RT8572 - Richtek

RT8572
High Voltage 8-CH LED Driver
General Description
Features
The RT8572 is an 8-CH LED driver capable of delivering
60mA for each channel. The RT8572 is a current mode
boost converter with an adjustable switching frequency
via the RT pin from 100kHz to 1MHz and a wide VIN
range from 9V to 28V.
z
The PWM output voltage loop selects and regulates the
LED pin with the highest voltage string to 0.6V, hence
allowing voltage mismatches between LED strings. The
RT8572 automatically detects and disconnects any
unconnected and/or broken strings during operation from
PWM loop to prevent VOUT from over voltage. The 1.5%
matched LED currents on all channels are simply
programmed with a resistor. A very high contrast ratio true
digital PWM dimming can be achieved by driving the PWM
pin with a PWM signal.
z
z
z
z
z
z
z
z
z
z
z
z
When an abnormal situation (open/short/thermal) occurs,
a status signal will be sent to the system to shut down
the IC.
The RT8572 is available in an SOP-24 package.
Ordering Information
RT8572
Package Type
S : SOP-24
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
RT8572GS : Product Number
RT8572
GSYMDNN
Wide Input Supply Voltage Range : 9V to 28V
Adjustable Boost Controller Switching Frequency
from 100kHz to 1MHz
Programmable Channel Current
Channel Current Matching : ±1.5%
External Dimming Control
Boost MOSFET Over Current Protection
Automatic LED Open/Short Protection to Avoid
Output Over Voltage
VCC Under Voltage Lockout
Adjustable Over Voltage Protection
Under Voltage Protection
Thermal Shutdown Protection
Abnormal Status Indicator for Open/Short/Thermal
Condition
RoHS Compliant and Halogen Free
Applications
z
z
z
LCD TV, Monitor Display Backlight
LED Driver Application
General Purpose Constant Current Source
Pin Configurations
(TOP VIEW)
LED1
OVP/UVP
RISET
NC
PWM
NC
STATUS
RT
VC
SS
EN
SEN
24
2
23
3
22
4
21
5
20
6
19
7
18
8
17
9
16
10
15
11
14
12
13
LED2
LED3
LED4
LED5
LED6
LED7
LED8
GND
VCC
CREG
DRV
PGND
SOP-24
YMDNN : Date Code
DS8572-01 March 2011
www.richtek.com
1
RT8572
Typical Application Circuit
VIN
9 to 28V
L1
10µH
D1
VOUT
CIN
47µF
OVP/UVP 2
16 VCC
CVCC
1µF
RF1
100
LED1
12
SEN
CF
1nF
RDRV
1k
PWM Signal
5
1k 11
Chip Enable
RC
560
CC
0.22µF
LED8
14 DRV
5
PWM
EN
18
RSTATUS
100k
STATUS 7
RISET
RT
VC
CREG
PGND
13
:
:
: …… :
:
:
:
:
1
RISET
9.09k
8
RRT 56k
15
VIN
FLT
3
SS 10
9
COUT
4.7µF x 6
LED2 24
……
RSENSE
100m
RF2
0
:
:
:
:
ROVP1
82k
RT8572
MSW
ROVP2
3.6M
CSS 0.1µF
CREG 1µF
GND
17
Figure 1. General Application
www.richtek.com
2
DS8572-01 March 2011
RT8572
FLT
VIN
9 to 28V
L1
10µH
D1
CIN
47µF
OVP/UVP 2
16 VCC
CVCC
1µF
RF1
100
SEN
CF
1nF
RSENSE
100m
RDRV
1k
PWM Signal
PWM
1k 11
EN
Chip Enable
RC
560
CC
0.22µF
5
9
:
:
: …… :
:
:
:
:
LED2 24
LED8
14 DRV
5
:
:
:
:
COUT
4.7µF x 6
LED1 1
……
RF2
0 12
ROVP2
3.6M
ROVP1
82k
RT8572
MSW
VOUT
45V
VC
PGND
13
18
RSTATUS
100k
STATUS 7
RISET
RT
CREG
FLT
3
RISET
9.09k
8
RRT 56k
SS 10
15
VIN
CSS 0.1µF
CREG 1µF
GND
17
Figure 2. External P-MOSFET Isolation Application
DS8572-01 March 2011
www.richtek.com
3
RT8572
Functional Pin Description
Pin No.
Pin Name
1
LED1
2
OVP/UVP
3
RISET
Pin Function
Channel 1 LED Current Sink. Leave this pin unconnected if it is not used.
NC
Over Voltage and Under Voltage Protection. PWM boost converter turns off when
VOVP or VUVP goes higher than 1.2V or lower than 0.6V, respectively.
LED Current Set Pin. A resistor or a current from DAC on this pin programs the
full LED current.
No Internal Connection.
5
PWM
Dimming Control Input.
7
STATUS
Boost Converter Operation Status Output.
8
RT
Switching Frequency Set. Connect a resistor between RT and GND to set the
boost converter switching frequency.
9
VC
10
SS
11
EN
Chip Enable. When EN is pulled low, chip will be in shutdown mode.
12
SEN
Current Sense Input. During normal operation, this pin senses the voltage across
the external inductor current sensing resistor for peak current mode control and
also to limit the inductor current during every switching cycle.
13
PGND
14
DRV
15
CREG
16
VCC
Power Supply of the Chip. For good bypass, a low ESR capacitor is required.
17
GND
LED8 to
LED2
Ground Pin.
Channel 8 to Channel 2 LED Current Sink. Leave the pins unconnected if not in
use.
4, 6
18 to 24
www.richtek.com
4
PWM Boost Converter Loop Compensation Node.
Soft-Start Pin. Place a capacitor of at least 10nF from this pin to GND to set the
soft-start time period.
Boost Converter Power Ground.
Boost Converter Power Switch Gate Output. This pin drives the external power
N-MOSFET device.
1μF capacitor should be placed on this pin to stabilize the 5V output of the
internal regulator. This regulator is for chip internal use only.
DS8572-01 March 2011
RT8572
Function Block Diagram
DRV SEN
OSC
RT
VCC
STATUS
OVP
-
6V
UVP
OTP
S
+
R
OVP/UVP
+
-
0.6V
+
R
LED Short
LED1
UVP
+
-
EN
-
5V
LDO
VC
Shutdown
+
CREG
+
-
1.2V
…………………
1.2V
LED8
VOUT
Regulation
Unit
+
5V
-
6µA
SS
PWM
GND
PGND
+
-
RISET
DS8572-01 March 2011
www.richtek.com
5
RT8572
Absolute Maximum Ratings
z
z
z
z
z
z
z
z
z
(Note 1)
Supply Voltage, VCC , STATUS ----------------------------------------------------------------------------------------LED1 to LED8 --------------------------------------------------------------------------------------------------------------PWM, EN, DRV, SEN, SS, VC, RT, CREG, OVP/UVP, RISET ------------------------------------------------Power Dissipation, PD @ TA = 25°C
SOP-24 ----------------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
SOP-24, θJA -----------------------------------------------------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) ----------------------------------------------------------------------------------------------MM (Machine Mode) -------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
z
z
z
z
33V
50V
5.5V
1.111W
90°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Voltage, VCC ------------------------------------------------------------------------------------------------------LED1 to LED8 --------------------------------------------------------------------------------------------------------------Junction Temperature Range --------------------------------------------------------------------------------------------Ambient Temperature Range ---------------------------------------------------------------------------------------------
9V to 28V
45V
−40°C to 125°C
−40°C to 85°C
Electrical Characteristics
(VCC = 12V, TA = 25°C, unless otherwise specified)
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Supply Voltage
Supply Current
IVCC
Switching Off
--
5
--
mA
Shutdown Current
ISHDN
VEN < 0.7V
--
--
10
μA
VDD LDO Output
VCREG
--
5
--
V
VDD LDO Capability
ICREG
30
--
--
mA
VCC UVLO Threshold
VUVLO
VCC Rising
--
--
8
Hysteresis
--
1.4
--
1.5
--
--
--
--
0.8
57
60
63
mA
--
±1.5
±3
%
EN Input
Threshold Voltage
Logic-High VENH
Logic-Low
VENL
V
V
LED Current Programming
LED Current
RISET = 9.09kΩ, V PWM > 1.2V
LED Current Matching
ILED = 60mA
LED1 to LED8 Regulation
Voltage
ILED = 60mA
--
0.6
--
V
VLED Threshold
No Connection
--
0.1
--
V
--
1.2
--
V
RISET Pin Voltage
I(MAX ) − I(MIN )
2 ×I( Avg)
×100%
To be continued
www.richtek.com
6
DS8572-01 March 2011
RT8572
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Dimming
PWM Input
Threshold Voltage
Logic-High
VPWMH
1.2
--
--
Logic-Low
VPWML
--
--
0.4
R RT = 24kΩ
--
1
--
MHz
R RT = Open
--
100
--
kHz
V
PWM Boost Controller
Switching Frequency
fSW
Minimum On Time
tON
--
100
--
ns
Maximum Duty Cycle
Dmax
80
--
--
%
--
0.5
--
V
Gate Driver Source
--
2.5
--
A
Gate Driver Sink
--
3
--
A
SEN Current Sense Limit
Input Current Limit
OVP, UVP, OTP and Soft-Start
OVP Threshold
VOVP
1.1
1.2
1.3
V
UVP Threshold
VUVP
0.57
0.6
0.63
V
SCP Threshold
VSCP
LED1 to LED8
--
4.3
--
V
Soft-Start Current
ISS
V SS < 2.5V
--
6
--
μA
Thermal Shutdown Temperature
TSD
Lockout Temperature Point
--
150
--
°C
Thermal Shutdown Hysteresis
ΔT SD
Resume Temperature Point
--
20
--
°C
STATUS Low Voltage
VSTATUS
Open Drain at 10mA
--
--
0.5
V
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. θJA is measured in natural convection at TA = 25°C on a low-effective thermal conductivity single-layer test board of JEDEC
51-3 thermal measurement standard.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
DS8572-01 March 2011
www.richtek.com
7
RT8572
Typical Operating Characteristics
LED Current vs. PWM Duty Cycle
LED Current vs. Input Voltage
60
70
65
60
LED1
LED2
LED3
LED4
55
LED Current (mA)
LED Current (mA)
50
LED5
LED6
LED7
LED8
40
PWM = 200Hz
PWM = 1kHz
PWM = 10kHz
30
20
10
96LEDs, RISET = 9.1kΩ
VIN = 12V, 96LEDs, RISET = 9.1kΩ
0
50
8
10
12
14
16
18
20
22
24
26
0
28
10
20
30
40
50
60
70
Input Voltage (V)
PWM Duty Cycle (%)
Efficiency vs. Input Voltage
Power On from VIN
80
90
100
100
Efficiency (%)
95
90
VIN
(5V/Div)
85
DRV
(5V/Div)
80
75
96LEDs, RISET = 9.1kΩ
70
8
10
12
14
16
18
20
22
24
26
I IN
(1A/Div)
VIN = 12V, CSS = 0.1μF,
96LEDs, RISET = 9.1kΩ
Time (5ms/Div)
28
Input Voltage (V)
Power On from EN
Power On from PWM
VEN
(2V/Div)
PWM
(2V/Div)
DRV
(5V/Div)
DRV
(5V/Div)
I IN
(1A/Div)
VIN = 12V, CSS = 0.1μF,
96LEDs, RISET = 9.1kΩ
Time (5ms/Div)
www.richtek.com
8
I IN
(1A/Div)
VIN = 12V, CSS = 0.1μF,
96LEDs, RISET = 9.1kΩ
Time (5ms/Div)
DS8572-01 March 2011
RT8572
Application Information
The RT8572 is an 8-CH driver controller that delivers well
matched LED current to each channel of LED strings. The
external N-MOSFET current source will accommodate the
power dissipation difference among channels resulting from
the forward voltage difference between the LED strings.
With high speed current source N-MOSFET drivers, the
RT8572 features highly accurate current matching, while
also providing very fast turn-on and turn-off times. This
allows a very narrow minimum on or off pulse. The RT8572
integrates adjustable switching frequency and soft-start
and provides circuitry for over temperature, over voltage,
under voltage and current limit protection.
Soft-Start
The RT8572 employs a soft-start feature to limit the inrush
current. The soft-start circuit prevents excessive inrush
current and input voltage droop. The soft-start time is
determined by a capacitor, CSS, connected between SS
and GND and charged with a 6μA constant current as shown
in the following equation.
tSS (max) = CSS x 4.8 x 105 (s)
The value of capacitor CSS is user-defined to satisfy the
designer' requirement.
Compensation
The regulator loop can be compensated by adjusting the
external components connected to the VC pin. The VC
pin is the output of the internal error amplifier. The
compensation capacitor will adjust the integrator zero to
maintain stability and the resistor value will adjust the
frequency integrator gain for fast transient response.
Typical values of the compensation components are RC =
510Ω, CC = 0.22μF.
LED Connection
The RT8572 equips 8-CH LED drivers and each channel
supports up to 15 LEDs. The LED strings are connected
from the output of the boost converter to pin LEDx (x = 1
to 8) respectively. If one of the LED channel is not in use,
the LEDx pin should be opened directly.
DS8572-01 March 2011
Setting and Regulation of LED current
The LED current can be calculated by the following
equation :
545.4
ILED ≅
RISET
where RISET is the resistor between the RISET pin and
GND. This setting is the reference for the LED current at
pin LEDx and represents the sensed LED current for each
string. The DC/DC converter regulates the LED current
according to the setting.
Over Voltage and Under Voltage Protection
The RT8572 integrates Over Voltage Protection (OVP) and
Under Voltage Protection (UVP). When the voltage at the
OVP/UVP pin rises above the threshold voltage of
approximately 1.2V or falls below the threshold voltage of
approximately 0.6V, the internal switch will be turned off
and STATUS pin will be pulled high. The internal switch
will be turned on again once the voltage at the OVP/UVP
pin returns to normal range. The output voltage can be
clamped at a certain voltage level and can be calculated
by the following equations :
⎛
⎞
R
VOUT(OVP) = VOVP × ⎜ 1 + OVP2 ⎟
ROVP1 ⎠
⎝
⎛
⎞
R
VOUT(UVP) = VUVP × ⎜ 1 + OVP2 ⎟
R
OVP1 ⎠
⎝
where ROVP1 and ROVP2 are the resistors in the resistive
voltage divider connected to the OVP/UVP 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 strings in normal operation.
Suggested value for ROVP2 is up to 3MΩ to prevent loading
effect.
LED Short Circuit Protection
The RT8572 integrates LED Short Circuit Protection (SCP).
If one of the LED1 to LED8 pin voltages exceeds a
threshold of approximately 4.3V during normal operation,
the STATUS pin will be pulled high for a fault signal.
www.richtek.com
9
RT8572
STATUS
Inductor Selection
After the IC is enabled, STATUS will output logic high if
LED short/OVP/UVP/OTP conditions exist. STATUS will
be reset after VIN or EN is re-applied.
The value of the inductance, L, can be approximated by
the following equation, where the transition is from
Discontinuous Conduction Mode (DCM) to Continuous
Conduction Mode (CCM) :
2
Setting the Switching Frequency
The RT8572 switching frequency is programmable from
100kHz to 1MHz by adjusting the oscillator resistor, RRT.
The switching frequency can be calculated by the following
equation :
21.6 × 109
fSW ≅ 100k +
RRT
Current Limit Protection
The RT8572 can sense the RSENSE voltage between the
SEN pin and GND to achieve over current protection. The
boost converter senses the inductor current during the on
period. The duty cycle depends on the current signal and
internal slope compensation compared with the error
signal. The external switch will be turned off when the
current signal is larger than the internal slope
compensation. In the off period, the inductor current will
decrease until the internal switch is turned on by the
oscillator. The current limit value can be calculated by the
following equation :
Current Limit (A) ≅
0.5V
RSENSE
L=
D × (1 − D ) × VOUT
2 × f × IOUT
The duty cycle can be calculated as the following
equation :
V
− VIN
D = OUT
VOUT
where VOUT is the maximum output voltage, VIN is the
minimum input voltage, f is the operating frequency, and
IOUT is the sum of current from all LED strings.
The boost converter operates in DCM over the entire input
voltage range when the inductor value is less than this
value, L. With an inductance greater than L, the converter
operates in CCM at the minimum input voltage and may
be discontinuous at higher voltages.
The inductor must be selected with a saturated current
rating that is greater than the peak current as provided by
the following equation :
IPEAK =
VOUT × IIOUT VIN × D × T
+
η × VIN
2×L
where η is the efficiency of the power converter.
Brightness Control
Diode Selection
The RT8572 features a digital dimming control scheme. A
very high contrast ratio true digital PWM dimming is
achieved by driving the PWM pin with a PWM signal. The
recommended PWM frequency is 200Hz to 10kHz, but
the LED current cannot be 100% proportional to duty cycle,
especially for high frequency and low duty ratio.
Schottky diodes are recommended for most applications
because of their fast recovery time and low forward voltage.
Power dissipation, reverse voltage rating, and pulsating
peak current are important parameters for consideration
when making a Schottky diode selection. Make sure that
the diode's peak current rating exceeds IPEAK and reverse
voltage rating exceeds the maximum output voltage.
Over Temperature Protection
The RT8572 has over temperature protection function to
prevent the IC from overheating due to excessive power
dissipation. The IC will shut down and the STATUS pin
will be pulled high when junction temperature exceeds
150°C. Main converter starts switching after junction
temperature cools down by approximately 20°C.
www.richtek.com
10
Capacitor Selection
The input capacitor reduces current spikes from the input
supply and minimizes noise injection to the converter. For
general applications, six 4.7μF ceramic capacitors are
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.
DS8572-01 March 2011
RT8572
It is recommended to choose a ceramic capacitor based
on the output voltage ripple requirements. The minimum
value of the output capacitor, COUT, can be calculated by
the following equation :
I
×D
COUT = OUT
ΔVOUT × f
Maximum Power Dissipation (W)1
1.2
where ΔVOUT is the peak-to-peak ripple voltage at the
output.
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 RT8572, the maximum junction temperature is 125°C
and TA is the ambient temperature. The junction to ambient
thermal resistance, θJA, is layout dependent. For SOP24 packages, the thermal resistance, θJA, is 90°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 :
Single-Layer PCB
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3. Derating Curve for RT8572 Packages
Layout Considerations
PCB layout is very important for designing switching power
converter circuits. The following layout guides should be
strictly followed for best performance of the RT8572.
`
The power components L1, D1, CIN, COUT must be placed
as close to the chip as possible to reduce current loop.
The PCB trace between power components must be as
short and wide as possible.
`
The compensation circuit should be kept away from the
power loops and shielded with a ground trace to prevent
any noise coupling. Place the compensation
components as close to pin 9 as possible, regardless
of whether it is RC or CC.
PD(MAX) = (125°C − 25°C) / (90°C/W) = 1.111W for
SOP-24 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. For the RT8572 package, the derating
curve in Figure 3 allows the designer to see the effect of
rising ambient temperature on the maximum power
dissipation.
DS8572-01 March 2011
www.richtek.com
11
RT8572
24
LED2
OVP/UVP
2
23
LED3
RISET
3
22
LED4
NC
4
21
LED5
PWM
5
20
LED6
NC
6
19
LED7
STATUS
7
18
LED8
RT
8
17
GND
VC
9
16
VCC
LED1
The compensation circuit should
be kept away from the power loops
and shielded with a ground trace to
prevent any noise coupling.
RC
CC
SS
10
15
CREG
EN
11
14
DRV
SEN
12
13
PGND
Place the power components as
close as possible to the IC. The
traces should be wide and short
especially for the high-current loop.
Place the CVCC as close
to VCC as possible.
COUT
VOUT
D1
CVCC
+
CIN
RDRV
MSW
RF2
VIN
L1
GND
RF1
CF
RSENSE
GND
Figure 4. PCB Layout Guide
www.richtek.com
12
DS8572-01 March 2011
RT8572
Outline Dimension
H
A
M
J
B
F
C
I
D
Dimensions In Millimeters
Dimensions In Inches
Symbol
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
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.
DS8572-01 March 2011
www.richtek.com
13