
+
-
GND
CTL
Operation
The RT8474A 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.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8474A-02
December 2013
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 RT8474A
provides protection functions which include overtemperature, and switch current limit to prevent abnormal
situations.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT8474A
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, VCC ---------------------------------------------------------------------------------------------SW Pin Voltage at Switching off, ISP, ISN, OVP -----------------------------------------------------------------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
4.5
VIN Falling
3.9
4.1
--
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
VC Threshold for CTL Switch Off
LED Dimming
0.2V VCTL 1.2V
Input Current of CTL Pin
ICTL
LED Current Off Threshold at
CTL
VCTL_OFF
0.15
0.2
0.25
V
LED Current On Threshold at
CTL
VCTL_ON
--
1.2
1.5
V
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
4
is a registered trademark of Richtek Technology Corporation.
DS8474A-02
December 2013
RT8474A
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
440
500
540
kHz
--
--
100
%
Minimum On-Time
--
100
200
ns
SW RDS(ON)
--
0.15
--
2
2.5
--
A
VISP VOVP
1.15
1.2
1.25
V
VOVP = 1.2V
--
1
--
V
VOVP 1.5V
--
30
--
A
(Note 6)
--
5.7
--
ms
PWM Converter
Switch Frequency
fSW
Maximum Duty Cycle
DMAX
SW Current Limit
ILIM_SW
Over-Voltage Protection and Soft-Start
OVP Threshold
VOVP
OVP Recovery
OVP Input Current
IOVP
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.
Note 6. Guarantee by design, not subject to production testing.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8474A-02
December 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT8474A
Typical Application Circuit
D1
VIN
4.5V to 50V
C1
Analog Dimming
or PWM Dimming
RT8474A
1 VCC
ISP 2
3
6 CTL
ISN
5 VC
R2
5.1M
R1
10k
C2
3.3nF
8 CREG
C3
1µF
RSENSE
100mV
SW 7
C4
L1
R4
R3
OVP 4
GND
9 (Exposed Pad)
Table 1. Suggested Components Selection
VIN
12V
24V
32V
42V
LED (pcs)
3
6
9
12
OVP SET (V)
R3
R4
ILED_spike (mA)
0.9
150k
10k
1100
1
120k
10k
1100
1.1
110k
10k
1000
0.9
316k
10k
1200
1
270k
10k
1200
1.1
232k
10k
1150
0.9
470k
10k
700
1
402k
10k
700
1.1
360k
10k
650
0.9
649k
10k
650
1
560L
10k
650
1.1
499k
10k
600
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
6
VOUT (V)
10
20
30
40
is a registered trademark of Richtek Technology Corporation.
DS8474A-02
December 2013
RT8474A
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
RSENS = 300mΩ, LED = 6pcs
0
50
5
10
15
20
25
30
35
40
45
0
50
0.5
1
1.5
2
2.5
3
VCTL (V)
Input Voltage (V)
Supply Current vs. VCC
ISP-ISN Threshold vs. Temperature
120
2.30
ISP-ISN Threshold (mV)
Supply Current (mA)
2.25
2.20
2.15
2.10
2.05
2.00
110
100
90
80
1.95
ICC
VCC = 24V
70
1.90
0
5
10
15
20
25
30
35
40
45
-50
50
-25
0
VCC (V)
50
75
100
125
Temperature (°C)
SW RDS(ON) vs. Temperature
SW RDS(ON) vs. VCC
0.30
0.180
0.178
0.26
RDS(ON) ( Ω)
RDS(ON) (Ω)
25
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)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8474A-02
December 2013
50
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT8474A
OVP Voltage vs. Temperature
Frequency vs. VCC
510
1.3
505
Frequency (kHz)
OVP Voltage (V)
VOVP
1.2
1.1
1.0
OVP Recovery
500
495
490
485
VCC = 24V
0.9
480
-50
-25
0
25
50
75
100
125
0
5
10
15
20
25
30
35
Temperature (°C)
VCC (V)
Power On from VCC
Power Off from VCC
VIN
(20V/Div)
VIN
(20V/Div)
VOUT
(20V/Div)
VOUT
(20V/Div)
IOUT
(200mA/Div)
IOUT
(200mA/Div)
VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs
40
45
50
= 24V,
24V, IIOUT
= 340mA,
340mA, LL == 47μH,
47μH, LED
LED == 6pcs
6pcs
VVIN
IN=
OUT =
Time (25ms/Div)
Time (50ms/Div)
OVP Turn On when LED Take Off and Restore
Switching
SW
(50V/Div)
VIN
(20V/Div)
VOUT
(20V/Div)
VOVP
(1V/Div)
VIN
(20V/Div)
VOUT
(20V/Div)
IOUT
(500mA/Div)
IOUT
(200mA/Div)
VIN = 30V, IOUT = 340mA, L = 47μH, LED = 7pcs
Time (250ms/Div)
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
8
VIN = 24V, IOUT = 340mA, L = 47μH, LED = 6pcs
Time (2.5μs/Div)
is a registered trademark of Richtek Technology Corporation.
DS8474A-02
December 2013
RT8474A
Application Information
The RT8474A 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.
Output Over-Voltage Setting
Frequency Compensation
Where the VOUT is the terminal voltage between LED string,
R3 and R4 are the voltage divider from VOUT with the divider
center node connected to the OVP pin. Normally, set R4
for 10kΩ in typical application.
The RT8474A 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
RT8474A is 10k and 3.3nF.
LED Current Setting
The LED current can be calculated by the following
equation :
ILED(MAX) =
VISP VISN
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 RT8474A 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 © 2013 Richtek Technology Corporation. All rights reserved.
DS8474A-02
December 2013
When using constant current to drive LEDs, removed or
turned LEDs on instantly can be dangerous for both the
LED and the LED driver. Harmful spikes are generated
when large currents are instantaneously turned on. The
RT8474A provided the Over-Voltage Protection (OVP)
function to detect the spike and switch off LED driver
immediately. When the voltage between the OVP pin and
ISP pin exceeds a threshold of approximately 1.2V, the
power switch is turned off. The power switch can be turned
on again once the voltage between the OVP pin and ISP
pin drops below 1V. For typical LED driver application,
the output voltage could be clamped at a certain voltage
level. If OVP happened,
ISP − VOVP > 1.2V
When OVP happened between VOUT, ISP − VOVP is 1.2V.
To avoid the inrush current during over-voltage protection,
we set the ISP-OVP voltage from 0.9 to 1.1V. Refer the
table 1, the R3 could be selected for different inrush current
concern.
Over-Temperature Protection
The RT8474A 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 4.7μH to 22μH inductor will meet the
demand of most of the RT8474A applications. Inductor
manufacturers specify the maximum current rating as the
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
9
RT8474A
current where the inductance falls to certain percentage
of its nominal value, typically 65%. In Buck 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 :
Thermal Considerations
VOUT
VOUT
L=
1
f IL(MAX) VIN(MAX)
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 can be calculated :
PD(MAX) = ( TJ(MAX) − TA ) / θJA
V
V
IL = OUT 1 OUT
VIN
f L
where,
VOUT = output voltage.
VIN = input voltage.
f = switching frequency.
Schottky Diode Selection
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for RT8474A
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).
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.
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 RT8474A 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
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
is a registered trademark of Richtek Technology Corporation.
DS8474A-02
December 2013
RT8474A
Layout Considerations
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guide lines are as follows :
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.
Place L1 and D1 as close to each other as possible.
The trace should be as short and wide as possible.
The input capacitor C1 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.
Keep the ISP and ISN with
the Kelvin sense connection.
ISP
VIN
RSENSE
R4
C1
D1
Locate input capacitor as
close to VCC as possible.
ISN
C4
R3
GND
...
C3
VCC
ISP
2
ISN
3
OVP
4
GND
8
CREG
7
SW
6
CTL
5
VC
9
L1
Place these components
as close as possible.
R1
Locate the compensation
components to VC pin as
close as possible.
OVP
R2
C2
GND
Figure 2. PCB Layout Guide
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8474A-02
December 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
RT8474A
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.
www.richtek.com
12
DS8474A-02
December 2013
工商网监
湘ICP备2023018690号
