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SC4540
High Efficiency Boost Converter for up to 10 LEDs
POWER MANAGEMENT Features
Input voltage range -- 4.5V to 12V 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 -- 38V (minimum) Under-voltage lockout (UVLO) Thermal shutdown Shutdown current -- <0.1A (typical) Ultra-thin package -- 2 x 2 x 0.6 (mm) Lead-free package, WEEE and RoHS compliant
Description
The SC4540 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. The wide input voltage range of the SC4540 makes it ideal for use with dual-cell Li-ion powered applications. 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 38V (guaranteed) to drive up to 10 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 SC4540 if the BL pin is floating or shorted to ground. The SC4540 is available in an MLPD-8 pin 2 x 2 x 0.6 (mm) package, with a rated temperature range of -40C to +85C.
Applications
DSLR, DSC, and Video Cameras Portable media players Set-top boxes Personal navigation devices
Typical Application Circuit
L1 5 6 H LX 4 .5 to 1 2 V IN
C IN 2.2F D 1 F C O UT
OU T
SC 4540
E n a b le a n d PWM d im m in g
R IS E T
EN IS E T C OMP
CC OM P
RC OM P 1 .2 7 k
47nF
BL GN D
July 18, 2008
(c) 2008 Semtech Corporation
1
SC4540
Pin Configuration Ordering Information
Device
SC4540ULTRT(1) (2) SC4540EVB
Package
MLPD-UT-8 2x2 Evaluation Board
LX EN IS E T COMP
1 2 3 4
TO P V IE W
8 7 6
IN GND BL OUT
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.
T
5
MLPD: 2mm x 2mm 8 Lead JA = 74C/W
Marking Information
FJ0 yw
FJ0 = Marking code for SC4540 yw = Date Code
2
SC4540
Absolute Maximum Ratings
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . -0.3 to +20.0 LX Voltage, Output Voltage (V) . . . . . . . . . . . . . -0.3 to +45 Current Sink Voltage (V) . . . . . . . . . . . . . . . . . . . . -0.3 to +45 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
Recommended Operating Conditions
Supply Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 to 12 Output Voltage (V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 to 38 Ambient Temperature Range (C) . . . . . . . . . . . . . -40 to +85
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 = 7.2V, CIN = 2.2F, COUT = 1F, CCOMP = 47nF, RCOMP = 1.27k, RISET = 5.76k, L = 56H, TA = -40 to 85C, typical values are at TA = 25C.
Parameter
UVLO Threshold UVLO Hysteresis Quiescent Supply Current Shutdown Supply Current EN Logic High Voltage EN Logic Low Voltage
Symbol
VUVLO VUVLO-HYS IQ ISHDN VIN VIL
Conditions
VIN rising
Min
2.40
Typ
2.60 120
Max
2.79
Units
V mV mA
Not switching EN tied to GND 1.80
1.9 0.1 3
A V
0.8 VIN = 12V, VEN = 0V to 1.8V 0.01 0.01 155 20 1
V
EN Logic Input Current
IIL, IIH VIN = 12V, VEN = 1.8V to 12V 5
A
Thermal Shutdown Temperature Thermal Shutdown Hysteresis Boost Converter Characteristics Switching Frequency Maximum Duty Cycle Minimum On-Time
TSD
TJ rising
C C
fSW DMAX tON(MIN) TA = 25C
680 92
800
920
kHz %
100
ns
3
SC4540
Electrical Characteristics (continued)
Parameter
Boost Converter Characteristics (continued) Switch Over-Current Protection Switch Leakage Current Switch Saturation Voltage COMP Sourcing Current ICOMP COMP Sinking Current OUT Over-Voltage Protection OUT Internal Pull-Down Current OUT Bias Current IOUT OUT Leakage Current PWM Dimming Frequency Range(1)(2) PWM Dimming Duty Cycle Range(1)(2) Current Sink Characteristics BL Current Setting Range(3) IBL BL Current Setting Accuracy BL Leakage Current BL Current Line Regulation BL Voltage ISET Bias Voltage ISET-to-IBL Gain Start-Up Time IL(BL) IBL/VIN VBL VISET AISET tstart-up TA= 25C VEN = 0V, VBL = 2V VIN = 3.0 to 5.5V -3.5 0.01 0.05 0.35 0.5 230 1 3.5 0.1 % A mA/V V V A/A s 1 30 mA fEN DEN VEN = 0V, VOUT = VIN = 12V Applied to EN pin 200Hz on EN pin 100 0 0.01 1 50k 100 A Hz % VOVP IOVP During OVP condition VEN = VIN, VOUT = 20V VCOMP = 0.9V, TA = 25C 38 1 50 70 6 43 V mA A IOCP IL(LX) VSAT VLX = 12V ILX = 0.3A VCOMP = 0.9V, TA = 25C 425 0.01 250 5 A 725 1 450 mA A mV
Symbol
Conditions
Min
Typ
Max
Units
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
SC4540
Typical Characteristics
All data taken with VOUT = 26.5V (8 white LEDs), RISET = 5.76k (IOUT = 20mA), VIN = 7.2V, L = 56H, and efficiency () = PLED/PIN unless otherwise noted.
Efficiency vs. IBL
90 V IN = 7.2 V 8 6 V = 8.4 V IN V IN = 5.0 V
86 90
Efficiency versus Output Voltage
V IN = 1 2V V IN = 8 .4 V
E ffic ie n c y (% )
E fficien cy (% )
V IN = 5 .0 V 82
V IN = 7 .2 V
82 V IN = 1 2V 78
78
74
74
70
70
10
15
IB L
20 (m A )
25
30
20
22
24
V O U T (V )
26
28
30
Efficiency versus Input Voltage
90 30 m A 88 20 m A
Efficiency versus Input Voltage
90
88
6 LEDs
E fficien cy (% )
E fficie n c y (% )
86
86 10 LEDs 84 8 LEDs
84 10 m A 82
82
80 4 .5
6 .0
7.5
V IN (V )
9 .0
1 0.5
1 2 .0
80 4 .5
6.0
7 .5
V IN (V )
9 .0
1 0 .5
1 2 .0
Efficiency versus Input Voltage
90
3 .5 2 .5 1 .5
N o rm a lize d I B L (% )
Normalized IBL versus Output Voltage
88
V O U T E fficie n cy L E D E fficie n cy
E ffic ie n c y (% )
86
0 .5 -0.5 -1.5 -2.5
V IN = 1 2 V V IN = 8 .4 V
84
V IN = 7 .2V
V IN = 5 .0V
82
80 4 .5
-3.5
6 .0
7 .5
V IN (V )
9 .0
1 0 .5
1 2 .0
20
22
24
V O U T (V )
26
28
30
5
SC4540
Typical Characteristics (continued)
Normalized IBL versus Duty Cycle
100
Typical Waveforms at VIN = 5.0V
P ercen tag e o f M a xim u m I B L (% )
80
VLX (20V/div)
60
VIN (50mV/div)
40
20
2 0 0H z 3 2kH z 0 20
5 0kH z
VOUT (100mV/div)
40 60 D u ty C y c le (m A ) 80 100
0
1s/div
Typical Waveforms at VIN = 7.2V
Typical Waveforms at VIN = 12V
VLX (20V/div)
VLX (20V/div)
VIN (50mV/div)
VIN (50mV/div)
VOUT (100mV/div) 1s/div
VOUT (100mV/div) 1s/div
Start-Up Response
PWM Operation at 200Hz and 1% Duty Cycle
VLX (20V/div)
VLX (20V/div)
VOUT (20V/div)
IOUT (10mA/div)
IOUT (10mA/div)
VEN (5V/div) 1ms/div
VEN (5V/div) 10s/div
6
SC4540
Typical Characteristics (continued)
PWM Operation at 20kHz and 10% Duty Cycle PWM Operation at 32kHz and 17% Duty Cycle
VLX (20V/div)
VLX (20V/div)
IOUT (10mA/div)
IOUT (10mA/div)
VEN (5V/div) 2s/div
VEN (5V/div) 2s/div
PWM Operation at 50kHz and 20% Duty Cycle
VLX (20V/div)
IOUT (10mA/div)
VEN (5V/div) 1s/div
7
SC4540
Pin Descriptions
Pin #
1 2 3 4
Pin Name
LX EN ISET COMP
Pin Function
Collector of the internal power transistor -- connect to the boost inductor and rectifying Schottky diode. Enable and brightness control pin for LED string Output current set pin -- connect a resistor from this pin to GND to set the maximum current. Output of the internal transconductance error amplifier -- this pin is used for loop compensation and soft-start. Connect a 1.27k 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 end 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.
5 6 7 8 T
OUT BL GND IN Thermal Pad
8
SC4540
Block Diagram
OUT 5 T herm al S hutdow n
U V LO
OVP 1
LX
IN
8
R eference and Internal R egulator
S R
Q
C om parator
RSENSE 7 GND
OSC
S ense A m p
E rror S ignal S election and S um m ation
4
COMP
EN BL
2 EN 6 ADJ LE D S etpoint ADJ
3 IS E T
9
SC4540
Applications Information
General Description
The SC4540 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 SC4540 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 methodology 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. 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 1A. This keeps the output capacitor charged and ready for instant activation of the LED string. 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. cycle PWM signal used for a few milliseconds provides the additional advantage of reduced in-rush 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, 10% for 32kHz PWM frequency and 1% for 200Hz PWM frequency. Refer to the Normalized IOUT versus Duty Cycle in the Typical Characteristics section for PWM performance across duty cycle for different PWM frequencies.
Zero Duty Cycle Mode
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 SC4540 provides several protection features to safeguard the device from catastrophic failures. These features include:
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 SC4540 provides dimming between 100Hz and 50kHz. The SC4540 is compatible with a wide range of devices by using dimming technology that avoids the audio band by using high frequency PWM dimming. A wide range of illumination can be generated while keeping the instantaneous LED current at its peak value for luminescent efficiency and color purity. The SC4540 can accommodate any PWM duty cycle between 0 and 100%. A low duty
* * * *
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 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
SC4540
Applications Information (continued)
must withstand 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 when 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 power is cycled on the part. Thermal Shutdown A thermal shutdown mode is included for protection in the event the junction temperature exceeds 155C. In thermal shutdown, the on-chip power switch is disabled. Switching and sinking resumes when the temperature drops by 20C. 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. where
IL VIN D L f osc 1 VIN VOUT
D
IIN
VOUT IOUT VIN
D is the duty cycle for continuous operation. Efficiency () can be approximated by using the curves provided in the Typical Characteristics section. Table 1 lists inductors that have been proven to work with SC4540. Table 1 -- Recommended Inductors
Part Number Coilcraft LPS6235 Coilcraft LPS4018-223ML Value (H) 56 22 DCR () 0.28 0.360 Rated Current (A) 1.1 0.70 Tolerance 20% 20% Dimensions (L x W x H) (mm) 6.2 x 6.2 x 3.5 3.9 x 3.9 x 1.7
Capacitor Selection
The input capacitor should be at least 2.2F. A larger capacitor will reduce the voltage ripple on the input. The output capacitor values can range from 0.22F to 1F. 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 recommended capacitors. Table 2 -- Recommended Capacitors
Part Number Input Capacitor Murata GRM219R61E225KA12 Output Capacitor Murata GRM21BR71H105KA12L Compensation Capacitor Taiyo Yuden EMK105BJ473KV-F 0.047 16 X7R 0402 1.0 50 X7R 0805 2.2 25 X5R 0805 Value (F) Rated Voltage (V) Type Case Size
Inductor Selection
The inductor value should be within the range of 22H to 68H. The DCR needs to be considered when selecting the inductor to ensure optimum efficiency. The largest inductor package that can be accommodated 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 IL 2
11
SC4540
Applications Information (continued)
Diode Selection
A Schottky diode with a reverse voltage of 60V and a forward current rating of 1A should be used with this device for optimum performance. The ST Microelectronics STPS05602 is a recommended diode for this application.
PCB Layout Considerations
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 4. The following design rules are recommended:
Selection of Other Components
RISET sets the maximum load current for the SC4540. Use the following equation to select the proper value: RISET = 230xVISET/ILOAD where VISET = 0.5V (typ). Refer to Figure 1 for selecting values for other current settings. Note that the error increases as the desired IBL current decreases.
100
* * *
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.
IB L (m A )
10
R IS E T T yp ica l
C u rre n t T o le ra n ce
1
1
10 R IS E T (k )
100
Figure 1 - Set Resistor Value Selection Graph
Figure 4- Layout
12
SC4540
Applications Information (continued)
Application Circuit for Alternate Input voltages
The typical applications circuit for the SC4540 is shown in Figure 2. The performance data and specifications included in this document are for the typical application circuit (Figure 2). The SC4540 can be used with input voltages lower than 4.5V (see Figure 3). The Inductor, L1, will need to be adjusted to allow proper operation for input voltages between to 2.8V to 5.5V. The SC4538 datasheet should be consulted for the performance of the lower input voltage circuit.
L1 5 6 H LX 4 .5 to 1 2 V IN
C IN 2.2F D 1 F C O UT
L1 2 2 H 2 .8 V to 5 .5 V IN
C IN 2.2F
D C O UT
LX OU T
1 F
OU T
SC 4540
E n a b le a n d PWM d im m in g
R IS E T
SC 4540
E n a b le a n d PWM d im m in g
R IS E T
EN IS E T C OMP
EN IS E T C OMP
CC OM P
C COM P
RC OM P 1 .2 7 k
RC OM P 1 .2 7 k
47nF
47nF
BL
BL GN D
GN D
Figure 2 -- Typical Application Circuit
Figure 3 -- Low Input Voltage Application Circuit
13
SC4540
Outline Drawing - MLPD-UT-8 2x2
A
D
B
D IM
P IN 1 IN D IC A T O R (LA S E R M A R K ) E A A1 A2 b D D1 E E1 e L N aaa bbb
D IM E N S IO N S IN C H E S M IL LIM E T E R S M IN N O M M A X M IN N O M M A X
.0 2 4 .0 0 2 (.0 0 6) .0 0 7 .0 1 0 .0 1 2 .0 7 5 .0 7 9 .0 8 3 .0 6 1 .0 6 7 .0 7 1 .0 7 5 .0 7 9 .0 8 3 .0 2 6 .0 3 1 .0 3 5 .0 2 0 B S C .0 1 2 .0 1 4 .0 1 6 8 .0 0 3 .0 0 4 .0 2 0 .0 0 0 0 .5 0 0 .0 0
-
-
A aaa C A1 A2 C
S E A T IN G P LA N E
(0 .1524) 0 .1 8 0 .2 5 0 .3 0 1 .9 0 2 .0 0 2 .1 0 1 .5 5 1 .7 0 1 .8 0 1 .9 0 2 .0 0 2 .1 0 0 .6 5 0 .8 0 0 .9 0 0 .5 0 B S C 0 .3 0 0 .3 5 0 .4 0 8 0 .0 8 0 .1 0
0 .6 0 0 .0 5
D1 1 E /2 L xN E1 2
N b xN e e /2 D /2 bbb CAB
NOTES:
1 . C O N T R O L L IN G D IM E N S IO N S A R E IN M ILLIM E T E R S (A N G LE S IN D E G R E E S ). 2 . C O P L A N A R IT Y A P P L IE S T O T H E E X P O S E D P A D A S W E LL A S T H E T E R M IN A LS .
14
SC4540
Land Pattern - MLPD-UT-8 2x2
R
H
D IM
C G H K P R X Y Z
D IM E N S IO N S IN C H E S M IL L IM E T E R S
(.0 7 7 ) .047 .067 .031 .020 .0 0 6 .012 .030 .106 (1.95) 1 .2 0 1 .70 0 .80 0 .5 0 0 .1 5 0 .3 0 0 .7 5 2 .7 0
(C ) K
G
Z
Y P X
NOTES: 1. 2.
C O N T R O L L IN G D IM E N S IO N S A R E IN M IL L IM E T E R S (A N G LE S IN D E G R E E S ). T H IS L A N D P A T T E R N IS F O R R E F E R E N C E P U R P O S E S O N LY . C O N S U L T Y O U R M A N U F A C T U R IN G G R O U P T O E N S U R E Y O U R C O M P A N Y 'S M A N U F A C T U R IN G G U ID E LIN E S A R E M E T . T H E R M A L V IA S IN T H E L A N D P A T T E R N O F T H E E X P O S E D P A D S H A L L B E C O N N E C T E D T O A S Y S T E M G R O U N D P LA N E . F A IL U R E T O D O S O M A Y C O M P R O M IS E T H E T H E R M A L A N D /O R F U N C T IO N A L P E R F O R M A N C E O F T H E D E V IC E .
3.
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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