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RT8453B
1.5A, 500kHz, 40V High Voltage Buck LED Driver
General Description
Features
The RT8453B is a buck current mode PWM regulator for
LED driving applications. It provides an integrated 1.5A
switch and can be operated with a wide input voltage range
from 4.5V to 40V. With a 500kHz operating frequency, the
RT8453B allows all external inductor and input/output
capacitors to be small. High efficiency is achieved with its
190mV current sensing capability. Dimming can be either
analog or PWM digital signal driven. The RT8453B is
available in a small SOP-8 (Exposed pad) package.
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Input Operating Range 4.5 to 40V
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1.5A Switching Current Limitation
10% Constant Current Output Accuracy
Current Mode PWM with 500kHz Switching
Frequency
Analog or PWM Control Signal for LED Dimming
Programmable Soft-Start
Input Under Voltage Lockout
Over Temperature Protection
RoHS Compliant and Halogen Free
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Ordering Information
RT8453B
Applications
Package Type
SP : SOP-8 (Exposed Pad-Option 2)
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Lead Plating System
G : Green (Halogen Free and Pb Free)
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General Illumination
Desk Lights and Room Lighting
Industrial Backlight/Lighting
Constant Current Source
Note :
Richtek products are :
`
Pin Configurations
RoHS compliant and compatible with the current require-
(TOP VIEW)
ments of IPC/JEDEC J-STD-020.
`
Suitable for use in SnPb or Pb-free soldering processes.
Marking Information
8
ISN
VC
2
ACTL
EN
3
GND
SW
6
GND
5
SS
9
4
VCC
7
RT8453BGSP : Product Number
RT8453B
GSPYMDNN
YMDNN : Date Code
DS8453B-01 May 2011
SOP-8 (Exposed Pad)
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RT8453B
Typical Application Circuit
VIN
4.5V to 40V
R2
190mV
C2
RT8453B
8 VCC
Analog
Dimming
or
ISN 1
R3
R4
D1
3 ACTL
C1
PWM
Dimming Control
4 EN
2
VC
5
SS
5V
R5
10k
C6
3.3nF
SW
GND
7
L1
22µH
6, 9 (Exposed Pad)
C3
10nF
Functional Pin Description
Pin No.
Pin Name
1
ISN
2
VC
Pin Function
Current Sense Amplifier Negative Input. Voltage threshold between VCC and ISN
is 190mV.
PWM Converter Loop Compensation Node.
3
ACTL
Analog Dimming Control. Effective programming range is between 0.3V and 1.2V.
4
EN
Chip Enable (Active High). When low, chip is in shutdown mode.
5
SS
6,
GND
9 (Exposed Pad)
Soft-Start. A capacitor of at least 10nF is required for soft-start.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
7
SW
PWM Converter Switch Node.
8
VCC
Power Supply of the Chip. For good bypass, a low ESR capacitor is required.
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DS8453B-01 May 2011
RT8453B
Function Block Diagram
SW
OSC
S
-
VCC
4.5V
R
+
R
EN
+
-
-
+
1.4V
Shutdown
VC
GM
+
6µA
ISN
VCC
SS
ACTL
+
-
GND
VCC – VISN
(mV)
190
0
0.3
1.2
VACTL (V)
Figure 1
DS8453B-01 May 2011
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RT8453B
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VCC ----------------------------------------------------------------------------------------------SW Pin Voltage at Switching Off, ISN -------------------------------------------------------------------------------ACTL ------------------------------------------------------------------------------------------------------------------------EN ----------------------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
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 Mode) ---------------------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
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45V
45V
8V (Note 2)
20V
1.333W
75°C/W
15°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 5)
Supply Input Voltage ------------------------------------------------------------------------------------------------------ 4.5V to 40V
Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VCC = 12V, No Load, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Overall
Supply Current
IVCC
VC ≤ 0.4V (Switching off)
--
4
6
mA
Shutdown Current
ISHDN
VEN ≤ 0.7V
--
250
--
μA
EN Input Threshold Logic-High
Voltage
Logic-Low
VEN_H
1.5
--
--
VEN_L
--
--
0.5
170
190
210
mV
V
Current Sense Amplifier
Sense Threshold Voltage
VISN)
(VCC −
4.5V ≤ common mode ≤ 40V
ISN Pin Input Current
IISN
VISN = 24V
--
40
--
μA
VC Pin Output Current
IVC
2.4V > VC > 0.2V
--
±20
--
μA
--
0.7
--
V
--
--
3
μA
--
0.2
--
V
400
500
600
kHz
VC Threshold for PWM Switch Off
LED Dimming
Analog Dimming ACTL Pin Input
Current
LED Current Off Threshold at
ACTL
IACTL
VACTL
0.3V ≤ VACTL ≤ 1.2V
PWM BOOST Converter
Switching Frequency
fSW
To be continued
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DS8453B-01 May 2011
RT8453B
Parameter
Maximum Duty Cycle
Symbol
Test Conditions
(Note 6)
SW On-Voltage
VSW
SW Current Limit
ILIM_SW
ISW = 0.5A
Min
Typ
Max
Unit
--
88
--
%
--
0.4
--
V
1.25
1.5
--
A
--
6
--
μA
--
150
--
°C
--
10
--
°C
Soft-Start
Soft-Start Current
ISS
VSS ≤ 2V
Over Temperature Protection
Thermal Shutdown Temperature T SD
Thermal Shutdown Hysteresis
ΔTSD
Note 1. Stresses listed as the above "Absolute Maximum Ratings" may cause permanent damage to the device. These are
for stress ratings. 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 for
extended periods may remain possibility to affect device reliability.
Note 2. If connected with a 20kΩ series resistor, this pin can go up to 40V.
Note 3. θJA is measured in natural convection at TA = 25°C on a high effective thermal conductivity four-layer test board of
JEDEC 51-7 thermal measurement standard. The measurement case position of θJC is on 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. When the natural maximum duty cycle of 500kHz 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.
DS8453B-01 May 2011
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RT8453B
Typical Operating Characteristics
Efficiency vs. Input Voltage
100
1.00
95
0.95
95
0.95
90
0.90
0.90
90
85
0.85
Efficiency (%)
Efficiency (%)
Efficiency vs. Input Voltage
100
1.00
3 LED
5 LED
7 LED
0.80
80
0.75
75
0.70
70
0.85
85
3 LED
5 LED
7 LED
0.80
80
0.75
75
0.70
70
0.65
65
0.65
65
TA = 25°C, ILED = 450mA, 3 LEDs
TA = 125°C, ILED = 450mA, 3 LEDs
0.60
60
0.60
60
10
15
20
25
30
35
40
10
15
20
Input Voltage (V)
30
35
40
LED Current vs. Temperature
200.0
197.5
197.5
195.0
195.0
LED Curret (mA)
LED Current (mA)
LED Current vs. Input Voltage
200.0
192.5
190.0
187.5
185.0
182.5
192.5
190.0
187.5
185.0
182.5
VIN = 24V, R2 = 1Ω, VEN = 3V
VEN = 3V, R2 = 1Ω
180.0
180.0
4
8
12
16
20
24
28
32
36
40
-50
-25
0
Input Voltage (V)
180
160
140
120
100
80
60
40
VIN = 24V, VOUT = 15V
0
0
0.25
0.5
0.75
1
ACTL Input Voltage (V)
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50
75
100
125
1.25
(VCC – ISN) Threshold Voltage vs. ACTL Duty
(VCC – ISN) Threshold Voltage (mV)
200
20
25
Temperature (°C)
(VCC – ISN) Threshold Voltage vs. ACTL Voltage
(VCC– ISN) Threshold Voltage (mV)
25
Input Voltage (V)
1.5
200
180
160
140
120
100
80
60
40
20
VIN = 24V, VOUT = 15V
0
0
10
20
30
40
50
60
70
80
90
100
ACTL Duty (%)
DS8453B-01 May 2011
RT8453B
Supply Current vs. Temperature
3.0
3.0
2.5
Supply Current (mA) 1
Supply Current (mA) 1
Supply Current vs. Input Voltage
3.5
2.5
2.0
1.5
1.0
0.5
2.0
1.5
1.0
0.5
VIN = 12V
TA = 25°C
0.0
0.0
0
6
12
18
24
30
36
42
-50
-25
0
25
50
75
100
Input Voltage(V)
Temperature (°C)
Continuous Mode Operation
Continuous Mode Operation
125
VLX
(10V/Div)
VEN
(10V/Div)
I LED
(200mA/Div)
I LED
(200mA/Div)
3 LEDs, VIN = 24V, IOUT = 350mA
3 LEDs, VIN = 12V, IOUT = 350mA
Time (1μs/Div)
Time (1μs/Div)
Power On from EN
Power Off from EN
VEN
(5V/Div)
VEN
(5V/Div)
VLED
(5V/Div)
VLED
(5V/Div)
I LED
(200mA/Div)
I LED
(200mA/Div)
3 LEDs, VIN = 12V
Time (50μs/Div)
DS8453B-01 May 2011
3 LEDs, VIN = 12V
Time (50μs/Div)
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RT8453B
Applications Information
The RT8453B 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 VCC 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.
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 ACTL pin.
The protection schemes in the RT8453B include over
temperature and switch current limit to prevent abnormal
situations.
Frequency Compensation
The RT8453B has an external compensation pin (VC),
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 RT8453B is 10k and 3.3nF.
Soft-Start
The soft-start of the RT8453B 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 which is 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.
LED Current Setting
The LED current can be calculated by the following
equation :
VCC − VISN
R2
where VCC − VISN is the voltage between VCC and ISN
(190mV typ. if ACTL dimming is not applied) and the R2
is the resister between VCC and ISN.
ILED(MAX) =
Current Limit Protection
The RT8453B 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 1.5A.
Over Temperature Protection
The RT8453B has an Over Temperature Protection (OTP)
function to prevent over heating caused by excessive power
dissipation. The OTP function will shut down switching
operation when the die junction temperature exceeds 150°C
. Once temperature falls below 150°C, the chip will
automatically resume operation again.
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 RT8453B 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 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 :
⎡ VOUT ⎤ ⎡
VOUT ⎤
L=⎢
⎥ × ⎢1 −
⎥
⎣ f × ΔIL(MAX) ⎦ ⎣ VIN(MAX) ⎦
The value of the soft-start capacitor is user-defined to
satisfy the designer's request.
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DS8453B-01 May 2011
RT8453B
The ripple current ΔIL can be calculated :
⎡ VOUT ⎤ ⎡ VOUT ⎤
ΔIL = ⎢
⎥ × ⎢1 − V ⎥
IN ⎦
⎣ 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 RT8453B
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).
the ambient temperature and θJA is the junction to ambient
thermal resistance.
For recommended operating conditions specification of
the RT8453B, the maximum junction temperature is 125°C
and TA is the ambient temperature. The junction to ambient
thermal resistance θJA is layout dependent. For SOP-8
(exposed pad) packages, the thermal resistance θJA is
75° 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 :
PD(MAX) = (125°C − 25°C) / (75°C/W) = 1.333W for
SOP-8 (exposed pad)
The maximum power dissipation depends on operating
ambient temperature for fixed T J(MAX) and thermal
resistance θJA. For RT8453B packages, the Figure 2 allows
the designer to see the effect of rising ambient temperature
on the maximum power dissipation allowed.
1.6
Maximum Power Dissipation (W)
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most RT8453B 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
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
1
⎡
⎤
ΔVOUT ≤ ΔIL × ⎢ESR +
⎥
8
×
f
×
C
OUT ⎦
⎣
Figure 2. Derating Curve for RT8453B Package
Thermal Considerations
Layout Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of IC
package, PCB layout, rate of surrounding airflow and
temperature difference between junction to ambient. The
maximum power dissipation can be calculated by following
the formula :
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guide lines are as follows :
PD(MAX) = (TJ(MAX) − TA) / θJA
Where TJ(MAX) is the maximum junction temperature, TA is
DS8453B-01 May 2011
`
The power components L1, D1 and C1 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.
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RT8453B
`
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 VCC
pin as possible.
`
Place the compensation components to VC pin as close
as possible to avoid noise pick up.
Locate input capacitor as
close VCC as possible.
C1
R2
C2
D1
8
ISN
R5
C6
VC
2
ACTL
3
EN
4
GND
7
SW
6
GND
5
SS
9
Locate the compensation
components to VC pin as
close as possible.
VCC
L1
Place these components
as close as possible.
C3
Figure 3. PCB Layout Guide
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DS8453B-01 May 2011
RT8453B
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
Richtek Technology Corporation
Headquarter
Taipei Office (Marketing)
5F, No. 20, Taiyuen Street, Chupei City
5F, No. 95, Minchiuan Road, Hsintien City
Hsinchu, Taiwan, R.O.C.
Taipei County, Taiwan, R.O.C.
Tel: (8863)5526789 Fax: (8863)5526611
Tel: (8862)86672399 Fax: (8862)86672377
Email: [email protected]
Information that is provided by Richtek Technology Corporation is believed to be accurate and reliable. Richtek reserves the right to make any change in circuit design,
specification or other related things if necessary without notice at any time. No third party intellectual property infringement of the applications should be guaranteed
by users when integrating Richtek products into any application. No legal responsibility for any said applications is assumed by Richtek.
DS8453B-01 May 2011
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