SC446TETRT [SAMTEC]
High Efficiency Integrated Driver for 3-Strings of 100mA LEDs; 高效集成驱动器，用于100mA的LED的3串
| 型号： | SC446TETRT |
| 厂家： | 
SAMTEC |
| 描述： | High Efficiency Integrated Driver for 3-Strings of 100mA LEDs |
| 文件： | 总19页 (文件大小：956K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
SC446
High Efficiency Integrated Driver for
3-Strings of 100mA LEDs
POWER MANAGEMENT
Features
Description
Wide input voltage range from 4.5V to 27V
36V maximum output voltage
Drives up to 30 WLEDs in 3 strings
Programmable LED current for up to 100mA per
string
+/- 2% string-to-string current matching
Up to 90% efficiency
Wide 0.2% to 100% PWM dimming range
The SC446 is a high-efficiency multiple string WLED driver
with an integrated boost converter. It operates over a
wide input range from 4.5V to 27V with a maximum out-
put voltage of 36V and a 2.5A internal power switch. It
can drive up to thirty WLEDs in 3 strings with current up
to 100mA per string. The string-to-string current match-
ing is 2% typical, 3% maximum. The overall efficiency is
greater than 90% due to the low current sense voltage
and a low-impedance internal power switch. The wide
PWM dimming range boasts a ratio of 500: 1.
Possible analog dimming
Integrated 2.5A power switch
700kHz switching frequency for small size
Adjustable OVP for cost-effective output cap selection
Open/short LED protection
Thermal protection with auto-recovery
Thermally enhanced TSSOP-16 EDP package
Pb Free, Halogen Free, andWEEE/RoHS Compliant
The 700kHz switching frequency enables the user to op-
timize the external component sizes for efficiency. When
there are fewer than 10 LEDs in each string, users can ad-
just the output voltage protection yielding an allowable
reduction in associated costs, size and voltage ratings of
the output capacitor.
The SC446 also features comprehensive open and short-
circuit LED protection functions. It disables the corre-
sponding strings with LED open or LED short conditions
while maintaining normal operation of other, unaffected
LED strings. This feature allows LCD panels to remain
viewable even under LED failure, wire disconnect, or
short-circuit conditions. The internal thermal shutdown
protects the IC from overheating at abnormal conditions.
Applications
Medium-sized LCD panel
Notebook Display
Automotive Car Navigation Display
Sub-Notebook and Tablet Computer Displays
Portable Media Players
The SC446 is available in a thermally-enhanced TSSOP-16
EDP package.
Typical Application Circuit
Vin (4.5V -27V)
1
2
SW
VIN
VOUT
3 Strings
EN
OVPIN
OVPRTN
PWM
10
SC446
3
IO1- 3
COMP
IOSET
AGND
SS
PGND
EDP
www.semtech.com
January 28, 2010
1
SC446
Pin Configuration
Ordering Information
Device
Package
SC446TETRT (1,2)
SC446EVB
TSSOP-16 EDP
1
16
IO2
IO1
Evaluation Board
IO3
AGND
Notes:
PGND
SW
SS
(1) Available in tape and reel only. A reel contains 2,500 devices.
(2) Available in lead-free package only. This product is fully WEEE/RoHS
compliant, Pb free and Halogen free.
COMP
PGND
EN
IOSET
OVPIN
OVPRTN
VIN
PWM
VOUT
8
9
θJA = 39º C/W
(TSSOP-16 EDP)
Marking Information
Top Mark
Marking for the TSSOP-EP 16 Lead package:
nnnnn = Part Number (Example: SC446)
yyww = Date (Example: 0952)
xxxx = Semtech Lot # (Example: E901)
© 2010 Semtech Corporation
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2
SC446
Absolute Maximum Ratings
Recommended Operating Conditions
VIN Pin: Supply Voltage …………………………… -0.3 to 30V
Maximum Output Power ……………………………… 22W
SW, OVPIN, OVPRTN, VOUT, IO1~IO3 Voltage …… -0.3 to 40V
IOSET Voltage ……………………………………… -0.3 to 2V
SS, COMP Voltage ………………………………… -0.3 to 4V
EN, PWM, Voltage …………………………… -0.3 to VIN +0.3V
Input Voltage Range ……………………………… 4.5V~27V
Maximum Output Voltage ……………………………… 36V
Maximum LED Current ……………………………… 100mA
Thermal Information
Junction to Ambient(1) ……………………………… 39°C/W
Maximum Junction Temperature ……………………… 150°C
Storage Temperature ………………………… -65 to +150°C
Lead Temperature (Soldering) 10 sec ………………… 260°C
PGND to AGND ………………………………………
Peak IR Reflow Temperature …………………………… 260°C
ESD Protection Level(2) ………………………………
3.5kV
0.3V
Exceeding the above specifications may result in permanent damage to the device or device malfunction. Operation outside of the parameters
specified in the Electrical Characteristics section is not recommended.
NOTES-
(1) Calculated from package in still air, mounted to 3”x 4.5”, 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
(2) Tested according to JEDEC standard JESD22-A114-B.
Electrical Characteristics
Unless otherwise specified: VIN =12V, -40°C < TA = TJ < 105°C, RIOSET=2.61kW.
Parameter
Symbol
Conditions
Min
Typ
Max
Units
Input Supply
UVLO-TH
UVLO-H
IIN_Q
Under-Voltage Lockout Threshold
UVLO Hysteresis
VIN rising
4.3
250
3
4.45
V
mV
mA
µA
VIN Quiescent Supply Current
VIN Supply Current in Shutdown
No switching
IIN_S
EN / PWM = low
1
Oscillator
FS
Switching Frequency
Internal Power Switcher
0.56
2.5
0.7
0.84
MHz
ISW
VSAT
Switch Current Limit
Switch Saturation Voltage
Switch Leakage Current
Maximum Duty Cycle
Minimum Duty Cycle
Minimum On-Time(1)
3.32
200
0.1
A
mV
µA
%
ISW = 1A
350
1
IS_LEAK
DMAX
DMIN
88
93
0
%
TON_MIN
100
ns
Compensation
IEA_SOURCE
IEA_SINK
Sourcing Current
Sinking Current
VCOMP = 0.5V
VCOMP = 2V
5
6
µA
µA
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© 2010 Semtech Corporation
SC446
Electrical Characteristics (continued)
Parameter
Symbol
Conditions
Min
Typ
Max
Units
Control Signals
VEN_H, VPWM_H
VEN_L, VPWM_L
IEN, IPWM
EN, PWM High Voltage
EN, PWM Low Voltage
EN, PWM Leakage Current
PWM Dimming Frequency(1)
PWM Dimming Minimum Duty Cycle
PWM Dimming Minimum Pulse-Width(2)
PWM Dimming Minimum Off Time
FFLAG Voltage
2
V
V
0.4
1
VEN, VPWM = 5V
0.1
0.2
µA
Hz
%
µs
ns
V
FDimming
50
50k
FDimming = 200Hz
DMIN_Dimming
TMIN_Dimming
TMIN_Off
5
200
0.25
4.5
1
300
VFFLAG
IFFLAG = 2 mA
VSS = 0V
ISS_SOURCE
ISS_SINK
VSS_Switching
VSS_END
SS Source Current
µA
SS Sink Current
VSS = 2V at OVP or OTP
VIN = 12V, TJ = 25 °C
VIN = 12V
SS Switching Threshold
SS End Value
0.5
0.7
2.5
0.85
V
V
Over-Voltage Protection
OVPIN Threshold
VOVPIN_TH
IOVPIN
OVPIN - AGND
VOVPIN = 20V
1.43
1.52
0.1
60
1.58
1
V
OVPIN Leakage Current
OVPRTN Saturation Voltage
OVPRTN Leakage Current
VOUT Internal Pull-down Current Source
VOUT Leakage Current
Current Source (IO1 ~IO3 )
Current Accuracy
µA
mV
µA
mA
µA
VOVPRTN
IOVPRTN
IOVP
IOVPRTN = 100 µA
VOVPRTN = 20V
VOUT = VIN + 3V
VOUT = 40V
0.1
0.9
0.1
IVOUT_
Leak
IO1~ IO3
TJ = 25 °C
TJ = 25 °C
93
100
107
mA
%
Current Matching
+/- 2
+/-3
IOMAX
VIO_SCP
IIO_LEAK
Maximum LED Current
LED Short-Circuit Protection
Leakage Current
150
2.2
mA
V
TJ = 25 °C, VIO1 ~ VIO3
EN = 0, VIO1 ~ VIO3 = VIN
TJ = 25 °C, VIO1 ~ VIO3
2.35
0.1
2.55
1
µA
0.963
1.07
1.07
100
1.177
V
V
Overshoot Protection Threshold
VIO1~VIO3
0.9065
1.2305
Any of IO1~ IO3
Overshoot Protection Hysteresis
Over-Temperature Protection
Thermal Shutdown Temperature
Thermal Shutdown Hysteresis
mV
TOTP
150
30
°C
°C
TOTP_H
Notes:
(1) Guaranteed by design.
(2) For achievable PWM dimming minimum pulse-width in applications, see the corresponding curves in Typical Characteristics.
© 2010 Semtech Corporation
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SC446
Typical Characteristics
VIN Quiescent Supply Current
UVLO Threshold vs. Temperature
UVLO Hysteresis vs. Temperature
vs. Temperature
4.24
285
275
265
255
245
235
3.40
3.35
3.30
3.25
3.20
3.15
3.10
3.05
3.00
4.23
4.22
4.21
4.20
4.19
-40
25
105
-40
25
105
-40
25
105
Temperature (oC)
Temperature (oC)
Temperature (oC)
COMP SINK / SOURCE Current
vs. Temperature
SS SINK / SOURCE Current
vs. Temperature
SW Saturation Voltage at 1A
vs. Temperature
230
220
210
200
190
180
6.5
6.0
5.5
5.0
4.5
4.0
6.0
5.0
4.0
3.0
2.0
1.0
VIN = 12V
VIN = 12V
VIN = 4.5V
VIN = 12V
VIN = 27V
SOURCE
SINK
SOURCE
SINK
-40
25
105
-40
25
105
-40
25
105
Temperature (oC)
Temperature (oC)
Temperature (oC)
LED Current Source Saturation Voltage
vs. LED Current
LED Current Source SCP Threshold
vs. Temperature
VOUT Pull Down Current Source
2.36
0.820
0.800
0.780
0.760
0.740
0.720
0.700
600
TA = 25 oC
VIN = 12V
VIN = 12V
VIN = 4.5V
500
400
300
200
100
0
2.35
2.34
2.33
2.32
2.31
VIN = 27V
VOUT = VIN + 3V
10
20
30
40
50
60
70
80
90
100
-40
25
105
-40
25
105
LED Current (mA)
Temperature (oC)
Temperature (oC)
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© 2010 Semtech Corporation
SC446
Typical Characteristics (continued)
OVPIN Threshold Voltage
vs. Temperature
OVPRTN Saturation Voltage
vs. Temperature
Efficiency ( PBOOST_OUTPUT / PINPUT
)
75
70
65
60
55
50
45
1.534
1.530
1.526
1.522
1.518
1.514
94
92
90
88
86
IOVPRTN = 100uA
27 VIN
21 VIN
12 VIN
84
82
80
78
76
74
72
5
VIN
VIN = 12V
4.5 VIN
-40
25
105
-40
25
105
30
60
90
120
150
180
210
240
270
300
Temperature (oC)
Temperature (oC)
Boost Section Output Current (mA)
Condition: VOUT = 36V, L1=B1000AS-100M
PWM Dimming Minimum Duty Cycle
vs. PWM Dimming Frequency
PWM Dimming Minimum Duty Cycle
PWM Dimming Minimum Pulse Width
vs. PWM Dimming Frequency
vs. PWM Dimming Frequency
12
50
45
10
40
35
8
6
4
2
0
30
25
20
15
10
5
0
0.1
0.2
0.5
1.0
2.0
5.0
10.0
20.0
30.0
40.0
50.0
0.1
0.2
0.5
1.0
2.0
5.0
10.0
20.0
30.0
40.0
50.0
PWM Dimming Frequency (kHz)
PWM Dimming Frequency (kHz)
PWM Dimming Frequency (kHz)
Condition: VIN = 12V, VOUT = 36V / 10mA x 3 strings
Condition: VIN = 12V, VOUT = 36V / 10mA x 3 strings
Condition: VIN = 12V, VOUT = 36V / 100mA x 3 strings
PWM Dimming Minimum Pulse Width
vs. PWM Dimming Frequency
0.1
0.2
0.5
1.0
2.0
5.0
10.0
20.0
30.0
40.0
50.0
PWM Dimming Frequency (kHz)
Condition: VIN = 12V, VOUT = 36V / 100mA x 3 strings
© 2010 Semtech Corporation
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6
SC446
Typical Characteristics (continued)
Start up
Shut Down
VIN
(12V/DIV)
VIN
(12V/DIV)
VOUT
(15V/DIV)
VSS
(1.2V/DIV)
VOUT
(15V/DIV)
VSS
(1.2V/DIV)
VSW
VSW
(20V/DIV)
(20V/DIV)
Time (2S/DIV)
Time (10mS/DIV)
Conditions: 20V, 36Vout / 100mA x 3 LED strings
Conditions: 20Vin, 36Vout / 100mA x 3 LED strings
Main Power Switching
Main Power Switching
VIN
(12V/DIV)
VIN
(12V/DIV)
VOUT
(100mV/DIV)
VOUT
(100mV/DIV)
VSW
VSW
(12V/DIV)
(12V/DIV)
Time (1uS/DIV)
Time (1uS/DIV)
Conditions: 20Vin, 36Vout / 100mA x 3 LED strings
Conditions: 12Vin, 36Vout / 100mA x 3 LED strings
Start up by PWM
PWM
(5V/DIV)
VOUT
(10V/DIV)
IO1
(10V/DIV)
VSW
(20V/DIV)
Time (5mS/DIV)
Conditions: 12Vin, 36Vout / 100mA x 3 LED strings,
200Hz PWM Dimming, 50% Duty Cycle
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© 2010 Semtech Corporation
SC446
Typical Characteristics (continued)
Over Temperature Protection
LED Short Circuit Protection
VOUT
(10V/DIV)
VIN
(10V/DIV)
VIN
(10V/DIV)
VSS
(1.5V/DIV)
VOUT
(10V/DIV)
IO1
(10V/DIV)
VSW
(20V/DIV)
IO2
(1V/DIV)
Time (10mS/DIV)
Time (100mS/DIV)
Conditions: 12V, 36Vout / 100mA x 3 LED strings,
IO1 has one LED short circuit
Conditions: 12Vin, 36Vout / 100mA x 3 LED strings
LED Open Circuit Protection
Adaptor Plug-in
VIN
(10V/DIV)
VIN
(10V/DIV)
VOUT
(1V/DIV)
VOUT
(20V/DIV)
IO1
(1V/DIV)
IO1
(1V/DIV)
SW
(20V/DIV)
IO2
(10V/DIV)
Time (50mS/DIV)
Conditions: 12Vin, 36Vout / 100mA x 3 LED strings,
IO1 LED String is open circuit
Time (50uS/DIV)
Conditions: 12Vin to 19Vin, 36Vout / 100mA x 3 LED strings
PWM Dimming
PWM Dimming
PWM
PWM
(5V/DIV)
(5V/DIV)
VOUT
(1V/DIV)
VOUT
(1V/DIV)
IO1
(10V/DIV)
IO1
(10V/DIV)
SW
(20V/DIV)
SW
(20V/DIV)
Time (5mS/DIV)
Conditions: 12Vin, 36Vout / 100mA x 3 LED strings,
200Hz PWM Dimming, 0.2% Duty Cycle
Time (5mS/DIV)
Conditions: 12Vin, 36Vout / 100mA x 3 LED strings,
200Hz PWM Dimming, 50% Duty Cycle
© 2010 Semtech Corporation
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8
SC446
Pin Descriptions
Pin #
Pin Name
Pin Function
1
2
3
IO1
AGND
SS
Provides constant current source to LED string 1.
Analog ground.
Soft-start pin.
4
5
COMP
EN
The output of the internal trans-conductance error amplifier.
Enable the device including regulator and LED drivers.
Current source IO value set pin. By selecting the resistor connected from this pin to GND,
the corresponding maximum current on all 4 strings are set.
6
IOSET
7
8
PWM
VOUT
VIN
PWM dimming control pin for LED strings.
Internal pull down current source in over voltage fault. Connect this pin directly to Boost output.
Power input voltage pin. Bypassed with capacitors close to the pin.
Over-Voltage Protection sense signal return path pin.
Over-Voltage Protection sense signal input.
9
10
11
12
13
14
15
16
EDP
OVPRTN
OVPIN
PGND
SW
Power ground.
Collector of the internal power switch.
PGND
IO3
Power ground.
Provides constant current source to LED string 3. Connect to VIN for 2 strings operation.
Provides constant current source to LED string 2.
Solder to the ground plane of the PCB.
IO2
Note: Any unused IO pin should be pulled up to VIN.
EN
0
STATUS
backlight disable
backlight enable
1
Note: When EN = 0; the boost is turned OFF and disabled.
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© 2010 Semtech Corporation
SC446
Block Diagram
SW
HICCUP
4
6
5
OSC
S
R
Q
FAULT-1
ONE IO CHANNEL SHOWN
LED OPEN / SHORT
CIRCUIT PROTECTION
-
-
ILIM
IO1
PGND
PGND
DISABLE1
ISENSE
-
LED CURRENT
SETTING
CURRENT
SOURCE
COMP
SS
IOSET
PWM
VOUT
OVPIN
OVP
Fault
OVPRTN
OVP
Detect
Fault
HICCUP
VIN
EN
0.9mA
CONTROL
LOGIC
3V3
UVLO
TSD
HICCUP
UVLO & TSD
Bandgap
4.5uA
SS
HICCUP
AGND
1uA
Figure 1. SC446 Block Diagram
© 2010 Semtech Corporation
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10
SC446
Applications Information
voltage increases. Proper decoupling is required on the
VIN pin, especially for a lower input voltage condition. A
22µF, 6.3V rated X5R ceramic capacitor is recommended
for a 5V input system.
SC446 Detailed Description
The SC446 contains a high frequency, current-mode
boost regulator and four programmable current sources.
The LED current source value is set using an external
resistor while the PWM controller maintains the output
voltage at a level keeping the current regulated through
the LEDs. Since the SC446 receives feedback from all of
the LED current sources, all LED strings can be turned on
at any given time. A typical application would use 3-10
backlight LEDs for each string, driven up to 100mA.
The internal LED current source (IO1 ~ IO3) helps to
regulate the LED current to its set point while the output
voltage increases; a suitable amount of error information
will be generated on the internal error amplifier.The COMP
pin voltage keeps rising and once the LED current reaches
its set point, the error information will not be generated
by the LED current source. The COMP pin voltage stays
level while keeping the LED current in its set point.
Operation
The SC446 controls the boost converter set point based
on instantaneous requirements of four current sources.
Therefore, only a single inductor and power switch is
needed to provide power to the entire lighting subsystem,
increasing efficiency and reducing part count. A digital
interface to output control is high-bandwidth, supporting
digital PWM dimming at 50Hz to 50kHz dimming
frequency, while aggressively shutting the entire supply
current down to 3mA (typical), when all LED strings are
turned off.
If the EN pin voltage is pulled below 0.4V, the SC446 will
stay in shutdown mode drawing less than 1µA from its
input power supply.
During the normal operation, when PWM pin is pulled
below 0.4V, the device operates in standby mode,
drawing 3mA (typical) current from the input. Under this
condition, since the EN pin is pulled high, soft-start is
initiated and the SS pin voltage is raised to its end value.
Following this, when the PWM signal goes high to enable
the SC446, the COMP pin voltage will rise quickly since it
is not limited by the SS pin. A proper capacitance (10nF ~
100nF) is required to prevent output voltage overshoot
on the COMP pin and its external RC network.
High frequency switching provides high output power
using a tiny 1.0mm high inductor, maximizing efficiency
for space-constrained and cost-sensitive applications.
Additionally, both converter and output capacitor are
protected from open-LED conditions by over-voltage
protection.
Shut Down
If the VIN pin voltage falls below its UVLO, or the voltage
on the EN pin goes low, the device will run in shutdown
mode as the internal switch and the LED current sources
will immediately turn off. The SS capacitor is discharged
by the internal current source of the SS pin. The SS pin
voltage decreases to 0V while the output voltage falls to
the same level as the input voltage.
LED Current Programming
The SC446 features programmable LED current regulators.
The LED current set points are chosen using external
resistors tied to the IOSET pin. The relationship between
the programming resistor value and the output current
set point can be described as follows:
If the PWM pin voltage goes low while SC446 is in normal
operation, then the SC446 will run in standby mode.
The Internal switcher and the LED current source will
immediately turn off.
RIOSET = (0.261V) / ILED
Where RIOSET is in kΩ. ILED is the LED current in Amperes.
The four output channels have the same output current.
NOTE–
Start-Up
The PWM signal does not affect the SS pin nor its final value.
During start-up, when the VIN pin voltage reaches its
UVLO threshold and both the EN and PWM signals are set
to high, the SS pin begins to source 4.5µA as its voltage
begins to rise from 0V to its end value of 2.6V. The output
voltage of the internal trans-conductance error amplifier
(COMP), increases and clamps to the SS pin voltage. When
the SS pin voltage reaches its switching threshold, output
Main Power Operation
SC446 is a 700kHz fixed-frequency, peak current-mode
step-up switching regulator with an integrated 2.5A
(minimum) power transistor.
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© 2010 Semtech Corporation
SC446
Applications Information (continued)
Referring to the Block Diagram, Figure 1, the clock from the
oscillation section resets the latch and turns on the power
transistor. Switch current is sensed with an integrated
sense resistor. The sensed current is summed with the
slope-compensating ramp and fed into the modulating
ramp input of the PWM comparator. The latch is set and
the power transistor conduction is terminated when
the modulating ramp intersects with the error amplifier
output (COMP).
there are some choices.
1) Leave some room between Vin range and Vo.
2) Operate the Boost converter at normal load (less Ro)
3) Increases the Boost inductance (L).
Over-Current Protection
SC446 provides cycle-by-cycle current limiting for its
internal switch. If the switch current exceeds 3.32A (the
typical current-limit trip point), then the current-limit
comparator ILIM, will set the latch immediately turning off
internal power. All LED current sources keep operating in
an over-current condition. Due to separate pulse-width
modulating and current limiting paths, the OCP trip point
is not affected by slope compensation (i.e. trip point is not
affected by switching duty cycle).
The current-mode switching regulator is a dual-loop
feedback control system. In the inner current loop, the
EA output (COMP) controls the peak inductor current. In
the outer loop, the error amplifier regulates the output
voltage to keep the LED current at setting point. The
double reactive poles of the output LC filter are reduced
to a single real pole by the inner current loop, allowing
the simple loop compensation network to accommodate
a wide range of input and output voltages.
Over-Voltage Protection (OVP)
SC446 includes an external programming over-voltage
protection circuit to prevent damage to the IC and output
capacitor in the event of an open-circuit condition. The
boost converter’s output voltage is detected at the OVPIN
pin. If the voltage at the OVPIN pin exceeds 1.52V (typical),
the boost converter will shut down and a 0.9mA pull-
down current will be applied to the VOUT pin to quickly
discharge the output capacitor. This added protection
prevents a condition where the output capacitor and
Schottky diode must endure high voltage for an extended
time, which can pose a reliability risk for the user’s system.
It is well known that, in Boost converter, Vo is greater than
or equal to Vin. In normal continuous conduction mode
(CCM) operation,
Vo
1
=
Vin 1− D
Where, D is the duty ratio of the PWM power switch. As
Vin increases, in order to regulate Vo to a given constant
value, D decreases. When Vin approaches Vo, D surely
leads to 0. In practice, due to the minimum on-time of
the PWM power switch, D usually could not approach 0
with infinitely small granularity. At some point, it either
produces one pulse with minimum on-time or generates
0 by skipping the pulse. Such point could be theoretically
calculated for SC446 as follows.
Refer to evaluation application circuit in page15. The
output over voltage trip point can be programmed by R5
and R7 resistor divider.
The relationship can be described as follows:
R5 + R7
R7
For CCM: Vin ≥ 0.92 Vo.
For DCM (Discontinuous conduction mode):
OVP _ trip = OVPIN _TH ×
Where OVPIN_TH is 1.52V typical.
2
V ≥
Vo
in
Ro
L
1+ 1+1.6*10−2 *
An OVP event causing a fault could disable the boost
converter enabling the device to a strong pull-down. This
event would cause the soft-start capacitor to discharge.
When the soft-start capacitor voltage falls below 0.5V,
and the output voltage falls to VIN, SC446 enters a soft-
start process.
Where, Ro is the Boost equivalent output resistance (=Vo/
Io), L is the Boost inductor (in uH).
In many Boost converter designs and operations, pulse
skipping is normally allowed at light load conditions.
Some designers even purposely let the Boost power con-
verter enter the pulse skipping in order to save power at
light load conditions. If some designers do not want pulse
skipping mode, based on the conditions provided above,
TheOVPdetectioncircuitryprovidesadisconnectfunction
during the shutdown state to prevent any leakage from
the output. The external OVP resistor divider should be
connected between VOUT and OVPRTN with the central
© 2010 Semtech Corporation
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12
SC446
Applications Information (continued)
tap connected to OVPIN.
the TSD trip point, SC446 will retry when SS falls below
0.5V and VOUT falls to VIN.
Note: If this disconnect function is not desired, bypass
the OVPRTN pin and connect the end of the OVP resistor
divider directly to GND. The OVPIN pin is sensitive to
noise, and a proper decoupling capacitor (1nF ~ 10nF) is
required. The combined impedance of the resistor divider
for OVPIN should be greater than 200kΩ.
PWM Dimming
ThePWMinputneedstobeheldhighfornormaloperation.
PWM dimming can be done by cycling the PWM input at a
given frequency where a“low”on the PWM input turns off
all IO current sources and a “high” turns on all IO current
sources. The short and open detection latches are blanked
for approximately 2µs as the PWM input transitions from
low-to-high to prevent a false fault detection during PWM
dimming.
LED Short-Circuit Protection
If one or more LEDs are detected as short-circuit, that
string will be latched off. Voltage is monitored if it exceeds
2.35V on the internal LED current source (IO pins). (The IO
voltage on an abnormal LED string will rise earlier than
other normal LED strings). If the voltage exceeds 2.35V on
any IO pin, the IO current source will latch off. The latch is
reset if VIN falls below UVLO or it will recycle the EN signal.
Other LED strings operate normally.
The PWM pin can be toggled by external circuitry to allow
PWM dimming. In a typical application, a microcontroller
sets a register – or counter, that varies the pulse-width
on a GPIO pin. The SC446 allows dimming over two
decades in frequency (50Hz–50kHz), in order to allow
compatibility with a wide range of devices, including the
newest dimming strategies that avoid the audio band
by using high frequency PWM dimming. In this manner,
a wide range of illumination can be generated while
keeping the instantaneous LED current at its peak value
for luminescent efficiency and color purity. Furthermore,
advanced lighting effects such as backlight dim-on can be
implemented as the SC446 can resolve 10µs (minimum),
PWM dimming pulse-width.
LED Open-Circuit Protection
If any LED string is detected as an open-circuit, that string
will latch off. If any given string is open, the IO current
source will go to deep saturation; the COMP pins will
be driven high and the boost converter duty cycle will
increase causing VOUT to rise. At some point VOUT will
rise high enough to cause all the IO pin voltages of the
intact strings to reach the shorted LED detection level and
latch off those strings. Because of the LED open string
VOUT will continue to rise until it reaches the programmed
OVP level.
As far as the maximum PWM dimming pulse-width, it
depends on the PWM dimming frequency. Clearly, it is
trivial to get 100% LED brightness by pulling PWM pin
“High” constantly. When the user tries to dim the LED
brightness using PWM signal from 100% down, he or she
needs to observe the following. When the PWM dimming
signal is actively switching from “High” to “Low” and to
“High”, there is a minimal OFF time (T_off_min, 200ns,
guaranteed by design) requirement of the PWM dimming
When OVP is reached, the voltage on the IO pins are
monitored and if any IO voltage is less than 0.2V that
string will be identified as open and will latch off.
Only when VIN falls below UVLO, or an EN signal is re-
cycled, and if thermal shutdown occurs, can this latch be
reset. A hiccup cycle is then initiated and the SS pin is
discharged slowly with a 1µA current source and a 0.9mA
discharge path (turned on to pull down VOUT). When SS
falls below 0.5V and VOUT falls below to VIN, the shorted
LED detection latches are reset and a new soft-start se-
quence is initiated to resume normal operation.
signal with this IC.
maximum PWM duty ratio before hitting to 100% in the
following way.
Such minimal OFF time sets the
Dmax =1−Toff _ min fPWM
For example, if the PWM dimming frequency f_
PWM=200Hz, the D_max=99.996%. If f_PWM=25kHz,
the D_max=99.5%. With most practical dimming
interfaces, the needed dimming steps and resolutions,
it is uncommon to run into the above D_max before
reaching 100%. While most applications will not run into
D_max, the designer should be aware of possible parasitic
Thermal Shutdown (TSD)
If the thermal shutdown temperature of 150°C is reached,
a hiccup sequence is initiated where the boost converter
and all IO current sources are turned off. SS is discharged
by a 1µA current source, and a 0.9mA discharge path is
turned on to pull down VOUT. As temperature falls below
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13
© 2010 Semtech Corporation
SC446
Applications Information (continued)
elements from PWM dimming interface to the PWM pin of
SC446. Usually, simply checking signal D_max at PWM pin
of SC446 is sufficient.
For some low LED current (e.g. 10mA) applications, it is
recommended to add 1M-10Mohm resistor from IO pin
to GND in order to reduce IO pin voltage during PWM
dimming.
Linear Dimming
The linear dimming control is available for SC446 by
applying an external control voltage on IOSET pin
through an external resistor-like circuit (shown below).
External environment brightness compensation can also
be achieved when the control voltage is generated by a
light sensor circuit.
Parallel Operation
When two or more SC446s are operating in parallel for a
large-sized panel application, audible noise may be ob-
served due to non-synchronous switching frequency. The
ripple voltage on the input voltage rail will be modulat-
ed by the beat frequency resulting in audible noise. This
situation can be resolved by adding an input inductor
between input voltage rail and the SC446 VIN pin. This
situation can also be improved by adding more input de-
coupling capacitors.
IOSET
R_EXT
V_EXT
R_IOSET
Inductor Selection
The inductance value of the inductor affects the convert-
er’s steady state operation, transient response, and its
loop stability. Special attention needs to be paid to three
specifications of the inductor, its value, its DC resistance
and saturation current. The inductor’s inductance value
also determines the inductor ripple current. The converter
can operate in either CCM or DCM depending on its work-
ing conditions. The inductor DC current or input current
can be calculated as,
The IOSET voltage is 0.5V when linear dimming is used and
the minimum IOSET current must be higher than 27µA
(i.e. 15mA per LED string). The external control voltage
slew rate must slow at 1V/10ms.
LED Strings Connection
Generally, LED strings are connected to IO1 ~ IO3 pins
through a mechanical connector which, generally, cannot
support an electrical connection thereby resulting in sig-
nificant noise. Consequently, the SC446 LED short-circuit
protection may false trip when the noise level is large.
Certain ceramic decoupling capacitor on pins IO1 ~ IO3 to
GND are useful to prevent the SC446 from noise influence.
9287 ,287
,
,1
9 Ș
,1
IIN - Input current;
IOUT – Output current;
As a general guideline, the decoupling capacitance should
be limited as follows.
VOUT – Boost output voltage;
VIN – Input voltage;
η – Efficiency of the boost converter.
0.6uS
Cdcple < ILED
*
Vo
Where, I_LED is the LED current per string, Vo is the Boost
output voltage and C_dcple is the suggested decoupling
capacitor value.
Then the duty ratio is,
9287 ꢁ 9,1 ꢀ 9'
'
9287 ꢀ 9'
For example, if I_LED=10mA, Vo=13.5V, the calculated
upper bound of C_dcple is about 444pF. One could use
390pF or less in the circuit. If I_LED=100mA, Vo=13.5V,
the calculated upper bound of C_dcple is about 4.44nF.
One may use 3.9nF or less in the circuit. In some applica-
tions, circuit designers tend to select the decoupling ca-
pacitors in the range of (100pF ~ 1nF).
VD – Forward conduction drop of the output rectifying
diode
WhentheboostconverterrunsinDCM(L<Lboundary),ittakes
the advantages of small inductance and quick transient
response while avoiding the bandwidthlimiting instability
of the RHP zero found in CCM boost converters.
© 2010 Semtech Corporation
www.semtech.com
14
SC446
Applications Information (continued)
The inductor peak current is,
the mechanics of regulation and frequency dependence
may be complex, actual selection of output capacitor can
be simplified because this capacitor is mainly selected
for the output ripple of the converter. Assume a ceramic
capacitor is used. The minimum capacitance needed for a
given ripple can be estimated by,
V ⋅D
IN
IL−peak
=
F ⋅L
S
The converter will work in CCM if L > Lboundary. Generally
the converter has higher efficiency under CCM and the
inductor peak current is,
(VOUT ꢀ V )xIOUT
IN
COUT
9 '
ꢁꢀ)6 /
,1
VOUT FS VRIPPLE
,
,,1 ꢀ
/ꢂSHDN
VRIPPLE – Peak to peak output ripple;
IOUT – Output current;
For many applications, an inductor with value of 4.7µH to
22µH should be fine, such as for the typical case shown
on page 1. The inductor peak current must be less than its
saturation rating. When the inductor current is close to the
saturation level, its inductance can decrease 20% to 35%
from the 0A value depending on the vendor specifications.
Using a small value inductor forces the converter under
DCM in which case the inductor current ramps down to
zero before the end of each switching cycle. It reduces the
boost converter’s maximum output current, and produces
large input voltage ripple. An inductor with larger
inductance will reduce the bandwidth of the feedback
loop, possibly higher DC resistance (DCR). Inductor’s DCR
plays a significant role for the total efficiency since the
power transistor is integrated inside the SC446. Of course,
there is a trade-off between the DCR and inductor size.
Table 2 lists recommended inductors and their vendors.
VOUT – Boost output voltage;
VIN – Input voltage;
FS – Switching frequency.
During load transient, the output capacitor supplies or
absorbs additional current before the inductor current
reaches its steady state value. Larger capacitance helps
withtheovershootandundershootsduringloadtransient,
and loop stability. Recommended ceramic capacitor
manufacturers are listed in Table 3.
Table 3. Recommended Ceramic Capacitor
Manufacturers
Vendor
Phone
Website
Kemet
408-986-0424
www.kemet.com
Table 2. Recommended Inductors
Murata
814-237-1431
408-573-4150
www.murata.com
www.t-yuden.com
Taiyo Yuden
Inductor
DR74, 4.7μH ~ 15μH
Website
www.cooperet.com
www.vishay.com
www.tokoam.com
Output Rectifying Diode Selection
IHLP-2525CZ-01, 4.7μ ~ 10μH
DS85LC, 6.8μH ~ 10μH
Schottky diodes are the ideal choice for SC446 due to their
low forward voltage drop and fast switching speed. Table
4 shows several different Schottky diodes that work well
with the SC446. Make sure that the diode has a voltage
rating greater that the possible maximum ouput voltage.
The diode conducts current only when the power switch
is turned off. A diode of 2A will be sufficient for most
designs.
Output Capacitor Selection
The next task in SC446 design is targeting the proper
amount of ripple voltage due to the constant-current
LED loads. The two error amplifiers that control the PWM
converter sense the delta between requested current
and actual current in each output current regulator. On
a cycle-by-cycle basis, a small amount of output ripple
ensures good sensing and tight regulation, while the
output current regulators keep each LED current at a fixed
value. Overall, this allows usage of small output caps while
ensuring precision LED current regulation. Although
Table 4. Recommended Rectifying Diodes
Part
Vendor
SS23
SS24
Vishay
www.vishay.com
www.semtech.com
15
© 2010 Semtech Corporation
SC446
Applications Information (continued)
Layout Guidelines
The SC446 contains a boost converter and the placements
of the power components outside the SC446 should fol-
low the layout guidelines of a general boost converter.
The evaluation application circuit on page 17 will be used
as an example. C2 and C3 are input decoupling capacitor
for SC446 VIN pin and main power input. C2,C3 should be
placed as close as possible to the VIN pin to achieve the
best decoupling performance.
To minimize the switching noise, The switching loop
formed by input decoupling capacitors, internal switch,
output Schottky diode and output capacitors must be
minimized. The LED current programming resistor(R6),
compensation network (R9,C5,C7) and soft start capaci-
tor (C6) should be placed as close as possible to SC446.
The C14~C17 are decoupling capacitors for LED cur-
rent source which prevent IO pins from noise influence.
C14~C17 should be placed close to each corresponding
IO pin.
Use an isolated local AGND plane underneath the con-
troller and tie it to the negative side of output capacitor
through R14 for better noise immunity.
© 2010 Semtech Corporation
www.semtech.com
16
SC446
Evaluation Application Circuit
VIN
VOUT
R1
0R
U1
R8
VIN
R2
10K
9
10
11
12
13
14
15
16
8
7
6
5
4
3
2
1
R4
10K
VIN
VOUT
PWM
IOSET
EN
1R
C1 4.7nF
R7 10K
PWM
OVPRTN
OVPIN
PGND
SW
C2
C3
C4
R6
RIOSET
N.P.
10uF/25V
L1
6.8uH
R5
243K
EN
SC446
VOUT
COMP
SS
VOUT
D1
R9
1.5K
PGND
IO3
C7
N.P.
R10
1R
C6
100nF
C5
IO3
IO2
AGND
IO1
C8
C9
22nF
4.7uF/50V
4.7uF/50V
IO1
IO2
R13
1R
R12
1R
C14
1nF
C15
1nF
C16
1nF
R14
0R
Evaluation Board Bill of Materials
Item
Reference
Quantity
Description
Package
Part
Vendor
1
2
C1
1
3
1
2
1
1
1
1
1
2
4
1
1
1
4
1
25V ceramic capacitor, X7R
50V ceramic capacitor, X7R
25V ceramic capacitor, X5R
50V ceramic capacitor, X5R
25V ceramic capacitor, X5R
6.3V ceramic capacitor, X7R
6.3V ceramic capacitor, X7R
60V, 2A Schottky diode
6.8μH, 3.67A
SM_0603
SM_0603
SM_1206
SM_1206
SM_0805
SM_0603
SM_0603
SMA
4.7nF
1nF
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
Panasonic
C14, C15, C16
3
C2
10uF
4.7uF
2.2uF
22nF
100nF
B260A
6.8μH
0R
4
C8, C9
5
C4
6
C5
7
C6
8
D1
Diodes or Any
Copper or Any
Any
9
L1
DR74
10
11
12
13
14
15
16
R1, R14
1% SMD resistor
SM_0603
SM_0603
SM_0603
SM_0603
SM_0603
SM_0603
R2, R3, R4, R7
5% SMD resistor
10K
Any
R5
1% SMD resistor
243K
RIOSET
1.5K
Any
R6
1% SMD resistor
Any
R9
R8, R10, R12, R13
U1
5% SMD resistor
Any
5% SMD resistor
1R
Any
Controller
EDP TSSOP-16
SC446
SEMTECH
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17
© 2010 Semtech Corporation
SC446
Outline Drawing - TSSOP-16 EDP
© 2010 Semtech Corporation
© 2010 Semtech Corporation
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18
SC446
Land Pattern -TSSOP-16 EDP
Contact Information
Semtech Corporation
Power Management Products Division
200 Flynn Road, Camarillo, CA 93012
Phone: (805) 498-2111 Fax: (805) 498-3804
www.semtech.com
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© 2010 Semtech Corporation
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