RICHTEK RT8577

RT8577
High Voltage 4-CH LED Driver
General Description
Features
The RT8577 is an 4-CH LED driver capable of delivering
200mA for each channel. The RT8577 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
Wide Input Supply Voltage Range : 9V to 28V
z
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
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
RT8577 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.
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When an abnormal situation (open/short/thermal) occurs,
a status signal will be sent to the system to shut down
the IC.
Applications
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Ordering Information
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RT8577
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Package Type
QW : WQFN-20L 5x5 (W-Type)
Pin Configurations
Note :
LED4
GND
GND
VCC
(TOP VIEW)
LED3
Lead Plating System
Z : ECO (Ecological Element with
Halogen Free and Pb free)
LCD TV, Monitor Display Backlight
LED Driver Application
General Purpose Constant Current Source
Richtek products are :
20
19
18
17
16
`
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
LED2
1
15
CREG
LED1
2
14
DRV
13
PGND
12
SEN
11
EN
GND
3
OVP/UVP
4
RISET
5
GND
21
6
7
8
9
10
RT
VC
SS
YMDNN : Date Code
STATUS
RT8577
ZQW
YMDNN
PWM
RT8577ZQW : Product Number
WQFN-20L 5x5
DS8577-00
November 2011
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1
RT8577
Typical Application Circuit
L1
10µH
VIN
9V to 28V
D1
CIN
47µF
CVCC
1µF
MSW
RT8577
12
2
LED1
1
LED2
SEN
CF
1nF
RDRV
5
External/Internal PWM
Chip Enable
LED4
RC
560
9
CC
0.22µF
RISET
6 PWM
1k 11
19
RSTATUS
100k
STATUS 7
14 DRV
1k
:
:
: …… :
:
:
:
:
:
:
:
:
COUT
4.7µF x 6
……
RSENSE
100m
RF2
0
ROVP1
82k
……
RF1
100
OVP/UVP 4
16 VCC
VOUT
45V
ROVP2
3.3M
RT
EN
5
8
RRT 56k
CREG 15
VC
PGND
13
FLT
RISET
9.1k
SS 10
VIN
CSS 0.1µF
CREG 1µF
GND
3, 17, 18,
21 (Exposed Pad)
Figure 1. General Application
FLT
VIN
9V to 28V
L1
10µH
D1
CIN
47µF
CVCC
1µF
MSW
External/Internal PWM
Chip Enable
RC
560
CC
0.22µF
RT8577
12
SEN
LED4
14 DRV
1k
6 PWM
1k 11
EN
:
:
: …… :
:
:
:
:
19
RSTATUS
100k
STATUS 7
RISET
RT
VC
PGND
13
VIN
FLT
5
RISET
9.1k
8
RRT 56k
SS 10
9
:
:
:
:
2
LED1
1
LED2
CF
1nF
RDRV
5
ROVP1
82k
COUT
4.7µF x 6
……
RSENSE
100m
RF2
0
16 VCC
……
RF1
100
OVP/UVP 1
VOUT
45V
ROVP2
3.3M
CSS 0.1µF
CREG 1µF
15
CREG
GND
3, 17, 18,
21 (Exposed Pad)
Figure 2. External P-MOSFET Isolation Application
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DS8577-00
November 2011
RT8577
Functional Pin Description
Pin No.
Pin Name
1
LED2
2
LED1
3, 17, 18,
GND
21 (Exposed pad)
Pin Function
Channel 2 LED Current Sink, Leave this pin unconnected if it is not used.
4
OVP/UVP
5
RISET
6
PWM
Channel 1 LED Current Sink, Leave this pin unconnected if it is not used.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum power dissipation.
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.
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
12
SEN
13
PGND
14
15
16
19, 20
DS8577-00
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.
Chip Enable. When EN is pulled low, the chip will be shut down.
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.
Boost Converter Power Ground.
Boost Converter Power Switch Gate Output. This pin drives the external power
DRV
N-MOSFET device.
1μF capacitor should be placed on this pin to stabilize the 5V output of the
CREG
internal regulator. This regulator is for chip internal use only.
Power Supply of the Chip. For good bypass, a low ESR capacitor close to the
VCC
pin is required.
LED4, LED3 Channel 4 and 3 LED Current Sinks. Leave the pins unconnected if not used.
November 2011
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RT8577
Function Block Diagram
DRV SEN
OSC
RT
VCC
STATUS
OVP
-
UVLO
UVP
OTP
S
+
R
OVP/UVP
+
-
0.6V
+
R
LED Short
LED1
UVP
+
-
EN
-
5V
LDO
VC
Shutdown
+
CREG
+
-
1.2V
…………………
1.2V
LED4
VOUT
Regulation
Unit
+
5V
-
6µA
SS
PWM
GND
PGND
+
-
RISET
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DS8577-00
November 2011
RT8577
Absolute Maximum Ratings
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(Note 1)
Supply Voltage, VCC, STATUS -----------------------------------------------------------------------------------------LED1 to LED4 --------------------------------------------------------------------------------------------------------------PWM, EN, DRV, SEN, SS, VC, RT, CREG, OVP/UVP, RISET ------------------------------------------------Power Dissipation, PD @ TA = 25°C
WQFN-20L 5x5 ------------------------------------------------------------------------------------------------------------Package Thermal Resistance (Note 2)
WQFN-20L 5x5, θJA -------------------------------------------------------------------------------------------------------WQFN-20L 5x5, θJC ------------------------------------------------------------------------------------------------------Junction Temperature -----------------------------------------------------------------------------------------------------Lead Temperature (Soldering, 10 sec.) -------------------------------------------------------------------------------Storage Temperature Range --------------------------------------------------------------------------------------------ESD Susceptibility (Note 3)
HBM (Human Body Mode) ----------------------------------------------------------------------------------------------MM (Machine Mode) -------------------------------------------------------------------------------------------------------
Recommended Operating Conditions
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33V
50V
5.5V
2.778W
36°C/W
6°C/W
150°C
260°C
−65°C to 150°C
2kV
200V
(Note 4)
Supply Voltage, VCC ------------------------------------------------------------------------------------------------------LED1 to LED4 --------------------------------------------------------------------------------------------------------------ILED1 to ILED4 ----------------------------------------------------------------------------------------------------------------Junction Temperature Range --------------------------------------------------------------------------------------------Ambient Temperature Range ---------------------------------------------------------------------------------------------
9V to 28V
45V
10mA to 200mA
−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
--
6.7
8
Hysteresis
--
1.4
--
EN Threshold
Voltage
V
Logic-High
VENH
1.5
--
--
Logic-Low
VENL
--
--
0.8
114
120
126
mA
--
±1.5
±3
%
V
LED Current Programming
LED Current Accuracy
RISET = 9.1kΩ, VPWM > 1.2V
RISET = 9.1kΩ, VPWM > 1.2V
LED Current Matching
ILEDx − ILED_AVE
ILED_AVE
×100%
LED1 to LED4 Regulation Voltage
ILED = 200mA
--
0.6
--
V
VLED Threshold
No Connection
--
0.1
--
V
--
1.2
--
V
RISET Pin Voltage
To be continued
DS8577-00
November 2011
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RT8577
Parameter
Symbol
Test Conditions
Min
Typ
Max
Unit
Dimming
PWM Threshold
Voltage
Logic-High
VPWMH
1.2
--
--
Logic-Low
VPWML
--
--
0.35
RRT = 24kΩ
--
1
--
MHz
RRT = 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, SCP, OTP and Soft-Start
OVP Threshold
VOVP
--
1.2
--
V
UVP Threshold
VUVP
--
0.6
--
V
SCP Threshold
VSCP
LED1 to LED4
--
4.3
--
V
Soft-Start Current
ISS
VSS < 2.5V
--
6
--
μA
Thermal Shutdown Temperature
TSD
Lockout Temperature Point
--
150
--
°C
Thermal Shutdown Hysteresis
ΔTSD
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 high-effective thermal conductivity four-layer test board of JEDEC
51-7 thermal measurement standard. The measurement case position of θJC is on the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution is recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
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DS8577-00
November 2011
RT8577
Typical Operating Characteristics
LED Current vs. Input Voltage
LED Current vs. PWM Duty Cycle
160
120
100
LED Current (mA)
LED Current (mA)
140
120
LED1
LED2
LED3
LED4
100
80
80
PWM = 200Hz
PWM = 1kHz
PWM = 10kHz
60
40
20
96LEDs, RISET = 9.1kΩ
60
VIN = 12V, 96LEDs, RISET = 9.1kΩ
0
8
10
12
14
16
18
20
22
24
26
28
0
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 = 18.2kΩ
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 = 18.2kΩ
Time (5ms/Div)
DS8577-00
November 2011
I IN
(1A/Div)
VIN = 12V, CSS = 0.1μF,
96LEDs, RISET = 18.2kΩ
Time (5ms/Div)
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RT8577
Application information
The RT8577 is an 4-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
RT8577 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 RT8577
integrates adjustable switching frequency and soft-start
and provides circuitry for over temperature, over voltage,
under voltage and current limit protection.
Soft-Start
The RT8577 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
Setting and Regulation of LED current
The LED current can be calculated by the following
equation :
1092
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 RT8577 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 ⎟
R
OVP1 ⎠
⎝
⎛
⎞
R
VOUT(UVP) = VUVP × ⎜ 1 + OVP2 ⎟
R
OVP1 ⎠
⎝
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 =
560Ω, CC = 0.22μF.
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 Connection
LED Short Circuit Protection
The RT8577 equips 4-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 4) respectively. If one of the LED channel is not in use,
the LED pin should be opened directly.
The RT8577 integrates LED Short Circuit Protection (SCP).
If one of the LED1 to LED4 pin voltages exceeds a
threshold of approximately 4.3V during normal operation,
the STATUS pin will be pulled high for a fault signal.
STATUS
After the IC is enable. STATUS will output logic high if
LED Short/OVP/UVP/OTP conditions exist. STATUS will
be reset after VIN or EN is re-applied.
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DS8577-00
November 2011
RT8577
Setting the Switching Frequency
The RT8577 switching frequency is programmable from
100kHz to 1MHz by adjusting the oscillator resistor, RRT.
The switching frequency can be calculated by the following
equation :
fSW ≅ 100k +
9
21.6 × 10
RRT
Current Limit Protection
The RT8577 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 :
0.5V
Current Limit (A) ≅
RSENSE
Brightness Control
The RT8577 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.
Over Temperature Protection
The RT8577 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.
Inductor Selection
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
L=
D × (1 − D ) × VOUT
2 × f × IOUT
DS8577-00
November 2011
The duty cycle can be calculated as the following
equation :
D=
VOUT − VIN
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 × IOUT VIN × D × T
+
η × VIN
2×L
where η is the efficiency of the power converter.
Diode Selection
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.
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.
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 :
COUT =
IOUT × D
ΔVOUT × f
where ΔVOUT is the peak-to-peak ripple voltage at the
output.
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9
RT8577
Thermal Considerations
Layout 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 :
Careful PCB layout is very important for designing
switching power converter circuits. The following layout
guidelines should be strictly followed for best performance
of the RT8577.
`
The power components L1, D1, CIN, COUT must be placed
as close as possible to the IC 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, RC and CC, as close as possible to pin 9.
`
The exposed pad of the chip should be connected to
ground plane for thermal consideration.
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
RT8577, the maximum junction temperature is 125°C and
TA is the ambient temperature. The junction to ambient
thermal resistance, θJA, is layout dependent. For WQFN20L 5x5 packages, the thermal resistance, θJA, is 36°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 :
PD(MAX) = (125°C − 25°C / (36°C/W) = 2.778W for
WQFN-20L 5x5 package
The maximum power dissipation depends on the operating
ambient temperature for fixed T J(MAX) and thermal
resistance, θJA. For RT8577 package, the derating curve
in Figure 3 allows the designer to see the effect of rising
ambient temperature on the maximum power dissipation.
Maximum Power Dissipation (W)1
3.20
Four-Layer PCB
2.80
2.40
2.00
1.60
1.20
0.80
0.40
0.00
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 3. Derating Curve for RT8577 Packages
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DS8577-00
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RT8577
Locate the CVCC as close to
VCC as possible.
AGND
LED3
LED4
GND
GND
VCC
CVCC
R1
20
19
18
17
16
VIN
1
15
CREG
LED1
2
14
DRV
GND
3
13
PGND
OVP/UVP
RISET
RISET
4
12
SEN
11
EN
GND
8
9
STATUS
RT
VC
MSW
RF2
CF
RSENSE
PGND
10
RC
RRT
AGND
RF1
D1
VOUT
RDRV
SS
7
PWM
21
6
COUT
L1
LED2
5
Place the power components as
Close as possible. The traces
should be wide and short especially
for the high current loop.
PGND
VIN
CIN
+
Separate power ground (PGND) and
analog ground (AGND). Connect AGND
and PGND islands at a single end. Make
sure there are no other connections
between these separate ground planes.
The PGND should be wide and short
enough to connect ground plane.
CSS
The exposed pad of the chip
should be connected to ground
plane for thermal consideration.
CC
The compensation circuit and RISET resistor
should be kept away from the power loops and
should be shielded with a ground trace to prevent
any noise coupling.
Figure 4. PCB Layout Guide
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November 2011
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11
RT8577
Outline Dimension
Dimensions In Millimeters
Dimensions In Inches
Symbol
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.250
0.350
0.010
0.014
D
4.900
5.100
0.193
0.201
D2
3.100
3.200
0.122
0.126
E
4.900
5.100
0.193
0.201
E2
3.100
3.200
0.122
0.126
e
L
0.650
0.550
0.026
0.650
0.022
0.026
W-Type 20L QFN 5x5 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.
www.richtek.com
12
DS8577-00
November 2011