RT8473 - Farnell

®
RT8473
1A, Hysteretic, High Brightness LED Driver with Internal
Switch
General Description
Features
The RT8473 is a high efficiency, continuous mode inductive
step-down converter, designed for driving single or multiple
series connected LEDs from a voltage source higher than
the LED voltage. It operates from an input voltage of 7V to
30V and employs hysteretic control with a high side current
sense resistor to set the constant output current.
z
The RT8473 includes an output switch and a high side
output current sensing circuit, which uses an external
resistor to set the nominal average output current. LED
brightness control is achieved with PWM dimming from
an analog or PWM input signal.
The RT8473 is available in a small TSOT-23-5 package.
Ordering Information
RT8473
Package Type
J5 : TSOT-23-5
Lead Plating System
G : Green (Halogen Free and Pb Free)
z
z
z
z
z
z
z
z
z
z
7V to 30V Input Voltage Range
Hysteretic Control with High Side Current Sensing
Internal N-MOSFET with 350mΩ
Ω Low RDS(ON)
1A Output Current
Up to 97% Efficiency
Typical ±5% LED Current Accuracy
Analog or PWM Control Signal for LED Dimming
300Hz On-Board Ramp Generator
Input Under Voltage Lockout
Thermal Shutdown Protection
RoHS Compliant and Halogen Free
Applications
z
z
z
z
z
z
Automotive LED Lighting
High Power LED Lighting
Indicator and Emergency Lighting
Architectural Lighting
Low Voltage Industrial Lighting
Signage and Decorative LED Lighting
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
05= : Product Code
05=DNN
DNN : Date Code
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8473-00 January 2012
Pin Configurations
(TOP VIEW)
VIN
SENSE
5
4
2
3
LX GND ADJ
TSOT-23-5
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RT8473
Typical Application Circuit
VIN
7V to 30V
C1
10µF/50V
optional
R1
100m
R2
RT8473
5
VIN
3 ADJ
optional
SENSE
4
optional
LED+
C3
C2
D1
SR26
LED2
GND
LX
1
L1
100µH
VIN = 7V to 30V
VOUT = 3.5V
IOUT = 1A
optional
R3
C4
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
LX
Switching Node. Open drain output of internal N-MOSFET.
2
GND
3
ADJ
Ground.
Dimming Control Input :
--- Analog signal input for analog PWM dimming.
--- PWM signal input for digital PWM dimming.
4
SENSE
Output Current Sense Terminal. Sense LED string current.
5
VIN
Supply Input Voltage.
Function Block Diagram
VIN
Regulator
Bandgap
SENSE
1.25V
VCC
Ramp Gen.
UVLO
Dimming
LX
GND
+
Dimming
-
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2
MOSFET
+
-
ADJ
+
UVLO
-
UVLO
is a registered trademark of Richtek Technology Corporation.
DS8473-00 January 2012
RT8473
Absolute Maximum Ratings
(Note 1)
Supply Input Voltage, VIN ------------------------------------------------------------------------------------Switch Voltage, LX --------------------------------------------------------------------------------------------z Sense Voltage, SENSE -------------------------------------------------------------------------------------z All Other Pins ----------------------------------------------------------------------------------------------------z Power Dissipation, PD @ TA = 25°C
TSOT-23-5 (Single-layer PCB) ------------------------------------------------------------------------------TSOT-23-5 (Four-layer PCB) --------------------------------------------------------------------------------z Package Thermal Resistance (Note 2)
TSOT-23-5, θJA (Single-layer PCB) ------------------------------------------------------------------------TSOT-23-5, θJC (Single-layer PCB) ------------------------------------------------------------------------TSOT-23-5, θJA (Four-layer PCB) --------------------------------------------------------------------------z Junction Temperature -----------------------------------------------------------------------------------------z Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------z Storage Temperature Range --------------------------------------------------------------------------------z ESD Susceptibility (Note 3)
HBM (Human Body Mode) ----------------------------------------------------------------------------------MM (Machine Mode) ------------------------------------------------------------------------------------------z
z
Recommended Operating Conditions
z
z
−0.3V to 33V
−0.3V to (VIN + 0.7V)
(VIN − 5V) to (VIN + 0.3V)
−0.3V to 6V
0.400W
0.625W
250°C/W
130°C/W
160°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VIN ------------------------------------------------------------------------------------- 7V to 30V
Junction Temperature Range --------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VIN = 12V, TA = 25°C, unless otherwise specified)
Parameter
Mean Current Sense Threshold
Voltage
Symbol
VSENSE
Test Conditions
Measure on SENSE Pin with
Respect to VIN. ADJ pin is
Floating.
Min
Typ
Max
Unit
95
100
105
mV
VLX = 5V
----
±5
350
0.01
--10
%
mΩ
μA
VIN Rising
--
5.2
--
V
Sense Threshold Hysteresis
MOSFET On-Resistance
MOSFET Leakage Current
VSENSEHYS
RDS(ON)
Under Voltage Lockout Threshold
VUVLO
Under Voltage Lockout Threshold
Hysteresis
Ramp Frequency
ΔVUVLO
--
400
--
mV
f RAMP
--
300
--
Hz
ADJ Input Threshold
Voltage
Logic-High
VADJ_H
1.4
--
5.5
Logic-Low
VADJ_L
--
--
0.2
0.4
--
1.2
V
Analog Dimming Range
V
Minimum Switch On Time
tON(MIN)
LX Switch On
--
210
--
ns
Minimum Switch Off Time
Quiescent Supply Current with
Output Off
tOFF(MIN)
LX Switch Off
--
170
--
ns
IVIN, Off
VADJ = 0V
--
450
--
μA
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8473-00 January 2012
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RT8473
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
ADJ Pin is Floating, fSW = 250kHz,
VIN = 8V
--
1000
--
μA
--
25
--
ns
--
300
--
nA
TSD
--
150
--
°C
ΔTSD
--
30
--
°C
Quiescent Supply Current with
Output Switching
Internal Propagation Delay
tPD
Sense Pin Input Current
ISENSE
Thermal Shutdown
Thermal Shutdown Hysteresis
IVIN, On
VSENSE = V IN – 0.1V
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. θJA is measured at TA = 25°C on a single-layer and four-layer test board of JEDEC 51. The measurement case position
of θJC is on the lead of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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RT8473
Typical Operating Characteristics
Output Current Deviation vs. Input Voltage
Efficiency vs. Input Voltage
100
100%
6%
6
Output Current Deviation (%)1
7 LED
95
95%
Efficiency (%)
3 LED
90
90%
85%
85
1 LED
80%
80
75%
75
VIN = 7V to 30V, ILED = 1A, L = 100μH
4
4%
2
2%
3 LED
0%
0
1 LED
-4%
-4
VIN = 7V to 30V, ILED = 1A, L = 100μH
-6
-6%
70%
70
0
7
14
21
28
0
35
7
Output Current vs. Input Voltage
1.01
1.00
Output Current Deviation (%)1
Output Current (A)
1.02
0.99
0.98
0.97
VIN = 7V to 30V, ILED = 1A, L = 100μH
0.96
28
35
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
3
3%
2
2%
1
1%
0%
0
-1%
-1
-2%
-2
-3%
-3
VIN = 7V to 30V, ILED = 1A, L = 100μH
-4%
-4
0
7
14
21
28
35
0
7
Input Voltage (V)
14
21
28
35
Input Voltage (V)
Switching Frequency vs. Input Voltage
Duty Cycle vs. Input Voltage
100%
100
800
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
700
600
500
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
90%
90
80
80%
Duty cycle (%)
Switching Frequency (kHz)1
21
Output Current Deviation vs. Input Voltage
4
4%
1 LED
2 LED
3 LED
4 LED
5 LED
6 LED
7 LED
8 LED
1.03
14
Input Voltage (V)
Input Voltage (V)
1.04
7 LED
-2%
-2
400
300
70
70%
60
60%
50
50%
40
40%
30
30%
20
20%
200
10
10%
VIN = 7V to 30V, ILED = 1A, L = 100μH
100
0
7
14
21
28
Input Voltage (V)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8473-00 January 2012
VIN = 7V to 30V, ILED = 1A, L = 100μH
0%0
35
0
7
14
21
28
35
Input Voltage (V)
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RT8473
Quiescent Input Current vs. Input Voltage
Quiescent Input Current vs. Input Voltage
495
Quiescent Input Current (μA)
Quiescent Input Current (μA)
1200
1000
800
600
400
200
490
485
480
Output Switching
Output Off
0
475
0
7
14
21
28
35
0
7
14
Input Voltage (V)
35
Output Current vs. PWM Duty Cycle
1200
1200
1000
1000
Output Current (mA)
Output Current (mA)
28
Input Voltage (V)
Output Current vs. PWM Duty Cycle
800
600
400
200
800
600
400
200
RS = 0.1Ω, fDIMMING = 10kHz
0
0
20
40
60
80
RS = 0.1Ω, fDIMMING = 500Hz
0
100
0
20
40
PWM Duty Cycle (%)
60
80
100
PWM Duty Cycle (%)
MOSFET On-Resistance vs. Temperature
LED Current vs. ADJ Voltage
1200
600
RS = 100mΩ
550
800
(mΩ))
On-Resistance (m
1000
LED Current (mA)
21
RS = 150mΩ
600
400
RS = 350mΩ
200
500
450
400
350
300
VIN = 12V, 1LED
0
0.2
0.5
0.8
1.1
1.4
ADJ Voltage (V)
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1.7
250
-50
-25
0
25
50
75
100
125
150
Temperature (°C)
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RT8473
Ramp Frequency vs. Temperature
Ramp Frequency vs. Input Voltage
350.0
350
Ramp Frequency (Hz)1
Ramp Frequency (Hz)
347.5
345.0
342.5
340.0
337.5
335.0
340
330
320
310
332.5
300
330.0
0
7
14
21
28
-50
35
-25
0
25
50
75
100
125
Input Voltage (V)
Temperature (°C)
Digital Dimming from ADJ On
Digital Dimming from ADJ Off
VADJ
(2V/Div)
VADJ
(2V/Div)
IOUT
(500mA/Div)
IOUT
(500mA/Div)
VIN = 12V, RS = 0.1Ω, fDIMMING = 500Hz, 1 LED
VIN = 12V, RS = 0.1Ω, fDIMMING = 500Hz, 1 LED
Time (5μs/Div)
Time (5μs/Div)
Power On from VIN
Power Off from VIN
VIN
(5V/Div)
VIN
(5V/Div)
IOUT
(500mA/Div)
IOUT
(500mA/Div)
RS = 0.1Ω, 1 LED
Time (500μs/Div)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8473-00 January 2012
150
RS = 0.1Ω, 1 LED
Time (500μs/Div)
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RT8473
Application Information
The RT8473 is a simple high efficiency, continuous mode
inductive step-down converter. The device operates with
an input voltage range from 7V to 30V and delivers up to
1A of output current. A high side current sense resistor
sets the output current and a dedicated PWM dimming
input enables pulsed LED dimming over a wide range of
brightness levels. A high side current sensing scheme
and an onboard current setting circuitry minimize the
number of external components. A 1% sense resistor
performs a ±3% LED current accuracy for the best
performance.
Under Voltage Lockout (UVLO)
The RT8473 includes a UVLO feature with 400mV
hysteresis. The internal MOSFET turns off when VIN falls
below 4.8V (typ.).
Setting Average Output Current
The RT8473 output current which flows through the LEDs
is set by an external resistor (RS) connected between the
VIN and SENSE terminal. The relationship between output
current (IOUT) and RS is shown as below :
IOUTavg = 0.1V ( A )
RS
Analog Dimming Control
The ADJ terminal can be driven by an external voltage
(VADJ) to adjust the output current to an average value set
by RS. The average output current is given by :
⎛
⎞ V
− 0.4
IOUTavg = ⎜ 0.1V ⎟ × ADJ
0.8
⎝ RS ⎠
where VADJ is ranged from 0.4V to 1.2V. When VADJ is
larger than 1.2V, the output current value will just be set
by the external resistor (RS).
Digital Dimming Control
A Pulse Width Modulated (PWM) signal can drive the ADJ
terminal directly. Notice that the PWM signal logic high
level must be above 1.4V and the logic low level must be
below 0.2V at the ADJ terminal. It's recommended to
maintain the PWM dimming at low frequency (ex. 500Hz
) in order to obtain a linear dimming curve.
PWM Soft-Start Behavior
The RT8473 features an optional PWM soft-start behavior
that allows for gradual brightness transition. This is
achieved by simply connecting an external capacitor
between the ADJ pin and GND. An internal current source
will then charge this capacitor for soft-start behavior,
resulting in steady LED current increase and decrease
during power on and power off, as shown in Figure 1.
1.2V
Internal
VRAMP
0.4V
VADJ 0V
1A
ILED
0A
Figure 1. PWM Soft-Start Behavior Mechanism
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RT8473
The capacitor can be selected according to below
equation :
-6
where
VD is the rectifier diode forward voltage (V)
C = 1.5 x 10 x tSS
VSEN is the voltage cross current sense resistor (V)
where tSS is the soft-start period.
RL is the inductor DC resistance (Ω)
LED Current Ripple Reduction
L is the inductance (H)
Higher LED current ripple will shorten the LED life time
and increase heat accumulation of LED. By adding an
output capacitor in parallel with the LED. This will then
allow the use of a smaller inductor.
The saturation current of the selected inductor must be
higher than the peak output LED current, and the
continuous current rating must be above the average output
LED current. In general, the inductor saturation current
should be 1.5 times the LED current. In order to reduce
the output current ripple, a higher inductance is
recommended at higher supply voltages. However, it could
also cause a higher line resistance and result in a lower
efficiency.
Inductor Selection
The inductance is determined by inductor current ripple,
switching frequency, duty ratio, circuit specifications and
component parameters, as expressed in the following
equation :
D
L > ⎡ VIN − VOUT − VSEN − RDS(ON) × IOUT ⎤ ×
⎣
⎦ f
SW × ΔIL
(
)
where
fSW is the switching frequency (Hz)
RDS(ON) is the on-resistance of internal MOSFET ( = 0.35Ω
typical)
Diode Selection
To obtain better efficiency, the Schottky diode is
recommended for its low reverse leakage current, low
recovery time and low forward voltage. With its low power
dissipation, the Schottky diode outperforms other silicon
diodes and increase overall efficiency.
D is the duty cycle determined by VOUT/VIN
Input Capacitor selection
IOUT is the required LED current (A)
Input capacitor has to supply peak current to the inductor
and flatten the current ripple on the input. The low ESR
condition is required to avoid increasing power loss. The
ceramic capacitor is recommended due to its excellent
high frequency characteristic and low ESR, which are
suitable for the RT8473. For maximum stability over the
entire operating temperature range, capacitors with better
dielectric are suggested.
ΔIL is the inductor peak-peak ripple current (internally set
to 0.1 x IOUT)
VIN is the input supply voltage (V)
VOUT is the total LED forward voltage (V)
Besides, the selected inductance has also to satisfy the
limit of the minimum switch on/off time. The calculated
on time must be greater than 210ns of the minimum on
time, and the off time must be greater than 170ns of the
minimum off time. The following equation can be used to
verify the suitability of the inductor value.
tON =
L × ΔIL
VIN − VOUT − IOUT (RSEN + RL + RDS(ON) )
> tON(MIN) (210ns typ.)
t OFF =
VOUT
Thermal Protection
A thermal protection feature is included to protect the
RT8473 from excessive heat damage. When the junction
temperature exceeds a threshold of 150°C, the thermal
protection will turn off the LX terminal. When the junction
temperature drops below 125°C, the RT8473 will turn back
on the LX terminal and return to normal operations.
L × ΔIL
+ VD + VSEN + (IOUT × RL )
> tOFF(MIN) (170ns typ.)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
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RT8473
Maximum Power Dissipation (W)1
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature. The
maximum power dissipation can be calculated by the
following formula :
PD(MAX) = (TJ(MAX) − TA) / θJA
where TJ(MAX) is the maximum junction temperature, TA is
the ambient temperature, and θJA is the junction to ambient
thermal resistance.
For recommended operating condition specifications of
the RT8473, the maximum junction temperature is 125°C
and TA is the ambient temperature. The junction to ambient
thermal resistance, θJA, is layout dependent. For TSOT23-5 package, the thermal resistance, θJA, is 250°C/W
on a standard JEDEC 51-3 single-layer thermal test board
and 160°C/W on a standard JEDEC 51-7 four-layer thermal
test board. The maximum power dissipation at TA = 25°C
can be calculated by the following formulas :
0.70
0.65
0.60
0.55
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
TSOT23-5 (Four-Layer PCB)
TSOT23-5 (Single-Layer PCB)
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curves for the RT8473 Packages
Layout Considerations
For best performance of the RT8473, please abide the
following layout guide.
`
The capacitor C1, C2 and external resistor, R1, R2 must
be placed as close as possible to the VIN and SENSE
pins of the device respectively.
PD(MAX) = (125°C − 25°C) / (250°C/W) = 0.400W for
`
The GND should be connected to a strong ground plane.
TSOT-23-5 package (single-layer PCB)
`
Keep the main current traces as short and wide as
possible.
`
The inductor (L1) should be mounted as close to the
device with low resistance connections.
`
The ADJ pin trace need to be kept far away from LX
terminal.
`
An example of PCB layout shown as Figure 3 and
Figure 4 for reference.
`
Rectifier diode D1 as close as possible to LX and VIN
pins.
PD(MAX) = (125°C − 25°C) / (160°C/W) = 0.625W for
TSOT-23-5 package (four-layer PCB)
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. For the RT8473 packages, the derating
curves in Figure 2 allow the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
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RT8473
Sense resistor R1 and R2 (optional)
with short loop between VIN and
SENSE pins.
Input capacitor C1 as
close as possible to VIN
and GND pins.
Place the capacitor C2
(optional) as close as
possible to the ADJ pin.
Rectifier diode D1 as close as
possible to LX and VIN pins.
Optional RC snubber R3 and
C4 for lowering EMI.
Sufficient copper at GND
pin for thermal cooling.
Figure 3. PCB Layout Guide (Top Layer)
Figure 4. PCB Layout Guide (Bottom Layer)
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RT8473
Outline Dimension
H
D
L
B
C
b
A
A1
e
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.700
1.000
0.028
0.039
A1
0.000
0.100
0.000
0.004
B
1.397
1.803
0.055
0.071
b
0.300
0.559
0.012
0.022
C
2.591
3.000
0.102
0.118
D
2.692
3.099
0.106
0.122
e
0.838
1.041
0.033
0.041
H
0.080
0.254
0.003
0.010
L
0.300
0.610
0.012
0.024
TSOT-23-5 Surface Mount Package
Richtek Technology Corporation
5F, No. 20, Taiyuen 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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