DS8489 00

®
RT8489
High Voltage 4-CH LED Driver Controller
General Description
The RT8489 is a 4-CH LED driver controller that delivers
well matched LED current to each channel of LED string.
With external current sources, the number of LEDs per
string is only limited by the current source and the VIN/
Other protection features include adjustable output over
voltage protection, PWM switch current limit and thermal
shutdown.
VOUT conditions. The current mode PWM Boost type
controller operates at an adjustable switching frequency
of up to 1MHz, with a wide VIN range covering from 7V to
28V. The switch driver is designed to drive industrial grade
high power MOSFETs.
Features
The PWM loop selects and regulates the LED string with
the highest voltage string to 0.7V, thus allowing voltage
mismatches between the LED strings. The RT8489
automatically detects and excludes any open and/or
broken strings during operation from the PWM loop to
prevent VOUT from over voltage.
The LED currents on all channels are simply adjustable
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 can regulate the duty cycle of
the LED current. 3. For noise free PWM dimming, use an
on board output clamping amplifier as a low pass filter to
convert PWM dimming signals into analog dimming
signals.
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Wide Operation Voltage Range : 7V to 28V
Programmable Channel Current
3% Current Matching Accuracy between Channels
Adjustable Switching Frequency
Easy Analog and Digital Dimming Control
Adjustable Soft-Start
Adjustable Output Over Voltage Protection
Under Voltage Lockout and Thermal Shutdown
SOP-24 package
RoHS Compliant and Halogen Free
Applications
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Building and Street Lighting
LED TV Backlight
LED Monitor Backlight
Industrial Display Backlight
Simplified Application Circuit
L1
VIN
VCC
CIN
R3
D1
GATE
VCC
CVCC
RSET
4 x N LEDs
LED1
VC
SS
M1
SENSE1
……
CSS
GATE1
DCTL
LED4
GATE4
ACTL
SENSE4
GND
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
November 2012
……
RSENSE
CVC
DS8489-00
MSW
ISENSE
RSET
RVC
VOUT
COUT
RT8489
RS1
M4
RS4
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1
RT8489
Ordering Information
Pin Configurations
RT8489
(TOP VIEW)
Package Type
S : SOP-24
Lead Plating System
G : Green (Halogen Free and Pb Free)
VCC
GBIAS
GATE
GND
ISENSE
RSET
EN
OVP
SS
VC
ACTL
DCTL
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
23
3
22
4
21
5
20
6
19
7
18
8
17
9
16
10
15
11
14
12
13
LED1
GATE1
SENSE1
LED2
GATE2
SENSE2
LED3
GATE3
SENSE3
LED4
GATE4
SENSE4
SOP-24
RT8489GS : Product Number
RichTek
RT8489
GSYMDNN
24
2
YMDNN : Date Code
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
VCC
Power Supply Input. For good bypass, a low ESR capacitor is needed between
this pin and GND.
2
GBIAS
Internal Gate Driver Bias Voltage (around 10V) Pin. A good bypass capacitor
between this pin and GND is needed.
3
GATE
Gate of External MOSFET. For the Boost PWM control loop.
4
GND
Ground.
5
ISENSE
Current Sense Input. Connect a sense resistor from this pin to GND. The switch
current sense signal is used for boost current mode PWM loop control and power
switch over current protection.
6
RSET
Switching Frequency Set Pin. Put a resistor from RSET to GND to program the
switching frequency. fSW = 280kHz when R SET = 40kΩ.
7
EN
Enable Control Input (Active High).
8
OVP
Over Voltage Protection Sense Input. OVP pin threshold is around 1.23V. Use a
resistor divider from Output to GND to program the OVP level.
9
SS
Soft-Start. Connect a soft-start capacitor from SS pin to GND to adjust the
soft-start time. An internal 8μA constant current charges external capacitor during
soft-start period.
10
VC
Loop Compensation.
11
ACTL
Analog/PWM Dimming Control Input. When used in analog dimming, ACTL
control range is from 0.4V to 1.4V.
12
DCTL
Digital PWM Dimming Control Input. By adding a 0.1μF filter capacitor on the
ACTL pin, the PWM dimming signal on the DCTL pin will be averaged out and
converted into analog dimming signal on the ACTL pin.
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is a registered trademark of Richtek Technology Corporation.
DS8489-00
November 2012
RT8489
Pin No.
Pin Name
Pin Function
13, 16, 19, 22 SENSEx
Source of External MOSFETx (x = 1 to 4).
The SENSEx pins are regulated around 225mV. Connect a sense resistor from
this pin to GND. The LED current is set by ILED = 225mV / (sense resistance)
when VACTL is greater than 1.4V.
14, 17, 20, 23 GATEx
Gate Drive Output for External MOSFETx (x = 1 to 4).
15, 18, 21, 24 LEDx
LED String Voltage Sense Input. Connect these pins to the Drain of External
MOSFETx (x = 1 to 4). Short the pin to GND if not used.
Function Block Diagram
VCC
10V
OSC
RSET
+
-
5.5V
GBIAS
+
-
1.23V
PWM
Control
-
OVP
GATE
ISENSE
+
3.2V
GBIAS
+
-
+
VC
LED2
3.2V
VOUT
Regulation
Unit
8µA
1.4V
+
3.2V
Shutdown
+
-
1.4V
+
-
ACTL
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
November 2012
LED3
GATE3
3.2V
SENSE3
-
1.4V
DS8489-00
GBIAS
+
LED2
GATE2
+
3.2V
DCTL
GBIAS
SENSE2
LED4
EN
SENSE1
3.2V
-
LED3
SS
GATE1
-
LED1
LED1
+
GBIAS
LED4
GATE4
-
SENSE4
GND
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RT8489
Operation
The internal gate driver circuit is powered from GBIAS pin
around 10V. The GBIAS voltage is generated by an internal
base driver to drive an external NPN emitter follower. The
OSC block generates a programmable frequency which
is set by an external resistor at RSET pin for RT8489
operation. At the beginning of the oscillator cycle, the
GATE turns high. The VOUT regulation unit automatically
detects the lowest sensed LEDx pin feedback voltage
among the 4 LED strings and compares to 0.7V. If the
lowest LEDx pin voltage is lower than 0.7V, the VC pin
(the output of the OP AMP in VOUT regulation unit) is
charged high. The ISENSE pin voltage is the triangular
feedback signal of the sensed switch current (which equals
inductor current ramp).The PWM comparator compares
ISENSE pin voltage to VC pin voltage. When ISENSE pin
voltage exceeds VC pin voltage, the PWM comparator
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resets the latch and turns off GATE. If ISENSE pin voltage
exceeds VC pin voltage by the end of the switching cycle,
the GATE will be turned off for minimum off-time. The cycle
repeats when the GATE is turned on at the beginning of
the next switching cycle. By this PWM closed loop control,
the lowest sensed LEDx pin voltage among the 4 LED
strings is regulated to 0.7V.
As the system starts, the cap at the soft-start pin is slowly
charged up by an internal current source around 8μA. The
slowly rising VC pin voltage allows the PWM duty to
increase gradually to achieve soft-start function.
The dimming can be set by varying ACTL pin analog or
PWM voltage signal. The internal sense threshold for the
4 current sources follows ACTL signal to achieve dimming
control.
is a registered trademark of Richtek Technology Corporation.
DS8489-00
November 2012
RT8489
Absolute Maximum Ratings
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(Note 1)
VCC --------------------------------------------------------------------------------------------------------------------------ISENSE
DC -----------------------------------------------------------------------------------------------------------------------------< 20ns -----------------------------------------------------------------------------------------------------------------------GBIAS ------------------------------------------------------------------------------------------------------------------------SENSE1 to SENSE4
DC -----------------------------------------------------------------------------------------------------------------------------< 20ns -----------------------------------------------------------------------------------------------------------------------LED1 - LED4 (Note 5) ---------------------------------------------------------------------------------------------------DCTL, ACTL, EN, OVP --------------------------------------------------------------------------------------------------Power Dissipation, [email protected] = 25°C
SOP-24 ----------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
SOP-24, θJA ----------------------------------------------------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Model) ----------------------------------------------------------------------------------------------MM (Machine Model) ------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
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−0.3V to 32V
−0.3V to 2V
−0.3V to 6V
−0.3V to 14V
−0.3V to 1V
−0.3V to 6V
−0.3V to 20V
−0.3V to 10V
1.3W
77°C/W
150°C
−65°C to 150°C
260°C
2kV
200V
(Note 4)
Supply Voltage, VIN ------------------------------------------------------------------------------------------------------- 7V to 28V
Junction Temperature Range --------------------------------------------------------------------------------------------- −40°C to 125°C
Ambient Temperature Range --------------------------------------------------------------------------------------------- −40°C to 85°C
Electrical Characteristics
(VIN = 12V, No Load, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Overall
Supply Current
IVCC
VVC ≤ 0.4V (Switching off)
--
5
8
mA
Shutdown Current
ISHDN
VEN ≤ 1.2V
--
5
--
μA
EN Input Voltage
Logic-High
VIH
2
--
--
Logic-Low
VIL
--
--
0.5
--
2
--
μA
214
225
236
mV
--
1.5
3
%
--
--
10
μA
--
0.4
--
V
EN Input Current
IEN
VEN ≤ 3.3V
V
LED Current Programming
SENSE1-SENSE4 Threshold
6V > VGATEx > 2V
V(MAX) − V(MIN)
SENSE Voltage CH to CH Matching
2 × V(avg)
Analog Dimming ACTL Input Current IACTL
LED Current Off Threshold at ACTL
VACTL_OFF
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8489-00
November 2012
VACTL ≤ 6V
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RT8489
Parameter
Symbol
Test Conditions
LED Current On Threshold at ACTL VACTL_ON
DCTL Input Current
Min
Typ
Max
Unit
--
1.4
--
V
--
--
1
μA
IDCTL
VDCTL ≤ 6V
fSW
RSET = 40kΩ
220
280
340
kHz
RSET = 40kΩ
--
300
--
ns
--
0.1
--
V
PWM Boost Converter
Switching Frequency
Minimum Off-Time
VLED Threshold for No Connection VLED
Regulated VLED
VLED
Highest Voltage LED String
--
0.7
--
V
Amplifier Output Current
IVC
2.4V > VVC > 0.2V
--
±30
--
μA
--
0.7
--
V
IGBIAS = 20mA
--
10
--
V
IGATE = −20mA
--
7.7
--
IGATE = −0.1mA
--
8.2
--
IGATE = 20mA
--
0.7
--
IGATE = 0.1mA
--
0.4
--
1nF load at Gate
--
20
--
IGATEx = −2mA
--
8.1
--
IGATEx = −0.1mA
--
8.3
--
IGATEx = 2mA
--
0.8
--
IGATEx = 0.1mA
--
0.6
--
--
1.23
--
V
VC Threshold for PWM Switch Off
Switch Gate Driver
GBIAS Voltage
VGBIAS
GATE High Voltage
VGATE_H
GATE Low Voltage
VGATE_L
GATE Drive Rise and Fall Time
V
V
ns
LED Current Sources Gate Driver
GATE1 to 4 High Voltage
VGATEx_H
GATE1 to 4 Low Voltage
VGATEx_L
V
V
OVP and Soft-Start
OVP Threshold
VOVP
OVP Input Current
IOVP
VOVP ≤ 1.23V
--
1
--
μA
Soft-Start Pin Current
ISS
VSS ≤ 3.2V
--
8
--
μA
Thermal Protection
Thermal Shutdown temperature
TSD
--
150
--
°C
Thermal Shutdown Hysteresis
ΔTSD
--
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. θJA is measured at TA = 25°C on a high effective thermal conductivity four-layer test board per JEDEC 51-7.
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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is a registered trademark of Richtek Technology Corporation.
DS8489-00
November 2012
RT8489
Typical Application Circuit
L1
VIN
CIN
RT8489
R3
*
12V
D1
1
VCC
GBIAS
CVCC
GATE
6 RSET
ISENSE
2
3
MSW
4 x N LEDs
RSENSE
VOUT
LED1
8
OVP
R2
10
24
GATE1 23
22
SENSE1
RS1
LED2
21
GATE2 20
CVC
SENSE2
9
M1
VC
RVC
M2
19
RS2
SS
CSS
Chip Enable
CB
5
RSET
R1
VOUT
COUT
LED3 18
7
12
11
4
GATE3 17
EN
M3
SENSE3 16
RS3
DCTL
ACTL
LED4
GATE4
GND
SENSE4
15
14
M4
13
RS4
* : If VIN is operated above 12V, it is recommended to keep V CC at 12V for optimal application
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8489-00
November 2012
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RT8489
Typical Operating Characteristics
LED Current vs. ACTL PWM Duty
Efficiency vs. Input Voltage
100
250
90
200
LEDx Current (mA)
Efficiency (%)
80
70
60
50
40
30
LED1
LED2
LED3
LED4
150
100
20
50
10
VIN = 24V, RSx = 0.9Ω
4 x 10LEDs, ILED = 100mA
0
0
8
12
16
20
24
28
0
20
Input Voltage (V)
LED Current vs. Input Voltage
80
100
1000
RSx = 2.2Ω
100
LED1
LED2
LED3
LED4
90
900
Frequency (kHz)1
110
800
700
600
500
400
300
200
VIN = 24V
100
80
10
13
16
19
22
10
25
20
30
40
50
Input Voltage (V)
60
70
80
90
100 110
RSET (kΩ)
Switching Frequency vs. Input Voltage
Supply Current vs. Input Voltage
840
10
9
RSET = 10kΩ
760
Supply Current (mA) 1
Switching Frequency (kHz)1
60
Frequency vs. RSET Resistance
120
LEDx Current (mA)
40
ACTL PWM Duty (%)
680
600
520
RSET = 20kΩ
440
360
280
RSET = 30kΩ
8
7
6
5
4
3
2
1
200
0
4
8
12
16
20
24
Input Voltage (V)
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28
4
8
12
16
20
24
28
Input Voltage (V)
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DS8489-00
November 2012
RT8489
Soft-Start Current vs. Input Voltage
Shutdown Current vs. Input Voltage
10.0
9.5
6
Soft-Start Current (μA)
Shutdown Current (μA)1
7
5
4
3
2
1
12
16
20
24
8.0
7.5
7.0
6.5
6.0
5.0
0
8
8.5
5.5
EN = 0
4
9.0
4
28
8
Input Voltage (V)
LED Regulated Voltage vs. Input Voltage
24
28
ISENSE Threshold Voltage vs. Input Voltage
ISENSE Threshold Voltage (mV)
LED Regulation Voltage(mV)
20
150
690
680
670
660
650
140
130
120
110
100
90
80
70
60
50
4
8
12
16
20
24
28
4
8
12
16
20
Input Voltage(V)
Input Voltage (V)
Load Transient Response
Power On from EN
VOUT
(50V/Div)
ACTL
(5V/Div)
24
28
EN
(5V/Div)
GATE
(5V/Div)
IOUT
(100mA/Div)
VIN = 24V, PWM = 1kHz, Duty = 50%
Time (250μs/Div)
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DS8489-00
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Input Voltage (V)
700
GATE
(5V/Div)
12
November 2012
VOUT
(50V/Div)
LED1
(500mA/Div)
VIN = 24V, ILED = 400mA, CSS = 0.1μF
Time (100μs/Div)
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RT8489
Application Information
The RT8489 is a 4-CH programmable current source
controller for LED backlight or lighting application.
By detecting the minimum voltage required to drive each
LED string and setting the Boost output accordingly, this
topology reduces power dissipation and increases overall
efficiency of the LED lighting system.
The individual current source channel regulates the current
flow to give accurate current sinking for each LED string.
The external N-MOSFET current source will accommodate
the power dissipation difference among channels resulting
from the forward voltage difference between the LED
strings.
Both digital PWM dimming signal and analog voltage
signal are used to control the LED current of each channel.
With high speed current source N-MOSFET drivers, the
RT8489 features highly accurate current matching of ±3
percent, while providing very fast turn-on and turn-off times.
This allows a very narrow minimum on or off pulse, which
increases dimming range and provides higher linearity.
Power On sequence
Power Off sequence
The RT8489 integrates adjustable switching frequency and
soft-start, and provides over temperature, over voltage and
current limit protection features.
Input UVLO
The input operating voltage range of the RT8489 is 7V to
28V. An input capacitor at the VCC pin can reduce ripple
voltage. It is recommended to use a ceramic 10μF or larger
capacitor 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 5.5V typically with a 0.7V hysteresis.
Power Sequence
Refer to below Figure 1 and 2. The recommended power
on sequence states that the PWM signal should be ready
before EN and/or VIN is ready. Otherwise, the soft-start
function will be disabled. As for power off sequence, EN/
VIN must be pulled low within 10ms to prevent “hardstart”as shown as Figure 3.
Abnormal Power-on sequence
UVLO
VIN
PWM
EN must be turned on later than VIN
and PWM signal
EN must be turned off earlier than
VIN and PWM signal
EN
No Soft-Start
Soft-Start
If PWM turns on late
VOUT
Figure 1. Power On Sequence Control by EN
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is a registered trademark of Richtek Technology Corporation.
DS8489-00
November 2012
RT8489
Power On sequence
Power Off sequence
Abnormal Power-on sequence
UVLO
VIN
PWM
EN
VIN must be turned off earlier than
EN and PWM signal
VIN must be turned on later than
EN and PWM signal
No Soft-Start
If PWM turns on late
Soft-Start
VOUT
Figure 2. Power On Sequence Control by VIN
EN/VIN
EN and/or VIN should be pulled low
once PWM pull low for over 10 ms
PWM
10ms
Figure 3. To Prevent “hard-start”Sequence
Soft-Start
The soft-start of the RT8489 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 8μA 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.
The soft-start interval is set by the soft-start capacitor
selection according
3.2V to the following equation :
tSS = CSS ×
(s)
8μ A
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8489-00
November 2012
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,
during an over-temperature event, or during a GBIAS under
voltage event.
GBIAS Regulator Operation
The GBIAS pin requires a capacitor for stable operation
and also to store the charge for the large GATE switching
currents. Choose a 10V rated low ESR, X7R or X5R
ceramic capacitor for best performance. The value of the
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11
RT8489
capacitor is determined primarily by the stability of the
regulator rather than the gate charge of the switching
N-MOSFET. A 1μF capacitor will be adequate for most
applications.
Place the capacitor close to the IC to minimize the trace
length to the GBIAS pin and also to the IC ground. An
internal current limit on the GBIAS protects the RT8489
from excessive on chip power dissipation.
If the input voltage, VIN, does not exceed 10V, then the
GBIAS pin should be connected to the input supply. Be
aware that a typical 20mA current will load the GBIAS to
shutdown.
Loop Compensation
The RT8489 uses an internal error amplifier, in which
through its compensation pin (VC) the loop response is
optimized for specific applications. The external inductor,
output capacitor, compensation resistor, and
compensation capacitor determine the loop stability. The
inductor and output capacitor are chosen based on
performance, size and cost. The compensation resistor
and capacitor at VC are selected to optimize control loop
response and stability.
The compensation resistor and capacitor are connected
in series from the VC pin to GND to provide a pole and a
zero for proper loop compensation. The typical
compensation values for RT8489 is 1.8kΩ and 3.3nF.
LED Current Setting
The maximum current of channel 1 to 4 is set by placing
an appropriate sense resistor for LED string. When the
voltage of ACTL is higher than 1.4V, the LED current can
be calculated by the following equation :
ILED, MAX =
225mV
RSx
(mA)
where, RSx is the resistor between external regulating
N-MOSFET and GND.
The ACTL pin should be tied to a voltage higher than 1.4V
to get the full scale 225mV (typical) threshold across the
sense resistor. The ACTL pin can also be used to dim the
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12
LED current to zero, although relative accuracy decreases
with the decreasing voltage sense threshold. When the
ACTL pin voltage is less than 1.4V, the LED current is :
ILED =
(VACTL − 0.4) × 225mV
RSx
(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 VIN to reduce
output power and switching current when VIN 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.
However, PWM dimming which switches the LED on and
off via different duty cycle to control the average LED
current is the better dimming method. 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 since
the LED current is either zero or at the programmed current.
Another advantage of PWM dimming is that a wider
dimming range is available.
The RT8489 features both analog and digital dimming
control. Analog dimming is linearly controlled by an
external voltage (0.4V to 1.4V) at the ACTL pin. A very
high contrast ratio is true digital PWM dimming which
can be achieved by driving the ACTL pin with a PWM signal
at a recommended PWM frequency of 100Hz to 10kHz.
The PWM dimming frequency can be sufficiently adjusted
from 100Hz to 30kHz. However, LED current cannot be
100% proportional to the duty cycle, especially for high
frequency and low duty ratio, because of physical
limitation caused by internal switching frequency. Referring
to Figure 4, the minimum dimming duty can be as low as
1% for the frequency range from 100Hz to 300Hz. For the
dimming frequency from 300Hz to 1kHz, the minimum
dimming duty is about 5%. If the frequency is increased
from 1kHz to 30kHz, the minimum dimming duty will be
about 10%.
is a registered trademark of Richtek Technology Corporation.
DS8489-00
November 2012
RT8489
LED Pin External Resistor Connection
LED Current vs. PWM Duty Cycle
160
The RT8489 equips 4 channel LED drivers and each channel
supports numerous LEDs. The 4 LED strings are
connected from VOUT to pin LEDx (x = 1 to 4) respectively.
If one of the LED channel is not used, the LEDx (x = 1 to
4) pins should be connected to ground directly.
LED Current (mA)
140
120
100
200Hz
1kHz
3kHz
80
40
In this case, there should be a current limiting resistor
between external MOSFET Drain node and LEDx pin to
limit the LEDx pin input current below 100μA.
20
The formula for this resistor is
60
0
0
20
40
60
80
100
Rx = (VOUT − |VLEDx(MAX)|) / 100μA
Duty Cycle (%)
VOUT
Figure 4. LED Current vs. PWM Dimming Duty
Cycle
RT8489
Programmable Switching Frequency
The RSET frequency adjust pin allows the user to program
the switching frequency from 100kHz to 1MHz in order to
optimize efficiency and performance or minimize external
component size. Higher frequency operation yields
smaller component size but increases switching losses
and gate driving current, and may not allow sufficiently
high or low duty cycle operation. Lower frequency
operation gives better performance, but is more costly
with larger external component size. An external resistor
from the RSET pin to GND is required, and do not leave
this pin open. For an appropriate RSET value, refer to Figure
5.
Frequency vs. RSET Resistance
1000
Frequency (kHz)1
900
800
700
600
500
400
300
VIN = 12V
200
100
10
20
30
40
50
60
70
80
90
100 110
RSET (kΩ)
LEDx
Rx
GATEx
SENSEx
RSx
GND
Figure 6. External Resistor Connection for LEDx Pin
Input Over Current Protection
The resistor, RSENSE, between the Source of the external
N-MOSFET (MSW) and GND should be selected to provide
adequate switch current.
The RT8489 senses the inductor current through ISENSE
pin in the switch on-period. The duty cycle depends on
the current sense signal summed with the internal slope
compensation and compared to the VC signal. The
external N-MOSFET will be turned off when the current
signal is larger than the VC signal. In the off-period, the
inductor current will descend. The external N-MOSFET is
turned on by the oscillator in the next beginning cycle.To
drive the application without exceeding the 120mV (typical)
current limit threshold on the ISENSE pin of the RT8489.
Select a resistor that gives a switch current of at least
20% greater than the required LED current according to :
⎛ V × 0.1V ⎞
RSENSE = ⎜ IN
⎟ (Ω )
⎝ VOUT × IOUT ⎠
Figure 5. Switching Frequence vs RSET
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8489-00
November 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT8489
The ISENSE pin input to RT8489 should be a kelvin
connection to the positive terminal of RSENSE.
IPEAK =
VOUT × IOUT VIN × T ⎛ VOUT − VIN ⎞
+
×⎜
⎟
η × VIN
2 × L ⎝ VOUT ⎠
where
Output Over Voltage Protection Setting
The RT8489 is equipped with Over Voltage Protection
(OVP) function. When the voltage at the OVP pin exceeds
threshold value, typically 1.23V, the power switch is turned
off. The power switch can be turned on once again after
the voltage at the OVP pin drops below 1.23V. The output
voltage can be clamped at a certain voltage level set by
the following equation :
⎛ R1 ⎞
VOUT,OVP = 1.23 × ⎜ 1+
⎟
⎝ R2 ⎠
where R1 and R2 make up the resistive voltage divider
from VOUT to GND with the divider center node connected
to the OVP pin.
As long as one string is opened, the controller will
automatically regulate the output voltage to OVP setting
level.
Over Temperature Protection
The RT8489 has an over temperature protection (OTP)
function to prevent overheating caused by excessive power
dissipation. The OTP function will shut down switching
operation when the die junction temperature exceeds
150°C. The chip will automatically start to switch again
once the die junction temperature starts cooling down by
approximately 20°C.
VOUT = maximum output voltage.
VIN = minimum input voltage.
f = operating frequency.
IOUT = sum of current from all LED strings.
η is the efficiency of the power converter.
The Boost converter operates in discontinuous conduction
mode over the entire input voltage range when the L1
inductor value is less than this value L. With an inductance
greater than L, the converter operates in continuous
conduction mode at the minimum input voltage and may
be discontinuous at higher voltages.
Power MOSFET Selection
For applications operating at high input or output voltages,
the power N-MOSFET switch is typically chosen for drain
voltage VDS rating and low gate charge. Consideration of
switch on resistance, R DS(ON), is usually secondary
because switching losses dominate power loss. The
GBIAS regulator on the RT8489 has a fixed current limit
to protect the IC from excessive power dissipation at high
VIN, so the N-MOSFET should be chosen such that the
product of QG at 7V and the switching frequency does not
exceed the GBIAS current limit.
Schottky Diode Selection
Inductor Selection
The inductor for the RT8489 should have a saturation
current rating appropriate to the maximum switch current.
Choose an inductor value based on the operating
frequency, input voltage and output voltage to provide a
current mode ramp during the MOS switching. Allow the
peak to peak inductor ripple to be ±30% of the output
current.The following equation is useful to estimate the
inductor value :
L=
(VOUT − VIN ) × (VIN )2
2 × IOUT × f × ( VOUT )2 x 0.3
The inductor must be selected with a saturation current
rating greater than the peak current provided by the
following equation :
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14
The Schottky diode, with their low forward voltage drop
and fast switching speed, is necessary for the RT8489
applications. In addition, power dissipation, reverse voltage
rating and pulsating peak current are important parameters
of the Schottky diode that must be considered. Choose a
suitable Schottky diode which reverse voltage rating is
greater than the maximum output voltage. 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).
If using the PWM feature for dimming, it is important to
consider diode leakage, which increases with the
temperature, from the output during the PWM low interval.
Therefore, choose the Schottky diode with sufficiently low
leakage current.
is a registered trademark of Richtek Technology Corporation.
DS8489-00
November 2012
RT8489
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
most applications, a 10μF 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 Boost applications, the output capacitor is typically a
ceramic capacitor selected based on the output voltage
ripple requirements. The minimum value of the output
capacitor, COUT, is approximately given by the following
equation :
I
× (VOUT − VIN )
COUT = OUT
η × VRIPPLE × VOUT × f
where V RIPPLE is rhe output voltage ripple, for LED
applications, the equivalent resistance of the LED is
typically low and the output filter capacitor should be sized
to attenuate the current ripple. Use of X7R type ceramic
capacitors is recommended. Lower operating frequencies
will require proportionately higher capacitor values.
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-24 package, the thermal resistance, θJA, is 77°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 formula :
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. The derating curve in Figure 7 allows the
designer to see the effect of rising ambient temperature
on the maximum power dissipation.
1.5
Maximum Power Dissipation (W)1
Capacitor Selection
Four-Layer PCB
1.2
0.9
0.6
0.3
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 7. Derating Curve of Maximum Power Dissipation
Layout Consideration
PCB layout is very important when designing power
switching converter circuits. Some recommended layout
guidelines are suggested as follows :
The power components L1, D1, CIN, MSW and COUT
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, which are connected to N-MOSFET,
as close as possible. The trace should be as short and
wide as possible.
The input capacitor, CVCC 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 pick up.
P D(MAX) = (125°C − 25°C) / (77°C/W) = 1.3W for
SOP-24 package
Copyright © 2012 Richtek Technology Corporation. All rights reserved.
DS8489-00
November 2012
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
15
RT8489
Outline Dimension
H
A
M
B
J
F
C
I
D
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
15.189
15.596
0.598
0.614
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
24–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.
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DS8489-00
November 2012