SC4539 Datasheet

SC4539
High Efficiency Boost Converter
for up to 6 LEDS
POWER MANAGEMENT
Features
Description
„
The SC4539 is designed to regulate current for a series
string of white LEDs in LCD backlighting applications
where small size and high efficiency are priorities. This
device integrates an 800kHz current-mode PWM boost
converter and a 30mA programmable low dropout current
sink regulator.
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Input voltage range — 2.8V to 5.5V
Programmable LED current up to 30mA
Current-mode PWM control — 800kHz
Soft-start to reduce in-rush current
PWM dimming — 100Hz to 50kHz
PWM dimming at ISET — Analog and filtered
Over-voltage protection — 22V (minimum)
Under-voltage lockout (UVLO)
Thermal shutdown
Shutdown current — <0.1μA (typical)
Ultra-thin package — 2mm x 2mm x 0.6mm
Lead-free package, WEEE and RoHS compliant
An external resistor sets the LED current up to 30mA.
PWM dimming (100Hz to 50kHz) may be applied directly
to the enable (EN) pin, or dimming can be controlled by
applying an analog signal to the ISET circuit. The boost
circuit can output up to 22V (guaranteed) to drive up to 6
LEDs in series. The current regulator protects against
shorts between the BL and OUT pins and also eliminates
backlight glow during shutdown when using LEDs with
high leakage. Under-voltage lockout and thermal shutdown provide additional protection. A small external
capacitor and series resistor control soft-start and loop
compensation. Over-voltage detection protects the
SC4539 if the BL pin is floating or shorted to ground.
Applications
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DSLR, DSC, and Video Cameras
Cellular handsets
Portable media players
Personal navigation systems
Satellite radio
Handheld video games
The SC4539 is available in an MLPD-8 pin 2mm x 2mm x
0.6mm package, with a rated temperature range of -40°C
to +85°C.
Typical Application Circuit
D
L1
LX
2.8 to
5.5V
COUT
OUT
IN
CIN
SC4539
Enable and
EN
PWM dimming
RISET
CCOMP
ISET
RCOMP
BL
COMP
GND
June 20, 2008
© 2008 Semtech Corporation
1
SC4539
Pin Configuration
LX
1
EN
2
ISET
3
COMP
4
Ordering Information
TOP
VIEW
T
8
IN
7
GND
6
BL
5
OUT
Device
Package
SC4539ULTRT(1) (2)
MLPD-UT-8 2x2
SC4539EVB
Evaluation Board
Note:
(1) Available in tape and reel only. A reel contains 3,000 devices.
(2) Lead-free package only. Device is WEEE and RoHS compliant.
MLPD: 2mm x 2mm 8 Lead
θJA = 74°C/W
Marking Information
FF0
yw
FF0 = Marking code for SC4539
yw = Date Code
2
SC4539
Absolute Maximum Ratings
Recommended Operating Conditions
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 5.5
LX Voltage, Output Voltage (V) . . . . . . . . . . . . . -0.3 to +45
Output Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 to 22
Current Sink Voltage (V) . . . . . . . . . . . . . . . . . . . . -0.3 to +45
Ambient Temperature Range (°C) . . . . . . . . . . . . . -40 to +85
Enable Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to (VIN + 0.3)
Compensation Voltage (V) . . . . . . . . . . . . . . . . . . -0.3 to +2.0
Current Set Voltage (V) . . . . . . . . . . . . . . . . . . . . . . -0.3 to +2.0
ESD Protection Level(1) (kV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Thermal Information
Thermal Resistance, Junction to Ambient(2) (°C/W) . . . . . 74
Maximum Junction Temperature (°C) . . . . . . . . . . . . . . +150
Storage Temperature Range (°C) . . . . . . . . . . . . -65 to +150
Peak IR Reflow Temperature (10s to 30s) (°C) . . . . . . . +260
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) Tested according to JEDEC standard JESD22-A114-B.
(2) Calculated from package in still air, mounted to 3 x 4.5 (in), 4 layer FR4 PCB with thermal vias under the exposed pad per JESD51 standards.
Electrical Characteristics
Unless otherwise noted: VIN = 3.6V, CIN = 2.2μF, COUT = 1μF, CCOMP = 47nF, RCOMP = 1.27kΩ, RISET = 5.76kΩ, L = 22μH, TA = -40 to 85ºC, typical values are at TA = 25ºC.
Parameter
Symbol
Conditions
Min
Typ
Max
Units
UVLO Threshold
VUVLO
VIN rising
2.40
2.60
2.79
V
UVLO Hysteresis
VUVLO-HYS
120
mV
mA
Quiescent Supply Current
IQ
Not switching
1.8
Shutdown Supply Current
ISHDN
EN tied to GND
0.1
EN Logic High Voltage
VIN
EN Logic Low Voltage
VIL
EN Logic Input Current
IIL, IIH
VIN = 5.5V, VEN = 0V or 5.5V
±0.01
Thermal Shutdown Temperature
TSD
TJ rising
155
°C
20
°C
1
1.80
Thermal Shutdown Hysteresis
μA
V
0.8
V
±1
μA
Boost Converter Characteristics
Switching Frequency
fSW
Maximum Duty Cycle
DMAX
Minimum On-Time
Switch Over-Current Protection
680
VIN = 3.2V, TA = 25°C
920
92
tON(MIN)
IOCP
800
%
100
425
kHz
ns
725
mA
3
SC4539
Electrical Characteristics (continued)
Parameter
Symbol
Conditions
Switch Leakage Current
IL(LX)
Switch Saturation Voltage
VSAT
Min
Typ
Max
Units
VLX = 5.5V
0.01
1
μA
ILX = 0.3A
250
450
mV
VCOMP = 0.9V, TA = 25ºC
5
VCOMP = 0.9V, TA = 25ºC
6
Boost Converter Characteristics (continued)
COMP Sourcing Current
ICOMP
COMP Sinking Current
OUT Over-Voltage Protection
VOVP
OUT Internal Pull-Down Current
IOVP
OUT Bias Current
μA
22
25
V
During OVP condition
1
mA
VEN = VIN, VOUT = 20V
50
70
μA
VEN = 0V, VOUT = VIN = 5.5V
0.01
1
μA
IOUT
OUT Leakage Current
PWM Dimming Frequency Range(1)(2)
fEN
Applied to EN pin
100
50k
Hz
PWM Dimming Duty Cycle Range(1)(2)
DEN
200Hz on EN pin
0
100
%
1
30
mA
-3.5
3.5
%
0.1
μA
Current Sink Characteristics
BL Current Setting Range(3)
IBL
BL Current Setting Accuracy
BL Leakage Current
BL Current Line Regulation
TA= 25°C
IL(BL)
VEN = 0V, VBL = 2V
0.01
ΔIBL/ΔVIN
VIN = 3.0 to 5.5V
±0.05
mA/V
BL Voltage
VBL
0.35
V
ISET Bias Voltage
VISET
0.5
V
ISET-to-IBL Gain
AISET
230
A/A
Start-Up Time
tstart-up
1.3
μs
Notes:
(1) Guaranteed by design.
(2) See PWM Dimming description in the Applications Information section for limitations at high PWM dimming frequencies and low PWM
dimming duty cycles.
(3) Not recommended to program below 1mA with RISET due to tolerance stackup. To produce output current less than 1mA, set the current >
1mA and use PWM dimming.
4
SC4539
Typical Characteristics
All data taken with VOUT = 20V (6 white LEDs), RISET = 5.76kΩ (IBL = 20mA), VIN = 3.6V, L = 22μH, and efficiency (η) = PLED/PIN unless otherwise noted.
Efficiency vs. IBL
Efficiency vs. Output Voltage
90
100
3.6VIN
86
2.8VIN
Efficiency (%)
Efficiency (%)
80
60
5.5VIN
40
5.5VIN
82
3.3VIN
78
2.8VIN
74
20
70
0
0
6
18
12
24
30
0
10
5
IBL (mA)
20
25
Efficiency vs. Input Voltage
Efficiency vs. Input Voltage
90
90
86
86
Efficiency (%)
Efficiency (%)
15
VOUT (V)
82
VOUT
LED
78
6 LED
82
4 LED
78
74
74
70
70
2.8
3.3
4.4
3.9
5.0
5.5
2.8
3.3
4.4
3.9
VIN (V)
5.0
5.5
VIN (V)
Normalized IBL vs. Output Voltage
Efficiency vs. Input Voltage
6.0
90
20mA
4.0
Normalized IBL (%)
Efficiency (%)
86
82
78
30mA
2.0
0.0
-2.0
10mA
74
-4.0
-6.0
70
2.8
3.3
4.4
3.9
VIN (V)
5.0
5.5
0
5
10
15
20
25
VOUT (V)
5
SC4539
Typical Characteristics (continued)
PWM Operation at 200Hz and 1% Duty Cycle
PWM Operation at 50kHz and 15% Duty Cycle
VLX (20V/div)
VLX (20V/div)
IOUT (20mA/div)
IOUT (20mA/div)
VOUT (10V/div)
VOUT (20V/div)
VEN (2V/div)
VEN (2V/div)
4μs/div
2ms/div
PWM Operation at 32kHz and 50% Duty Cycle
VLX (20V/div)
PWM Operation at 32kHz and 10% Duty Cycle
VLX (20V/div)
IOUT (20mA/div)
IOUT (20mA/div)
VOUT (20V/div)
VOUT (20V/div)
VEN (2V/div)
VEN (2V/div)
10μs/div
10μs/div
IBL vs. Duty Cycle
IBL Error vs. PWM Frequency at 50% Duty Cycle
6.0
4.0
80
2.0
60
IBL Error (%)
Percentage of Maximum IBL (%)
100
40
200Hz
0.0
-2.0
20
-4.0
50kHz
32kHz
-6.0
0
0
20
40
60
PWM Duty Cycle (%)
80
100
0.1
1
10
100
PWM Frequency (kHz)
6
SC4539
Typical Characteristics (continued)
Typical Waveforms at VIN = 5.5V
Typical Waveforms at VIN = 2.8V
VLX (20V/div)
VLX (20V/div)
VIN (50mV/div)
VIN (50mV/div)
VOUT (50V/div)
VOUT (50V/div)
1μs/div
1μs/div
Typical Waveforms at VIN = 3.6V
Start-Up Response
VLX (20V/div)
VLX (20V/div)
VIN (50mV/div)
IOUT (20mA/div)
VOUT (10V/div)
VOUT (50mV/div)
VEN (2V/div)
1μs/div
100μs/div
Waveforms During Over-Voltage Protection
Normalized IBL vs. Input Voltage
3.5
2.3
VLX (20V/div)
IBL Error (%)
1.2
0.0
-1.2
VOUT (5V/div)
-2.3
VEN (5V/div)
-3.5
40μs/div
2.8
3.25
3.7
4.15
4.6
5.05
5.5
VIN (V)
7
SC4539
Pin Descriptions
Pin #
Pin Name
Pin Function
1
LX
Collector of the internal power transistor — connect to the boost inductor and rectifying Schottky
diode.
2
EN
Enable and brightness control pin for LED string
3
ISET
Output current set pin — connect a resistor from this pin to GND to set the maximum current.
4
COMP
5
OUT
6
BL
7
GND
8
IN
T
Thermal Pad
Output of the internal transconductance error amplifier — this pin is used for loop
compensation and soft-start. Connect a 1.27kΩ resistor and 47nF capacitor in series to GND.
Boost output voltage monitor pin — internal over-voltage protection monitors the voltage at this
pin. Connect this pin to the output capacitor and the anode of the LED string.
LED constant current sink — connect this pin to the cathode of the LED string
Ground
Power supply pin — bypass this pin with a capacitor close to the pin
Pad for heatsinking purposes — connect to the ground plane using multiple vias.
Not connected internally.
8
SC4539
Block Diagram
OUT
5
UVLO
IN
8
Thermal
Shutdown
OVP
S
Reference and
Internal
Regulator
7
GND
4
COMP
R
RSENSE
Sense Amp
OSC
Error Signal
Selection
and Summation
2
ADJ
EN
BL
LX
Q
Comparator
EN
1
6
ADJ
LED
Setpoint
3
ISET
9
SC4539
Applications Information
General Description
The SC4539 contains an 800kHz fixed-frequency currentmode boost converter and an independent LED current
regulator. The LED current set point is chosen using an
external resistor, while the PWM controller operates independently to keep the current in regulation. The SC4539
receives information from the internal LED current regulator and drives the output to the proper voltage with no
user intervention.
The current flowing through the LED string is independently controlled by an internal current regulator, unlike
the ballasting resistor scheme that many LED current
regulators use. The internal current regulator can be shut
off entirely without leaking current from a charged
output capacitor or causing false-lighting with low LED
count and high VIN. The backlight current (IBL) is programmed using an external resistor.
seconds provides the additional advantage of reducing
in-rush current at start up.
The start-up delay time between the enable signal going
high and the activation of the internal current regulator
causes nonlinearity between the IBL current and the duty
cycle of the PWM frequency seen by the EN pin. As the
PWM signal frequency increases, the total on time per
cycle of the PWM signal decreases. Since the start up
delay time remains constant, the effect of the delay
becomes more noticeable, causing the average IBL to be
less predictable at lower duty cycles. Recommended
minimum duty cycles are 20% for 50kHz PWM frequency,
15% for 32kHz PWM frequency and 2% for 200Hz PWM
frequency. Refer to the IBL vs. Duty Cycle in the Typical
Characteristics section for PWM performance across duty
cycle for different PWM frequencies.
Zero Duty Cycle Mode
The path from the EN pin to the output control is a high
bandwidth control loop. This feature allows the PWM
dimming frequency to range between 100Hz and 50kHz.
In shutdown mode, leakage through the current regulator output is less than 1μA. This keeps the output
capacitor charged and ready for instant activation of the
LED string.
Zero duty cycle mode is activated when the voltage on
the BL pin exceeds 1.3V. In this mode, the COMP pin
voltage is pulled low, suspending all switching. This allows
the VOUT and VBL voltages to fall. The COMP voltage is held
low until the VBL falls below 1V, allowing VCOMP to return to
its normal operating voltage and switching to resume.
Protection Features
The 800kHz switching speed provides high output power
while allowing the use of a low profile inductor, maximizing efficiency for space constrained and cost-sensitive
applications. The converter and output capacitor are
protected from open-LED conditions by over-voltage
protection.
PWM Dimming
The enable pin can be toggled 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
device is compatible with a wide range of applications by
allowing dimming strategies that avoid the audio band
by using a frequency between 100Hz and 50kHz for PWM
dimming. Various intensity levels can be generated while
keeping the instantaneous LED current at its peak value
for luminescent efficiency and color purity. The SC4539
can accommodate any PWM duty cycle between 0 and
100%. A low duty cycle PWM signal used for a few milli-
The SC4539 provides several protection features to safeguard the device from catastrophic failures. These features
include:
•
•
•
•
Over-Voltage Protection (OVP)
Soft-start
Thermal Shutdown
Current Limit
Over-Voltage Protection (OVP)
A built-in over-voltage protection circuit prevents damage
to the IC and output capacitor in the event of an opencircuit condition. The output voltage of the boost
converter is detected at the OUT pin and divided internally. If the voltage at the OUT pin exceeds the OVP limit,
the boost converter is shut down and a strong pull down
is applied to the OUT pin to quickly discharge the output
capacitor. This additional level of protection prevents a
condition where the output capacitor and Schottky diode
10
SC4539
Applications Information (continued)
must endure high voltage for an extended period of
time.
Soft-Start
The soft-start mode reduces in-rush current by utilizing
the external compensation network. As the error amplifier
slowly charges the COMP node voltage, the duty cycle of
the boost switch ramps from 0% to its final value once in
regulation. The gradual increase of the duty cycle slowly
charges the output capacitor and limits in-rush current
during start up. Soft-start is implemented only when the
input power is cycled.
Thermal Shutdown
A thermal shutdown system is included for protection in
the event the junction temperature exceeds 155°C. In
thermal shutdown, the on-chip power switch is disabled.
Switching and sinking resumes when the temperature
drops by 20°C.
Current Limit
The power switch of the boost converter is protected by an
internal current limit function. The switch is opened when
the current exceeds the maximum switch current value.
Inductor Selection
The inductor value should be within the range of 4.7μH to
22μH. The DCR needs to be considered when selecting
the inductor to ensure optimum efficiency. The largest
inductor package that can be tolerated in the circuit area
should be used since the DCR generally decreases with
increasing package size.
The saturation current of the inductor should be much
higher than the peak current of the internal boost switch
to ensure that the inductor never enters saturation during
normal operation of the part. The equation to calculate
the peak inductor current is
IL(Peak)
IIN ǻIL
VIN
VOUT
D
1
IIN
VOUT u IOUT
Ș u VIN
D is the duty cycle for continuous operation. Efficiency (η)
can be approximated by using the curves provided in the
Typical Characteristics section. Table 1 lists inductors that
have been proven to work with SC4539.
Table 1 -- Recommended Inductors
Value
(μH)
DCR
(Ω)
Rated
Current (A)
Tolerance
Dimensions
(L x W x H)
(mm)
Coilcraft
LPS4018-223ML
22
0.360
0.70
±20%
3.9 x 3.9 x 1.7
Murata
LQH43CN150K03
15
0.320
0.570
±10%
4.5 x 3.2 x 2.6
Murata
LQH32CN150K53
15
0.580
0.300
±10%
3.2 x 2.5 x 1.55
Part Number
Capacitor Selection
The input capacitor should be at least 2.2μF. A larger
capacitor will reduce the voltage ripple on the input. The
output capacitor values can range from 0.22μF to 1μF. The
compensation capacitor value should be 47nF. Capacitors
of X5R type material or better can be used for any of the
capacitors. See Table 2 for the recommended capacitors.
Table 2 -- Recommended Capacitors
Part Number
Value
(μF)
Rated
Voltage (V)
Type
Case Size
2.2
6.3
X7S
0603
1.0
50
X7R
0805
0.047
16
X7R
0402
Input Capacitor
Murata
GRM188C70J225KE20
Output Capacitor
Murata
GRM21BR71H105KA12L
Compensation Capacitor
2
Taiyo Yuden
EMK105BJ473KV-F
where
ǻIL
VIN u D
L u f osc
11
SC4539
Applications Information (continued)
Diode Selection
PCB Layout Considerations
For optimum performance, it is recommended that a
Schottky diode with a reverse voltage of 40V and a forward
current rating of 1A like the Central Semiconductor
Corporation CMOSH-4E be used. Diodes with lower
voltage ratings can be used, but performance should be
compared to the performance with this 40V part to ensure
stable operation is maintained.
Poor layout can degrade the performance of the DC-DC
converter and can be a contributory factor in EMI problems, ground bounce, thermal issues, and resistive voltage
losses. Poor regulation and instability can result. A typical
layout is shown in Figure 2.
The following design rules are recommended:
•
Selection of Other Components
RISET sets the maximum load current for the SC4539. Use
the following equation to select the proper value:
•
RISET = 230 × VISET/ILOAD
•
where
Place the inductor and filter capacitors as close
to the device as possible and use short, wide
traces between the power components.
Route the output voltage feedback path away
from the inductor and LX node to minimize
noise and magnetic interference.
Use a ground plane to further reduce noise
interference on sensitive circuit nodes.
VISET = 0.5V (typ).
Refer to Figure 1 for selecting values for other current settings. Notice that the error increases as the desired IBL
current decreases.
IBL (mA)
100
10
RISET
Typical
Current
Tolerance
1
1
10
RISET (kΩ)
100
Figure 1 – Set Resistor Value Selection Graph
Figure 2– Layout
12
SC4539
Outline Drawing – MLPD-UT-8 2x2
B
D
A
DIMENSIONS
INCHES
MILLIMETERS
DIM
MIN NOM MAX MIN NOM MAX
E
PIN 1
INDICATOR
(LASER MARK)
A
aaa C
A2
A1
SEATING
PLANE
C
A
A1
A2
b
D
D1
E
E1
e
L
N
aaa
bbb
. 024
. 002
(.006)
.007 . 010 . 012
. 075 .079 . 083
. 061 .067 . 071
. 075 .079 . 083
. 026 .031 . 035
. 020 BSC
. 012 .014 . 016
8
.003
.004
.020
. 000
0.60
0.50
0.00
0.05
(0. 1524)
0.18 0.25 0.30
1.90 2.00 2.10
1.55 1.70 1.80
1.90 2.00 2.10
0.65 0.80 0.90
0. 50 BSC
0.30 0.35 0.40
8
0.08
0.10
D1
1 2
LxN
E/2
E1
N
bxN
bbb
e
C A B
e/2
D/2
NOTES:
1.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS ( ANGLES IN DEGREES).
2.
COPLANARITY APPLIES TO THE EXPOSED PAD AS WELL AS THE TERMINALS.
13
SC4539
Land Pattern – MLPD-UT-8 2x2
H
DIMENSIONS
R
(C)
G
K
Z
Y
P
DIM
INCHES
C
(.077)
G
.047
1.20
H
.067
1.70
K
.031
0.80
P
.020
0.50
R
.006
0.15
X
.012
0.30
Y
.030
0.75
Z
.106
2.70
MILLIMETERS
(1.95)
X
NOTES:
1.
2.
CONTROLLING DIMENSIONS ARE IN MILLIMETERS (ANGLES IN DEGREES).
THIS LAND PATTERN IS FOR REFERENCE PURPOSES ONLY.
CONSULT YOUR MANUFACTURING GROUP TO ENSURE YOUR
COMPANY'S MANUFACTURING GUIDELINES ARE MET.
3.
THERMAL VIAS IN THE LAND PATTERN OF THE EXPOSED PAD
SHALL BE CONNECTED TO A SYSTEM GROUND PLANE.
FAILURE TO DO SO MAY COMPROMISE THE THERMAL AND/OR
FUNCTIONAL PERFORMANCE OF THE DEVICE.
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
14