RT8464 - Richtek

®
RT8464
Isolated Secondary Side 6-CH LED Driver with Optocoupler
Driver
General Description
The RT8464 is an isolated secondary side 6-CH LED driver
that delivers well matched LED current to each channel of
LED strings. A built-in optocoupler driver supports the
transformer primary side control circuit to regulate the
LED current.
The LED current is linearly controlled by a high gain amplifier
that drives the optocoupler to control the primary side,
thus making the conventional secondary DC/DC PWM
switching stage unnecessary. The RT8464 selects and
regulates the LED strings with the highest voltage, hence
it allows voltage mismatches between LED strings. The
RT8464 automatically detects and disconnects any
unconnected and/or broken strings during operation from
the PWM loop to prevent VOUT from over voltage.
The LED currents on all channels can be simply
programmed with a resistor on each channel. Three
convenient dimming methods are provided : 1. Analog
dimming is linearly controlled by an external voltage; 2.
True digitally controlled PWM dimming controls duty cycle
of LED current; 3. PWM dimming signal can also be easily
filtered into analog dimming signal for noise-free PWM
dimming by the low pass filter with an external capacitor
at ACTL pin. Other protecting features include output over
voltage protection and thermal shutdown.
The RT8464 is available in the SOP-28 package.
Features
z
z
z
z
z
z
z
z
z
z
High Voltage : VCC up to 32V
Channel Current Programmable
3% Current Sense Threshold Voltage
Easy Analog and Digital Dimming Control
Adjustable Soft-Start to Avoid Inrush Current
Automatic Detecting Unconnected Channel
Adjustable Over Voltage Protection to Limit Output
Voltage
Thermal Shutdown Under Voltage Lockout
SOP-28 Package
RoHS Compliant and Halogen Free
Applications
z
z
z
z
Building and Street Lighting
LED TV Backlight
LED Monitor Backlight
Industrial Display Backlight
Simplified Application Circuit
Isolation Barrier
VIN
EN
RT8464
Primary Side
Controller
VCC
LEDx
x6
GATEx
x6
SENSEx
x6
VC
SS
Dimming Signal
ACTL
RSENSE
OPTO
x6
GND
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8464-00
November 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT8464
Ordering Information
Pin Configurations
RT8464
(TOP VIEW)
Package Type
S : SOP-28
Lead Plating System
G : Green (Halogen Free and Pb Free)
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
VCC
OPTO
CL
EN
OVP
SS
VC
ACTL
DCTL
SENSE6
GATE6
LED6
SENSE5
GATE5
RT8464GS : Product Number
RichTek
RT8464
GSYMDNN
28
2
27
3
26
4
25
5
24
6
23
7
22
8
21
9
20
10
19
11
18
12
17
13
16
14
15
GND
LED1
GATE1
SENSE1
LED2
GATE2
SENSE2
LED3
GATE3
SENSE3
LED4
GATE4
SENSE4
LED5
SOP-28
YMDNN : Date Code
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VCC
Power Supply Input of the Chip. For good bypass, a low ESR capacitor is needed
between this pin and GND.
2
OPTO
Optocoupler Driver. It is an open collector output of the internal NPN transistor.
3
CL
Current Limit. Current limit threshold is 100mV.
4
EN
Chip Enable. The chip is active when VEN > 1.4V.
5
OVP
Over Voltage Detection Input. OVP pin threshold is 1.2V (typ.).
6
SS
Soft-Start. There is an internal constant current (6μA) to the SS pin during startup.
Connect a capacitor to set soft-start time.
7
VC
PWM Loop Compensation.
8
ACTL
Analog/PWM Dimming Control Input. When using in analog dimming, ACTL control
range is from 0.4V to 1.2V.
9
DCTL
Digital Dimming Control Input. By adding a 0.1μF filter capacitor at ACTL, the digital
dimming signal at DCTL pin will be averaged out and converted into analog
dimming signal at ACTL pin.
10, 13, 16,
19, 22, 25
11, 14, 17,
20, 23, 26
12, 15, 18,
21, 24, 27
28
SENSE6 to
SENSE1
GATE6 to
GATE1
LED6 to
LED1
Current Sense Input for LED Current. The threshold is 215mV and ILED = 0.215V /
RSENSE
GND
Ground.
Gate Drive Output for External Current Source MOSFETs.
LED String Voltage Sensing Input. This pin is connected to the Drain of external
current source MOSFET.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
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is a registered trademark of Richtek Technology Corporation.
DS8464-00
November 2012
RT8464
Function Block Diagram
OVP
+
1.2V
-
VCC
OPTO
1.2V
+
+
0.7V
Min
Select
+
VC
-0.1V
CL
6
LEDx
6
GATEx
-
+
6
-
SENSEx
2.5V/6µA
SS
1.4V
+
EN
-
Shutdown
5V
1.2V
DCTL
+
-
+
-
GND
ACTL
Operation
The RT8464 regulates the lowest cathode voltage of LED strings and generates a feedback control signal to a primary
controller to regulate LED current. Each LED channel current is accurately matched and controlled by sensing an
external resistor in series with the MOSEFT. LED current dimming in all six channels can be precisely controlled by
either a PWM signal via the DCTL input pin or by an analog dimming voltage applied at the ACTL pin. Internal protection
is provided for over current and over voltage at the secondary output. The device can be operated in low current shutdown
mode by pulling the EN pin low.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8464-00
November 2012
is a registered trademark of Richtek Technology Corporation.
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3
RT8464
Absolute Maximum Ratings
(Note 1)
Supply Voltage, VCC ----------------------------------------------------------------------------------------------------SENSEx --------------------------------------------------------------------------------------------------------------------(1)
z LEDx
---------------------------------------------------------------------------------------------------------------------(2)
z DCTL, ACTL, EN, OVP
------------------------------------------------------------------------------------------------z CL ----------------------------------------------------------------------------------------------------------------------------z Power Dissipation, PD @ TA = 25°C
SOP-28 ---------------------------------------------------------------------------------------------------------------------z Package Thermal Resistance (Note 2)
SOP-28, θJA ----------------------------------------------------------------------------------------------------------------z Junction Temperature ----------------------------------------------------------------------------------------------------z Lead Temperature (Soldering, 10 sec.) ------------------------------------------------------------------------------z Storage Temperature Range -------------------------------------------------------------------------------------------z ESD Susceptibility (Note 3)
HBM (Human Body Model) ---------------------------------------------------------------------------------------------MM (Machine Model) ----------------------------------------------------------------------------------------------------Notes :
(1) Add a series resistor of at least 30kΩ for higher pin voltage.
(2) Add a series resistor of at least 20kΩ for higher pin voltage.
z
z
Recommended Operating Conditions
z
z
z
−0.3V to 34V
−0.3V to 12V
−0.3V to 20V
−0.3V to 10V
−1V to 0.3V
1.31W
76.5°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Voltage, VCC ----------------------------------------------------------------------------------------------------- 10V to 32V
Junction Temperature Range -------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range -------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VCC = 24V, No load on any output, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
OVERALL
VCC Supply Current
IVCC
VVC ≤ 0.4V (Not Switching)
--
5
8
mA
Shutdown Current
ISHDN
VEN ≤ 0.7V
--
15
--
μA
Shutdown Threshold at EN
VEN
--
1.4
1.6
V
Input Current at EN
IEN
--
--
0.1
μA
204
215
226
mV
--
--
0.5
μA
--
0.4
0.5
V
--
--
0.5
μA
--
0.1
--
V
Highest Voltage LED String
--
--
0.1
V
2.4V > VVC > 0.2V
--
±30
--
μA
VEN ≤ 5V
LED Current Programming
SENSEx Threshold
Analog Dimming Input Current at
ACTL
LED Current Off Threshold at ACTL
6V > VGATEx > 2V
VACTL
Input Current at DCTL
IDCTL
0.3V ≤ VACTL ≤ 1.3V
0.3V ≤ VDCTL ≤ 6V
VSENSEx Threshold for No Connection
Regulated VLEDx
Amplifier gm Output Current
IVC
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is a registered trademark of Richtek Technology Corporation.
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RT8464
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
--
0.15
--
V
IGATEx = −1mA
4.2
4.8
--
IGATEx = −0.1mA
4.5
5
--
IGATEx = 1mA
--
0.8
1.1
IGATEx = 0.1mA
--
0.7
1
1.17
1.2
1.25
V
Optocoupler Driver
OPTO Low Voltage
IOPTO = 2mA, VVC = 1.6V
LED Current Sources Gate Driver
GATEx High Voltage
GATEx Low Voltage
V
V
OVP and Soft-Start
OVP Threshold
VOVP
OVP Input Current
VOVP ≤ 1.2V
--
--
−100
nA
Soft-Start Current at SS
VSS ≤ 2.5V
--
6
--
μA
--
150
--
°C
Thermal Protection
Thermal Shutdown Temperature
TSD
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.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8464-00
November 2012
is a registered trademark of Richtek Technology Corporation.
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RT8464
Typical Application Circuit
10V to 32V
AC Power
Input
Primary
Side
Control
Circuit
R1
5 OVP
RT8464
6 x N LEDs
R2
1 VCC
LED1
27
GATE1 26
25
SENSE1
2
Optocoupler
3
CVC
RS1
OPTO
CL
LED2 24
GATE2
RVC
CSS
7
VC
M1
23
M2
SENSE2 22
RS2
6 SS
LED3 21
GATE3 20
Chip Enable
4
EN
9 DCTL
Dimming Signal
8 ACTL
28
GND
M3
SENSE3 19
RS3
LED4
18
GATE4 17
SENSE4
M4
16
RS4
LED5
15
GATE5 14
M5
SENSE5 13
RS5
LED6 12
GATE6 11
10
SENSE6
M6
RS6
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RT8464
Typical Operating Characteristics
OVP vs. Input Voltage
1.240
9
1.235
8
1.230
7
OVP (V)
Supply Current (mA)
Supply Current vs. Input Voltage
10
6
5
4
3
1.225
1.220
OVP_H
1.215
1.210
OVP_L
2
1.205
1
VIN = 10V to 32V
VIN = 10V to 32V
1.200
0
10
12
14
16
18
20
22
24
26
28
30
10
32
12.75 15.5 18.25
Input Voltage (V)
LED Current vs. ACTL PWM Duty
225
225
125
LED Current (mA)
LED Current (mA)
LED1 (mA)
LED2 (mA)
LED3 (mA)
LED4 (mA)
LED5 (mA)
LED6 (mA)
150
100
75
50
LED1 (mA)
LED2 (mA)
LED3 (mA)
LED4 (mA)
LED5 (mA)
LED6 (mA)
200
175
150
125
100
75
50
25
25
VIN = 15V, PWM = 100Hz, RSENSE = 0.95Ω
0
VIN = 15V, RSENSE = 0.95Ω
0
0
10
20
30
40
50
60
70
80
90
100
0.4
0.6
0.7
0.8
0.9
100
7.2
95
6.4
Gate Voltage (V)
90
85
80
75
70
VIN = 10V to 32V, IOPTO = 1mA
60
14
16
18
20
22
24
26
28
Input Voltage (V)
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
November 2012
1.2
1.3
1.4
4.8
4.0
3.2
2.4
Low
0.8
VIN = 10V to 32V
0.0
12
1.1
High
5.6
1.6
65
10
1
Gate Voltage vs. Input Voltage
OPTO Voltage (Low) vs. Input Voltage
OPTO Voltage Low (mV)
0.5
ACTL Voltage (V)
ACTL PWM Duty (%)
DS8464-00
32
LED Current vs. ACTL Voltage
250
175
23.75 26.5 29.25
Input Voltage (V)
250
200
21
30
32
10
12.75 15.5 18.25
21
23.75 26.5 29.25
32
Input Voltage (V)
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RT8464
Application Information
The RT8464 regulates the lowest cathode voltage of LED
strings and generates a feedback control signal to a
primary controller to regulate LED current. Each LED
channel current is accurately matched and controlled by
sensing an external resistor in series with the MOSEFT.
LED current dimming in all six channels can be precisely
controlled by either a PWM signal via the DCTL input pin
or by an analog dimming voltage applied at the ACTL pin.
Internal protection is provided for over current and over
voltage at the secondary output. The device can be
operated in low current shutdown mode by pulling the EN
pin low.
Under Voltage Lockout
The input operating voltage range of the RT8464 is from
10V to 32V. Placing an input capacitor at the VCC pin can
reduce ripple voltage. It is recommended to use a 10μF
ceramic capacitor or larger capacitance as the input
capacitor. This IC provides an Under Voltage Lockout
(UVLO) function to enhance the stability when start up.
The UVLO rising input voltage threshold is set at 7V
typically with a 0.7V hysteresis.
Soft-Start
Soft-start of the RT8464 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 charged by an internal 6μA constant
charging current. The SS pin directly limits the rate of
voltage rise at the VC pin, which in turn limits the peak
switch current.
The soft-start interval is set by the soft-start capacitor
selection according to the following equation :
tSS = CSS x
2.5V
6μ A
A typical value for the soft-start capacitor is 0.1μF. The
soft-start pin reduces the oscillator frequency and the
maximum current in the switch. The soft-start capacitor
is discharged when EN/UVLO falls below its threshold or
during an over temperature event.
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Optocouple Driver
The OPTO output directly drives the diode side of an
optocoupler to give isolated feedback control of a primary
side PWM controller. The guaranteed sink current of the
opto pin is 1mA.
When the LEDx (x = 1 to 6) voltage decreases to below
0.7V, the error amplifier reacts by lowering the VC pin
voltage, thereby decreasing the current from optocoupler.
The decreased optocoupler bias signals the primary side
controller to increase the amount of power and then raise
the output voltage back to its regulated value.
The output voltage of OPTO also responds to different
protection conditions. When OVP or current limit occurs,
the OPTO pin will sink more current immediately from
the optocoupler. The increased optocoupler bias signal
on the primary side controller will decrease secondary
side output power.
Compensation
The RT8464 uses an internal error amplifier, in which
through its compensation pin (VC) allows the loop
response to be optimized for specific application. The error
amplifier is a true voltage mode error amplifier and
frequency compensation is performed around the amplifier.
VC also ties to the overshoot control amplifier logic that
detects if the VC pin is at its high clamp level. 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.
LED Current Setting
The maximum current of channel 1 to 6 is programmed by
placing an appropriate sense resistor for each LED string.
When the voltage of ACTL is higher than 1.4V, the LED
current can be calculated by the following equation :
215mV
ILED, MAX =
(mA)
RSENSE
where R SENSE is the resistor between the external
regulating N-MOSFET and GND.
The ACTL pin should be tied to a voltage higher than 1.4V
to get the full-scale 215mV (typical) threshold across the
sense resistor. The ACTL pin can also be used to dim the
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November 2012
RT8464
LED current to zero, although relative accuracy decreases
with decreasing voltage sense threshold. When the ACTL
pin voltage is less than 1.4V, the LED current is :
ILED =
(VACTL − 0.4) × 215mV
RSENSE
(mA)
The ACTL pin can also be used in conjunction with a
thermistor to provide over temperature protection for the
LED load, or with a resistive voltage divider to VCC to
reduce output power and switching current when VCC is
low.
Brightness Control
For LED applications where a wide dimming range is
required, two methods are available: analog dimming and
PWM dimming. The easier method is to simply vary the
DC current through the LED by analog dimming.
The other dimming method is PWM dimming, which turns
the LED on and off by different duty cycle to control the
average LED current. The PWM dimming offers several
advantages over analog dimming and is more preferred by
LED manufacturers. One advantage is the chromaticity of
the LEDs which remains unchanged in this scheme since
the LED current is either zero or at a programmed current.
Another advantage of PWM dimming is that a wider
dimming range is available.
The RT8464 features both analog and digital dimming
control. Analog dimming is linearly controlled by an
external voltage (0.4V to 1.2V) at the ACTL pin. A very
high contrast ratio can be obtained by true digital PWM
dimming which is achieved by driving the ACTL pin with a
PWM signal. The recommended PWM frequency is from
100Hz to 10kHz.
The PWM dimming frequency can be sufficiently adjusted
from 100Hz to 30kHz. However, the LED current cannot
be 100% proportional to duty cycle, especially for high
frequency and low duty ratio because of physical limitation
caused by the internal switching frequency.
Output Over Current Protection
The current limit amplifier senses the voltage drop across
an external sense resistor via the CL pin to implement
over current protection. A voltage drop of 100 mV, gives a
maximum current limit of 2A with a 0.5Ω sense resistor.
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8464-00
November 2012
Output Over Voltage Protection
The RT8464 is equipped with Over Voltage Protection
(OVP) function. When the voltage at the OVP pin exceeds
threshold value typically 1.2V, the power switch is turned
off. The power switch can be turned on again once the
voltage at the OVP pin drops below 1.2V. The output
voltage can be clamped at a certain voltage level set by
the following equation :
VOUT, OVP = 1.2 × (1+
R1
)
R2
where R1 and R2 are the resistors in the resistive voltage
divider from VOUT to GND with the divider center node
connected to the OVP pin.
If at least one string is in normal operation, the controller
will automatically ignore the open strings and continue to
regulate the current for the string(s) in normal operation.
Over Temperature Protection
The RT8464 has Over Temperature Protection (OTP)
function to prevent excessive power dissipation from
overheating the device. 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 down by
approximately 20°C.
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, the
maximum junction temperature is 125°C. The junction to
ambient thermal resistance, θJA, is layout dependent. For
SOP-28 package, the thermal resistance, θ JA , is
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RT8464
76.5°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 :
Layout Considerations
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guidelines are as follows :
PD(MAX) = (125°C − 25°C) / (76.5°C/W) = 1.31W for
SOP-28 package
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.
`
The input capacitor CVCC must be placed as close to the
VCC pin as possible.
`
Place the compensation components as close to the
VC pin as possible to avoid noise pick up.
1.4
Four-Layer PCB
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 1. Derating Curve of Maximum Power Dissipation
Power Path should be Wide as possible
Isolation Barrier
VIN
EN
Primary Side
Controller
RT8464
GND
VCC
Place the
compensation
components
as close to
the VC pin as
possible
GND
Dimming Signal
VC
LEDx
x6
GATEx
x6
SENSEx
x6
SS
ACTL
OPTO
GND
RSENSE
x6
GND
Figure 2. PCB Layout Guide
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is a registered trademark of Richtek Technology Corporation.
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RT8464
Outline Dimension
H
A
M
J
B
F
C
I
D
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
17.704
18.110
0.697
0.713
B
7.391
7.595
0.291
0.299
C
2.362
2.642
0.093
0.104
D
0.330
0.508
0.013
0.020
F
1.194
1.346
0.047
0.053
H
0.229
0.330
0.009
0.013
I
0.102
0.305
0.004
0.012
J
10.008
10.643
0.394
0.419
M
0.381
1.270
0.015
0.050
28–Lead SOP Plastic 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.
DS8464-00
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