RT8456 - Farnell

®
RT8456
Isolated Secondary Side LED Lighting Current Controller
General Description
Features
The RT8456 is an isolated secondary LED current controller
designed specifically for lighting fixtures such as E27,
GU10, T5 and T8. The IC is suitable for applications that
require power line isolation for safety and reliability, high
conversion efficiency, as well as high LED current accuracy.
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Secondary Side LED Current Regulation
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4.75V to 48V Operation Range
High Accuracy 59mV LED Current Threshold Control
0.6mA Operating Current
Programmable LED Current
Output Over Voltage Protection
Small SOT-23-6 Package
RoHS Compliant and Halogen Free
The RT8456 contains 1) a constant current regulating
amplifier with 59mV threshold used to program the LED
string current with a simple resistor to within 5% LED
current accuracy; 2) an over voltage comparator to protect
the output when LED string is open or broken; 3) and an
opto-coupler driver to control the primary side of the
transformer to complete the system loop.
The RT8456 is offered in a small SOT-23-6 package.
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Applications
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Isolated LED Lighting Fixtures
E27/GU10/T5/T8
Pin Configurations
(TOP VIEW)
Ordering Information
RT8456
VCC OUT CP
6
Package Type
E : SOT-23-6
Lead Plating System
G : Green (Halogen Free and Pb Free)
5
4
2
3
CN GND OVP
SOT-23-6
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
EM= : Product Code
EM=DNN
DNN : Date Code
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8456-05 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT8456
Function Block and Typical Application Circuit
D1
VOUT
Optocoupler
VCC
RLED
R1
RT8456
Secondary
side winding
CVCC
LED String
OUT
2.5V
- CV
+
RVC1 CVC1
OVP
59mV
- CC
RIC1
+
R2
CIC1
CP
CN
GND
RCN
RSENSE
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
CN
Non-inverting Input of the CC Regulating Amp. It has a 59mV offset from the CP pin.
The CN pin should be connected to the "Current In" node of the current sensing
resistor, RSENSE .
2
GND
Ground of the RT8456 and secondary side.
3
OVP
Output Over Voltage Protection Pin with Threshold of 2.5V.
4
CP
5
OUT
6
VCC
Inverting Input of the CC Regulating Amp. It has a −59mV offset from CN pin. CP pin
connects to a resistor to set LED current.
Open-collector Output. Connects this pin to an opto-coupler with a current limiting
resistor.
Supply Voltage Input. A 0.1μF bypass capacitor should be connected between VCC
and GND.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8456-05 April 2013
RT8456
Absolute Maximum Ratings
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(Note 1)
Supply Input Voltage, VCC ----------------------------------------------------------------------------------------------CP -----------------------------------------------------------------------------------------------------------------------------CN -----------------------------------------------------------------------------------------------------------------------------OUT ---------------------------------------------------------------------------------------------------------------------------Power Dissipation, PD @ TA = 25°C
SOT-23-6 -------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
SOT-23-6, θJA ---------------------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ----------------------------------------------------------------------------------------------MM (Machine Model) ------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
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−0.3V to 52V
−0.6V to 1V
−0.3V to 1V
−0.3V to 52V
0.4W
250°C/W
260°C
150°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Input Voltage, VCC (Note5) --------------------------------------------------------------------------------- 4.75V to 48V
Junction Temperature Range --------------------------------------------------------------------------------------------- −40°C to 125°C
Electrical Characteristics
(VCC = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Quiescent Current
ICC
VCN = VCP = 0V
--
550
--
μA
GND Pin Current
IGND
VCN = VCP = 0V, GND = 0V
--
530
--
μA
OVP Voltage
VOVP
VCN = VCP = 0V
2.46
2.5
2.54
VCN = VCP = 0V, T A = −25°C to 105°C
2.45
--
2.55
OVP Input Bias Current
IOVP
VOVP = 2.4 to 2.6V
--
--
100
LED Current Sensing
Amplifier Threshold
VCN−CP
VOVP = 2.4V
56
59
62
VOVP = 2.4V, TA = −25°C to 105°C
55
--
63
--
--
200
nA
--
8
--
mA
CN Input Bias Current
ICN
OUT Maximum Pin Sink
IOUTH
Current
VOUT = 1.5V
V
nA
mV
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 low effective thermal conductivity single-layer test board per JEDEC 51-3.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Note 5. RT8456 starts regulation at VCC ≥ 4.5V, and meets all parameter specs at VCC ≥ 4.75V.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8456-05 April 2013
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT8456
Typical Operating Characteristics
VOVP vs. Temperature
ICC vs. Temperature
2.52
680
VCC
VCC
VCC
VCC
640
45V
36V
12V
4.75V
2.51
VCC
VCC
VCC
VCC
2.50
VOVP (V)
I CC (µA)
600
=
=
=
=
560
520
=
=
=
=
45V
36V
12V
4.75V
2.49
2.48
480
2.47
440
2.46
400
-50
-25
0
25
50
75
100
-50
125
-25
0
50
75
100
125
Temperature (°C)
Temperature (°C)
IOVP vs. Temperature
VOUTL vs. Temperature
50
1.0
45
VCC
VCC
VCC
VCC
40
35
30
=
=
=
=
0.9
45V
36V
12V
4.75V
0.8
0.7
VOUTL (V)
I OVP (nA)
25
25
20
0.5
0.3
10
0.2
5
0.1
0
0.0
-25
0
25
50
75
100
=
=
=
=
45V
36V
12V
4.75V
0.4
15
-50
VCC
VCC
VCC
VCC
0.6
125
IOUT = 2mA
-50
-25
0
Temperature (°C)
25
50
75
100
125
Temperature (°C)
IOUTH vs. Temperature
VCN-CP vs. Temperature
61
10
9
60
8
59
VCN-CP (mV)
I OUTH (mA)
7
6
VCC
VCC
VCC
VCC
5
4
3
=
=
=
=
45V
36V
12V
4.75V
58
VCC
VCC
VCC
VCC
57
56
=
=
=
=
45V
36V
12V
4.75V
2
55
1
54
0
-50
-25
0
25
50
75
100
Temperature (°C)
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4
125
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
DS8456-05 April 2013
RT8456
Application Information
Output Voltage Setting
Compensation
The voltage control loop is controlled via the first transconductance operational amplifier. An optocoupler which
is directly connected to the output and an external resistor
bridge is connected between the output positive line and
the ground reference. The middle point is to be connected
to the OVP pin of the RT8456, where R2 is the upper
resistor and R1 the lower resistor of the bridge. The
relationship between R2 and R1 is shown below
(R1 + R2 )
VOUT = VOVP x
R2
( VOUT - VOVP )
R1 = R2 x
VOVP
where VOUT is the desired maximum output voltage. To
avoid discharge of the load, the resistor bridge R1, R2,
should be highly resistive. For this type of application a
total value of 100kΩ (or more) would be appropriate for
the resistors R1 and R2.
Both the voltage control trans conductance amplifier and
the current control trans-conductance amplifier can be fully
compensated. The output and negative inputs are directly
accessible for external compensation components, as
shown in the Typical Application Circuit.
As an example, with R1 = 80kΩ and R2 = 20kΩ,
VOUT = 12.5V
Output Current Setting
The current control loop is controlled via the second
transconductance operational amplifier. An optocoupler and
the sense resistor, RSENSE, is placed in series on the output
negative line. VCN−CP threshold is achieved externally by
a resistor bridge tied to the reference voltage, VREF. Its
middle point is tied to the positive input of the current
control operational amplifier and its foot is connected to
the lower potential point of the sense resistor. The resistors
of the bridge are matched to provide the best precision.
With VCN−CP and RSENSE, the expected output current,
IOUT, can be obtained as below equation :
IOUT
V
= CN−CP
RSENSE
where IOUT is the desired maximum output current, and
VCN−CP the threshold voltage for the current control loop.
Note that the sense resistor, RSENSE, should be chosen
taking into account its maximum power dissipation (PLIM)
during full load operation.
Copyright © 2013 Richtek Technology Corporation. All rights reserved.
DS8456-05 April 2013
The typical component values for the compensation
network of voltage control loop is CVC1 = 2.2nF and RVC1
= 22kΩ. The typical component values for the
compensation network of current control loop is CIC1 =
2.2nF, RIC1 = 22kΩ and RCN = 1kΩ. However, in many
application conditions, the current control loop can be
stable without compensation network (RCN = 0, no CIC1
nor RIC1).
When the voltage control loop is used as the voltage limit
protection or the current control loop is used as the current
limit protection, no compensation network is needed for
the protecting control loop.
A resister, ROPT, must be connected in series with the
opto-coupler since it is part of the compensation network.
Although the value of ROPT is not critical, it's recommended
to be in the range from 0.33kΩ to (VOUT − 2) / (0.005)Ω.
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.
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RT8456
For recommended operating condition specifications, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOT-23-6 packages, the thermal resistance, θJA, is 250°C/
W on a standard JEDEC 51-3 single-layer thermal test
board. The maximum power dissipation at TA = 25°C can
be calculated by the following formula :
P D(MAX) = (125°C − 25°C) / (250°C/W) = 0.4W for
SOT-23-6 package
Layout Consideration
For the best performance of the RT8456, the following
PCB Layout guidelines must be strictly followed.
`
Place the RSENSE resistor as close to the IC as possible.
`
Keep the input/output traces as wide and short as
possible.
Secondary
side winding
D1
VOUT
Maximum Power Dissipation (W)1
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 1 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
Optocoupler
RLED
R1
RVC1 CVC1
CVCC
0.5
RIC1
:
: LED String
:
CIC1
CN
Single-Layer PCB
VCC OUT CP
0.4
6
0.3
5
4
2
3
CN GND OVP
0.2
R2
RCN
0.1
RSENSE
0.0
0
25
50
75
100
125
Figure 2. PCB Layout Guide
Ambient Temperature (°C)
Figure 1. Derating Curve of Maximum Power Dissipation
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is a registered trademark of Richtek Technology Corporation.
DS8456-05 April 2013
RT8456
Outline Dimension
H
D
L
C
B
b
A
A1
e
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.889
1.295
0.031
0.051
A1
0.000
0.152
0.000
0.006
B
1.397
1.803
0.055
0.071
b
0.250
0.560
0.010
0.022
C
2.591
2.997
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
SOT-23-6 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.
DS8456-05 April 2013
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